Dear Friends,

Twenty years ago today, our cameras on Cassini captured a set of images that stunned all those following our exploration of the Saturn system. It was a set of images that capped a thrilling 11 months in 2005 that brought us the slow reveal of the small icy moon, Enceladus. As was very often the case during Cassini’s time at Saturn, the story had began decades before.

When the Voyager 2 spacecraft sped through the Saturnian system in August 1981, it came within 90,000 kilometers of the moon Enceladus, its cameras returning a handful of images that confounded planetary scientists for years.

Even by the diverse standards of Saturn’s satellites, Enceladus was an outlier. Its icy surface was as white and bright as fresh snow, and whereas the other airless moons were heavily pocked with craters, Enceladus was mantled in places with extensive plains of smooth, uncratered terrain, a clear sign of past internally driven geologic activity. At just over 500 kilometers across, Enceladus seemed far too small to generate much heat on its own. Yet something unusual had clearly happened to this body to erase vast tracts of its cratering record so completely.

Caption: Highest resolution Voyager 2 image of Enceladus, ~1 kilometer per pixel.

Voyager’s brief encounter allowed no more than a cursory look, and, in hindsight, its imaging coverage of Enceladus was terribly unfortunate: a few medium­ resolution images of the northern hemisphere, some low ­resolution coverage in the south, and none of the south pole. We had no idea what we had missed. But the interest generated by Voyager’s visit made a comprehensive examination of Enceladus a cardinal goal of the Cassini mission to Saturn.

The first hints of what awaited us on Cassini came in images that were taken in the opening months of 2005 that had been deliberately planned to search for plumes of ice particles ejected from the moon’s surface.

The E ring of Saturn was long believed to be created by Enceladus material. It was, after all, at the orbit of Enceladus that the E ring reached its densest and thickest. But after Voyager’s encounter, there were two opposing proposals for its origin: one stated that particles were being ejected from Enceladus by impacts on the surface from high-energy particles, and the other stated that they were created by geysers. Geysers, of course, would require sub-surface liquid water. But attempts to try to theoretically explain how such a small moon like Enceladus could be warm enough to have an interior liquid water layer in the present day, billions of years after its formation, failed badly, even after adding an anti-freeze like ammonia.

Yet, Enceladus had obviously undergone in the recent geological past an episode of internal activity, strongly implicating liquid water. In fact, in the figure caption written by the Voyager imaging team for the Voyager 2 Enceladus image above (https://ciclops.org/view/3183/Voyager-2-Color-Image-of-Enceladus-Almost-Full-Disk.html), it was stated:

“It is possible that the high reflectivity of Enceladus’ surface results from continuous deposition of icy particles from Saturn’s E-ring, which in fact may originate from icy volcanoes on Enceladus’ surface.”

… and …

“The implication carried by Enceladus’ surface is that this tiny ice ball has been geologically active and perhaps partially liquid in its interior for much of its history. The heat engine that powers geologic activity here is thought to be elastic deformation caused by tides induced by Enceladus’ orbital motion around Saturn and the motion of another moon, Dione.”

Very prescient suggestions, indeed, and in fact eventually confirmed by Cassini.

All considered, before we ever got to Saturn, I was of the mind that a sub-surface water ocean, and geysers, were very good possibilities, so much so that in our 2004 Space Science Reviews publication in which my team and I described the capabilities of the Cassini imaging experiment, as well as the scientific goals we aimed to achieve -- https://ciclops.org/sci/docs/CassiniImagingScience.pdf -- I wrote, ‘Enceladus could be the Europa of the Saturn system’. Accordingly, our first Enceladus imaging sequences, planned for the first 3 months of 2005, had the words ‘plume’ in the title and were taken as the moon was backlit by the Sun, a viewing geometry that planetary astronomers call high solar phase.

Just as the dust that coats your car’s windshield becomes dramatically more visible when you drive into the sun, so do the very fine particulates that are spread throughout the solar system when you look through them at high solar phase. These viewing circumstances had proven very successful throughout the Voyager mission in revealing hard-­to­-see structures in rings and atmospheres of the outer planets and their moons, and they were key to our imaging investigation of Enceladus.

Such images taken in January, February and March 2005 all revealed a flare protruding from the moon’s south polar limb.

Caption: The first Cassini sightings of the Enceladus plume in early 2005 … b&w on the left and enhanced, false-color-coded on the right … on Jan 16 (at 1.3 kilometers/pixel) and Feb 17 (at 1.8 kilometers/pixel).

We Voyager veterans on the team were immediately reminded of the volcanic plumes rising above Jupiter’s volcanic moon Io and the gossamer hazes in the atmosphere of Neptune’s moon Triton.

In addition, during the same flybys, the magnetometer instrument team noticed that Saturn’s magnetic field lines were being distorted as the planet’s rotation carried them passed Enceladus—a sign that the field lines were picking up heavy ions. The source of the ions appeared to be a tenuous atmosphere. Based on these results, the principal investigator of the magnetometer experiment presented the case to the Cassini project managers to get a better look at what might be causing such an atmosphere —specifically, to lower the altitude of the upcoming July 2005 Enceladus flyby.

The request was granted and on July 14, Cassini flew closer to Enceladus than originally planned, under the moon and over its high southern mid-latitudes, giving our cameras for the first time a clear shot of the south pole where a landscape, as astonishing and geologically distinct as any seen anywhere in our solar system, was discovered.

Caption: https://ciclops.org/view/1878/Enceladus-the-Storyteller.html

The terrain surrounding the south pole, we came to find, is a roughly circular region, completely crater­-free and prominently etched by a handful of deep, parallel cracks we had earlier dubbed the “tiger stripes” … because in much lower resolution images they looked like the stripes on the back of a tiger. Nearly evenly spaced by ~35 km, they each extend ~130 kilometers within a region sharply demarcated at 55⁰S latitude by a contiguous and meandering circumpolar boundary of concentric mountains and valleys.

There is obviously a tale writ on the countenance of this little moon that tells of dramatic events in its past, but its present, we were about to find out, is more stunning by far. In its excursion over the outskirts of the south polar terrain during the July 2005 flyby, Cassini’s dust analyzer also picked up tiny particles, apparently coming from the region of the tiger stripes. Two other instruments detected water vapor, and one of them delivered the signature of carbon dioxide, nitrogen and methane. Cassini had passed through a tenuous cloud, confirming the imaging and magnetometer results returned earlier in the year. Only now, it was beginning to look like maybe the basic organic ingredients necessary for life might be present!

What is more, a combination of our images and the results of the thermal infrared detector showed elevated temperatures along the fractures—possibly as high as 180 kelvins, well above the 70 kelvins that would be expected from simple heating by sunlight.

We could not believe our good fortune to have stumbled on such a fascinating place. In quick response, the imaging team planned a special series of images to be taken in four months’ time—late November 2005—looking over the south pole at very high resolution and very high solar phase.

And so it happened that 20 years ago, on November 27, a series of striking images of a crescent Enceladus arrived on Earth showing, as clear as day, the actual geophysical origins of the material that our Cassini instruments had detected: a dozen or more distinct and narrow fountains of fine, icy particles jetting into space and feeding a faint but giant, flame­-shaped plume towering over the south polar terrain.

Caption: https://ciclops.org/view/1688/Fountains-of-Enceladus.html A time-lapse video clip was also released: https://ciclops.org/view.php%3Fid=1702.html

Later analysis by my research group found that the jet sources, which the imaging team had shown to be geysers sourced in liquid water, coincide with the hottest locations on the tiger stripe fractures … the first definitive evidence of a connection between the fractures, warmth, and active venting.

By anyone’s measure, these images were a dramatic find: an incontrovertible indicium of current internal activity on an otherwise cold little moon. I could not help but feel an immediate kinship with those long ago who first set eyes on the geysering turmoil of the Yellowstone region of the American west.

In the 20 years since, much has been learned about Enceladus. What Cassini eventually found on this tectonically wracked little world has been a planetary explorer’s dream come true, and as a result, this tiny outpost tucked deep within a magnificent planetary system clear across the solar system has taken on a significance that belies its diminutive size. Energy, organics, and liquid water are three main requisites for life as we know it, and all have been found in significant quantities on Enceladus, marking this tiny moon as a potential host for living organisms.

Many of us are eager to see a return to Enceladus to search for definitive signs of life. And though it has become obvious that if and when it occurs, it will be so far in the future that those of us original Voyager and Cassini scientists won’t live to see it, it has nevertheless been enormously gratifying and pride-making that we were there at the very beginning. And that’s plenty good for one small, human life.

Earlier this month, and all these years after our first Cassini Enceladus images and our astonishing discovery of its geysering basin and the plume it creates, my research colleagues and I submitted for publication what will likely be my final, magnum opus on Enceladus … a very detailed look at the structure of its plume, its variability, and the ground motions along its fractures that are likely responsible for producing it. Be assured that I will send out the word when that treatise is finally published.

In the meantime, I hope you, here in America, are enjoying a lovely Thanksgiving 2025, and I wish all of you around the world a safe and happy holiday season.

Carolyn Porco

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Dear Friends,

One of the most extraordinary events in the history of our travels among the planets in orbit around our Sun took place twenty-years ago today.

The scene was a cold, haze-enshrouded moon in the outer solar system. With over 7 years of travel behind it, a human-made device entered the top of its atmosphere, drifted downwards on a piece of fabric, chronicled its 2.5 hour descent with onboard environmental sensors and panoramic cameras, came to permanent rest on its surface, and for the next 90 minutes relayed its findings back to Earth.

And so went the landing of the European-built Huygens Probe on the surface of Titan, the most highly anticipated event of the entire Cassini mission to the Saturnian system, the moment that would reveal what had been hitherto unseen on Saturn’s largest and most mysterious satellite. We knew that what we learned from this experience would be enlightening and serve as indisputable ground truth, a guide that would inform and assist us in making sense of the remainder of our time exploring Titan from the Cassini orbiter, and we were eager for the outcome. It made our wait all the more anxious.

Three days earlier, in preparation for the arrival of the Huygens probe at Titan, we at CICLOPS had crafted our usual coverage map, utilizing of course the best Titan images that we had available from orbit, to illustrate the plans for descent imaging of the surface that we on Earth were about to see.

Caption: This map illustrates the planned imaging coverage for the Descent Imager/Spectral Radiometer, or DISR, onboard the European Space Agency's Huygens probe during the probe's descent toward Titan's surface on January 14, 2005. For more information, see the full caption.

And to herald the occasion, I was moved to write a Captain’s Log. I reproduce it here to remind us all how it felt to be expecting that weighty moment when, in a flash, the solar system would become a very much smaller place.

“January 11, 2004

“Separated from Cassini 3 weeks earlier, the probe is now racing silently towards Saturn’s most mysterious moon, Titan, at a swift 7 kilometers per sec, or 17,000 miles per hour. In just 3 days, it will impact the top of the atmosphere and rapidly decelerate, making a glowing fireball that no one will see and a sonic boom no one will hear ... shock waves spread 'round a world in an alien planetary system far, far away.

”The probe will descend through the thick layered smog of the upper stratosphere and eventually dive below the haze into the dimly lit environment near the surface. It will drift with the wind and spin as it falls. It will record panoramic images of the scene below, sift through the electromagnetic radiation reflecting off the surface, and sense the properties of the Titan atmosphere, in many ways sister to our own. Should it touchdown in a lake of liquid hydrocarbons or land on hard frozen ice, it - and we - will know.

”All these precious, hard-earned bits of knowledge, encoded into a stream of ones and zeroes that will ride the telemetry signals back to the orbiter, will find their way to Earth three hours later.

”The first images taken from 150 kilometers above the surface will likely look very much like the landing site images we've already seen, but subsequent images will grow rapidly in detail. The landing site itself is diverse, with regions of bright and dark and surface streaks, similar to other areas on Titan. This is good, for what we learn from this thrilling two-hour voyage will guide us in understanding Titan in all its complexity.

”We are about to enter a cold and misty wilderness, never before touched by anything human. This will be a tale to tell, of exploration, discovery and intrigue, not unlike those told by Jules Verne a century and a half ago. Only this time, it will be real.

””We are capable of extraordinary achievements, and this will surely be one of them.

Ladies and gentlemen ... prepare to make contact.”

I personally will never forget being in Darmstadt, Germany, where the European Space Agency operations center for the Huygens portion of the Cassini mission was sited, and seeing the first images to be publicly released by the Huygens imaging team (below) after the probe’s landing. Despite all the thinking, anticipation, and consideration of what we might see, what we saw floored us.

All at once, we found ourselves staring at unfiltered, exquisitely detailed views of the Titan's surface. They were unambiguous in their account. They told of a bright terrain, bounded by what looked for all the world like a shoreline, and etched by a complex, dendritic drainage system that could only have been made by a fluid, either at present or sometime in the past, flowing downhill, draining through narrow ravines and canyons, and discharging onto the formless, dark, adjoining plain.

Caption: This mosaic of three frames from the Huygens Descent Imager/Spectral Radiometer (DISR) instrument take from an altitude of ~10 miles provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources.

It was an unmistakable Earthlike process at work on an exceedingly cold and strange extraterrestrial body … a startling conjunction of the familiar and the alien. Science fiction had become reality. I felt disoriented … in one universe one instant, in another the next. It was an exhilarating rite of passage that forever left all of us altered and wanting more.

And there was, of course, a great deal more to come. There was, immediately following, the Huygens image taken after it reached the surface, shown here in color.

Caption: Under a cloudy sky and across a cobble-strewn ground, the Huygens probe, having just landed on Titan’s surface, looks to the moon's horizon in the distance. The icy cobbles in the foreground are roughly a foot across.

And then there was the slow and suspenseful reveal of the entire Titan surface over the next dozen years by the Cassini mother ship and its cargo of scientific instruments.

With perhaps the exception of the first sighting of the geysers on the surface of the small icy moon, Enceladus, which signaled what has come to be regarded as the most promising place in the solar system to search for extraterrestrial life, there was no other single occasion during our 13 years at Saturn that carried the same acute, emotional resonance as making landfall in the outer solar system for the first time.

An experience like that happens only once, and I will die a grateful woman that I lived to see that sacred day all those years ago.

Happy Huygens Landing anniversary to all!

Carolyn Porco

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Dear Friends,

Twenty years ago today was the day that we on Cassini first began to unravel one of the longest standing mysteries in the study of the bodies in orbit around our Sun. It was the day we got our first close-up view of Saturn’s moon, Iapetus.

In the late 17^th century, the Italian astronomer Giovanni Domenico Cassini discovered four moons around the planet Saturn. The first was Iapetus, sighted in 1671. (The others were Rhea, Tethys and Dione.) Iapetus was only the 6^th extraterrestrial moon to be discovered, after Galileo’s 1610 discovery of the (ahem!) Galilean moons of Jupiter, and Christian Huygens’ 1655 discovery of Saturn’s largest, Titan.

Cassini found soon that as Iapetus traveled in its orbit, it could be observed on one side of Saturn but not on the other. He conjectured that three conditions must be in play for that to make sense: Iapetus must have one face always pointed at Saturn (as our Moon always has one face pointed to the Earth), it must be very much darker on one side than the other, and such a pronounced brightness dichotomy could be explained if the moon were covered with a bright material on one side and a very dark material on the other. We know today that he was exactly right on all counts.

No other body in our solar system at the time of Cassini was known to have such an extreme contrast in brightness between one hemisphere and another, and that remains true even today. Iapetus, at ~900 miles across and ~40% the size of our Moon, is the yin/yang moon of our solar system. How it got that way was a great mystery for 350 years.

In 1972, planning for the Voyager mission (then called Mariner Jupiter Saturn, or MJS) to the outer solar system began. At that time, the trajectories of the two Voyager spacecraft were not yet determined. But there was hope, and even reasons to expect, that at Saturn, imaging of at least 2 satellites at resolutions of 0.6 miles or better would be possible. Experience with imaging Mars – from the Mariner missions in the 1960s and Mariner 9 in 1971/72 – suggested that resolutions of that order, together with extensive spatial coverage, would allow discrimination between external and internal processes in modifying satellite surfaces.

The Saturnian moon of greatest interest at that time was Titan, Saturn’s largest and known to have a substantial atmosphere laced with simple hydrocarbon compounds, like methane. But Iapetus, with the most intriguing brightness distribution in the solar system, was a close second.

The first proposed explanation for Iapetus’ yin/yang appearance came in 1974, when planetary physicist Steve Soter, showed how impact ejecta released from the surface of Phoebe … the largest and very dark, retrograde, irregular moon of Saturn, and the next one outward beyond Iapetus … would spiral inwards and form a ring of dark debris that extended all the way down to the orbit of Iapetus. In its never-ending motion around Saturn, Iapetus would sweep up this dark material, which had the potential to darken its leading hemisphere. It was hoped that Voyager would reveal whether this or some other mechanism was the answer to the riddle of Iapetus.

Finally, early in the 1980s, and 309 years after the discovery of Iapetus, the two Voyager spacecraft sped through the Saturn system and gave humankind its first close glimpse of this strange body.

Caption: Voyager 1 image of Iapetus, November 12, 1980, from a distance of 1.9 million miles, yielding an image scale of 17 miles/pixel.

Caption: A Voyager 2 image of Iapetus, taken on August 22, 1981 from a distance of ~600,000 miles, yielding an imaging scale of ~ 6 miles/pixel.

Voyager 1’s images revealed a roughly elliptical shape to the boundary between the dark and light terrains on Iapetus, as well as some large surface features. Images taken by Voyager 2 from three times closer than Voyager 1, made clear that the bright side of Iapetus was heavily cratered and the boundary between light and dark was graduated and feathery in places. But within the dark terrain, no surface features were seen in either image, and no other topographic information was forthcoming either. Its reflectivity was found to be a paltry 3% … roughly half the reflectivity of the dark areas on our Moon and comparable to that of coal. And it was clear that the colors of the bright and dark areas were not the same: the dark side was reddish, the bright material less so. From ground-based telescopic observations made after the Voyager Saturn encounters, it was suspected that the dark material contained, in large part, organic substances, probably compounds such as frozen hydrogen cyanide polymers.

But most notable, upon closer examination, were the elliptical reflectivity contours in the dark area: they were centered on the apex of orbital motion. That is the locale on the surface that leads in the moon’s orbit around Saturn. The bright material, on the other hand, was seen everywhere else … on the trailing hemisphere and at the poles. This seemed to align very well with the Soter hypothesis of emplacement of externally sourced, impact-derived debris from a more distant retrograde moon, like Phoebe.

The trouble, however, was that what we knew then about the composition of Phoebe did not match what was found on Iapetus. And the vacuum created by a lack of an explanation of course invited other proposals. The leading alternate suggestion at the time was that Iapetus’s leading hemisphere was the site of a major cataclysmic event long ago that caused sub-surface material, unusually dark and rich in organic substances, to gush out onto its surface and coat its leading hemisphere.

So, in the early years after the Voyager flybys, there were two distinctly different origin scenarios – one internal and one external -- for the dramatically split countenance of Iapetus: it was either material rich in organics derived from within the moon that was coating the surface, or material from elsewhere had moved into the Iapetan orbital corridor, was swept onto Iapetus, and darkened its leading hemisphere. This riddle would remain unsolved until, two decades later, Cassini arrived at Saturn.

We on the Cassini imaging team knew we had the most sophisticated imaging system ever installed on a planetary mission and we intended to make the most spectacular use of it at Saturn we could muster. If we had anything to do about it, any object that could be imaged would be imaged. Among satellites, Iapetus was one of our prime interests; thankfully, it was also of prime interest to other Cassini teams.

The problem, however, was that getting to Iapetus was a huge investment in spacecraft resources: it was the most distant of the major satellites … 3 times farther out from Saturn than Titan … and getting extremely close to it would mean taking time and fuel to climb up Saturn’s gravity well and then carefully move in close. And most likely, all that maneuvering would necessitate giving up a Titan close-pass to do it. Almost all agreed, that was not an acceptable choice.

So, the decision was made that there would be only one extremely close Iapetus ‘targeted’ flyby that would take us within ~1000 miles of the surface. It wouldn’t occur until September 2007, near the end of the nominal mission.

However, we were able to execute two fantastic, better-than-Voyager flybys of Iapetus in which we came within 100,000 miles and 250,000 miles of Iapetus, respectively, producing images with spatial scales down to ~0.6 miles/pixel. The first of these occurred 20 years ago today, on New Year’s Eve, 2004.

All flybys of alien bodies are thrilling events … when you lose track of human time, when one astonishment is immediately replaced by another. Iapetus was a mind blower from the moment our cameras began to produce images sharper than Voyager’s. That moment occurred on the long trek out to Iapetus, in October 2004, two months before our New Year’s Eve rendezvous.

In those October images (below), we were approaching from the south, looking at the transition between the dark and bright sides, and immediately saw a portion of a big basin in the southern hemisphere, and recognized a line of white features that had been discovered in Voyager images. These were bright mountain peaks, aligned remarkably close to the equator, which were believed at the time of Voyager to vie in height with the tallest peaks in the solar system. But there wasn’t much else to be deduced from Voyager. Flybys always leave more questions than they answer, and Voyager left us with the need to go back to Saturn and have a longer, deeper look. And our new Cassini October images were great but not yet great enough to be very helpful.

Caption: Theses images were taken with the narrow angle camera between October 15-20, 2004, from distances of 746,000, 684,000 and 808,000 miles from Iapetus, respectively. The Sun-Iapetus-spacecraft, or phase, angle changes from 88 to 144 degrees across the three images. Images obtained using ultraviolet, green and near-infrared filters were combined to produce the enhanced color views at left and center; the image at right was obtained in visible white light. The images on the bottom row are identical to those on top, with the addition of an overlying coordinate grid. The image scale is ~4.5 miles/pixel.

To prepare for satellite flybys, we Cassini imaging folks got into the habit of making maps utilizing the best available images, to both plan our imaging campaigns and to illustrate to others what our coverage would be. Members of the public who followed our adventures with Cassini loved these maps, as it put them in the right reference frame to anticipate and contextualize our discoveries on their own. For this first Iapetus flyby, we naturally used the only good images we had … the Voyager images.

Caption: Map of Iapetus, based on a Voyager 1 image, used to plan our Cassini imaging campaign. The yellow and green areas show the areas we intended to cover with both the narrow- and wide-angle cameras. The pixel scales shown apply only to the narrow angle camera.

Eventually, our trajectory brought us up close and face-to-face with the beating heart of Iapetus’ mystery … its leading hemisphere! In the image below, with an image scale of 0.6 miles/pixel, we are looking at exactly 90 degrees West longitude or, in other words, the apex of Iapetus’ motion around Saturn and the center of the dark region, now called Cassini Regio (after the man, not the spacecraft). Within this region, and especially near the equator, dark deposits with a visual reflectivity of only ~4% coat nearly everything with remarkable uniformity. However, at latitudes poleward of about 40 degrees, the surface transitions to a much brighter, icy terrain near the pole where the brightest icy materials have reflectivities over 60%. This region is not uniform: close inspection reveals that the surface is stained by crudely north-south trending wispy streaks of darker material, typically a few miles wide and sometimes tens of miles long. These are very much the signs of an emplacement process, not a volcanic eruption.

Caption: A mosaic of 4 narrow-angle camera images, taken on December 31, 2004 and stitched together to form a global view of Iapetus. This particular view looks right in the face at the heretofore never seen, dark, leading hemisphere of Iapetus. Cassini’s distance was ~100,000 miles; the image scale in the full image is 0.6 miles/pixel. The mosaic has been contrast-enhanced to aid visibility of surface features. [Full Image Here]

A heavily cratered, and therefore ancient, 250 mile-wide impact basin appears just above the center of the disk. The basin is overprinted by more recent, smaller impact craters, and its rim is delineated by steep cliffs that descend to the basin floor. Many of these cliffs, as well as walls of nearby craters, appear bright, probably due to exposed outcrops of relatively clean ice. Particularly at the northern mid-latitudes seen in the New Year’s Eve images, the brightest cliff exposures appear to face away from the equator (i.e. toward the pole). Often, the opposite south-facing cliffs are stained with the lower-albedo material. This particular set of relationships was the clue that moved us imaging team members to realize that at least on small spatial scales, a runaway thermal effect was in play that caused dark areas to get warm and lose their ice through sublimation and thus leave behind any dark non-ice contaminants and thereby get darker and warmer, lose more ice, leave behind more dark contaminants, and so on, until the dark areas were completely ice-free at the surface. Such a runaway thermal cycle is now believed to be of major importance in making Iapetus look the way it does, both on small and very large scales. I will leave the telling of this particular part of the story of Iapetus for another time.

But in the meantime, what to our wondering eyes should appear … the most unique, and perhaps most remarkable feature that we discovered on Iapetus … an unmistakable, 12-mile wide, over 800-mile-long topographic ridge that coincides almost exactly with the geographic equator. [It had actually been discovered on December 25, 2004 … 6 days earlier.] On the left horizon, the peak of the ridge reaches at least 8 miles above the surrounding terrain. Along the entire length that it can be traced in this picture, it remains almost exactly parallel to the equator within a couple of degrees. In places the ridge morphs into mountainous peaks. Some are bright white – see the October 2004 images (above) – while others have exactly the same reddish-brown color as the dark material. In height, these peaks rival Olympus Mons on Mars, which is surprising for a small body like Iapetus which is 5x smaller than Mars.

The origin of the ridge is still not known, even now that the Cassini mission is over. Back when this image was first taken, we thought it might have something to do with the unusual nature of the dark terrain. Now, 20 years later, that notion is considered unlikely.

The uniform appearance of the dark materials at the equator, the apparent thinning and spottiness of the low albedo materials at progressively higher latitudes, and dark wispy streaks near the distal margins of Cassini Regio strongly suggest that dark material was ballistically emplaced as a coating. One important finding from 20 years ago was the lack of any evidence for resurfacing of Cassini Regio by erupted fluids. Another was the high density of impact craters, indicating that the terrain underlying the dark coating is relatively ancient and has not been eradicated by whatever turned Cassini Regio dark. These simple observations argued for the Soter emplacement model, though we know now that simple emplacement is only part of the story. We wouldn’t learn the full story until later in the mission, when we had the chance to examine Iapetus even closer.

It is obvious, though, that Iapetus has a very ancient surface … heavily marked by craters of all sizes, and even sporting large impact basins. In time, we would count 9 of them on this moon.

One of my favorite images from the New Year’s Eve flyby of 2004 is this one (below), … of 75-mile-wide Malun crater, seen on the edge of, and interior to, Turgis basin, near the equator and on the right hand side of the image above. (Turgis is the largest basin on Iapetus, about 375 miles across.) It’s such a clear image of the end result of a common solar-system-wide, geologic process … a massive landslide, likely caused by the impact that created the crater, which destabilized the 9-mile-high cliff through its seismic effects, thereby causing part of it to shake, disintegrate and fall. It happened no doubt billions of years ago, but looks like it could have been yesterday.

Caption: Image of Malun, an Iapetan crater, lies up against the 9-mile-high cliff of a large 375-mile-wide basin within Cassini Regio, imaged at 0.4 miles/pixel. Solar illumination is coming from the right. The image has been contrast-enhanced and magnified by a factor of two to aid visibility.

And for fun … if you have some green/red stereo eyeglasses, go find them and take a look at the stereo image below. It was created to dramatize the topography of Iapetus and uses two images taken on Christmas Day 2004, in which we first discovered Iapetus’ equatorial ridge. I imagine Cassini, the man, would have given a great deal to time-travel into the future and see for himself what he first sighted 350 years ago.

Caption: Get your red/green stereo eyeglasses out! This is a stereo view of Iapetus, obtained with the Cassini narrow-angle camera on December 25, 3004 while on approach to its New Year’s Eve encounter. North is towards upper left. Cassini's distance from Iapetus differed between the two images by 914 miles, and the Sun-Iapetus-spacecraft, or phase, angle changed from 21 to 22 degrees. Resolution achieved in the original images was 3.2 and 2.7 miles/pixel

In the years following the 2004 New Year’s Eve encounter, we learned a lot more about Iapetus from our own images and from several of Cassini’s other instrument teams about the composition of Cassini Regio. From Voyager days, all the way up to Cassini’s Iapetus-targeted flyby in late 2007, it was believed that organics were the primary constituent covering the dark side of Iapetus. And since the particular organics proposed were also found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust, it was a reasonable conclusion. But it became clearer after the September 2007 flyby and towards the end of the mission in 2017 that this interpretation was almost certainly wrong.

In the end, two Cassini remote-sensing infra-red instruments, the Visual and Infrared Imaging Spectrometer (VIMS), and the Cassini Infrared Spectrometer (CIRS), both capable of determining surface composition, agreed that, in fact, both within Cassini Regio and on the moon Phoebe, the dominant colorant is actually silicate, or rocky, material. Particular molecular constituents of Cassini Regio seem now to be metallic iron, nanometer-size iron oxide (hematite), CO2, H2O ice, and possible signatures of ammonia, bound water, H2 or OH-bearing minerals, trace organics, and as-yet-unidentified materials.

Another Cassini instrument, the Cosmic Dust Analyzer, reported tiny dust particles streaming through the Saturn system that contained oxygen, silicon and iron … that is, iron-bearing silicates.

To cap it off, the Cassini spectral information was good enough to show that the overall shape and multiple spectral features within the spectrum of the dark material on Iapetus match those seen not only on Phoebe but on Hyperion, Dione, Epimetheus, Saturn’s rings’ Cassini Division, and the F-ring. This indicates that the material has a common composition throughout the Saturn system and likely a common source.

Where does it come from? Its ultimate source might simply be meteoritic, external to the Saturn system and landing everywhere. And because, as we now know, there exists a special mechanism for directing impact-ejected surface material from Phoebe, into an extended dust ring and ultimately directly onto the leading side of Iapetus, it is Cassini Regio where the greatest concentration of this material can be found. Mystery solved!

I hope you all have enjoyed this lovely story of a wondrous thing that happened 20 years ago today and a billion miles from here.

But now, here on Planet Earth, I’m wishing you all a very Happy New Year!

See you all in 2025!

Carolyn Porco

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Sad news. Today, Jimmy Carter, the 39th President of the United States, has died at the age of 100. He served only 1 presidential term, losing a second chance to Ronald Reagan in 1980.

Caption: Carter, with his wife Rosalynn and daughter Amy, waves to the crowd on Pennsylvania Avenue en route from the Capitol to the White House, January 20, 1977. Photo: Suzanne Vlamis/AP

People often speak of his presidency as a failed one. Someone on Blue Sky (@timlongman.bsky.social), a political science professor, paints a completely different picture, arguing that Carter's term as President was in fact 'groundbreaking':

. He promoted environmentalism, establishing the Department of Energy and expanding the National Park system.

Caption: Dec. 2, 1980: President Jimmy Carter holds up the Alaska National Interest Lands Conservation Act, which declared 104 million acres in Alaska as national parks, wildlife refuges and other conservation categories, after signing it into law at the White House in Washington, D.C. (The Associated Press)

. He made human rights an element of US foreign policy.

. He promoted international peace, particularly promoting nuclear nonproliferation, the Panama Canal treaty, and the Camp David Accords (between Israel and Egypt).

Caption: Carter, Egyptian president Anwar Sadat and Israeli PM Menachem Begin join hands at the White House in September 1978 after the Camp David accords. Photo: David Hume Kennerly/Getty Images.

. He promoted diversity in government hiring.

. He created the Department of Education.

And in his Substack post earlier today, Robert Reich, one of my favorite political commentators who served in 3 presidential administrations, including Carter's, reminds us that Carter was President of the US when the Voyager spacecraft were launched in 1977 to the outer solar system and beyond.

Read Carter's immortal words to our fellow galactic citizens, included on Carl Sagan's Golden Record, attached to the Voyager spacecraft. [Reich’s brief tribute to Carter can be found here].

This Voyager spacecraft was constructed by the United States of America. We are a community of 240 million human being among the more than 4 billion who inhabit the planet Earth. We human beings are still divided into nation states, but these states are rapidly becoming a single global civilization.

We cast this message into the cosmos. It is likely to survive a billion years into our future, when our civilization is profoundly altered and the surface of the Earth may be vastly changed. Of the 200 billion stars in the Milky Way galaxy, some – perhaps many – may have inhabited planet and spacefaring civilizations. If one such civilization intercepts Voyager and can understand these recorded contents, here is our message:

“This is a present from a small distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours. We hope someday, having solved the problem we face, to join a community of galactic civilizations. This record represents our hope and our determination, and our good will in a vast and awesome universe.”

-- Jimmy Carter, President of the United States of America, the White House, June 16, 1977

Caption: This gold aluminum cover was designed to protect the Voyager 1 and 2 "Sounds of Earth" gold-plated records from micrometeorite bombardment. The two identical, 12 inch gold-plated copper discs contain, among other information, greetings in 60 languages, samples of music from different cultures and eras, and natural and man-made sounds from Earth. Along with a message from President Carter, a message from U.N. Secretary-General Kurt Waldheim was also included.

Rest in peace, Jimmy Carter. A more unselfish, giving, compassionate American President we're not likely to see again soon.

Best wishes to everyone,

Carolyn Porco

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Dear Friends,

I’ve been very busy lately, working on a couple of science papers and preparations for the writing of a major book (or two!) Hence, my silence. But I did take time out two weeks ago to attend one of Taylor Swift’s final concerts of her Eras Tour and thought perhaps some of you ... those who are fans as well as those unfamiliar with Swift and wondering why she generates so much attention ... might be interested in a post about her.

Some of you may also be wondering ‘What does Taylor Swift have to do with our coming of age in the solar system?’

Well, a lot.

Never before has there been a woman who has achieved such an extreme level of global acclaim or commercial success in the realms of music and popular culture, all the while being a perfectly lovely individual. As a result, she has been winning awards and breaking records all her professional career, beginning from the time she was 17 years old, continuing to TIME magazine’s choice of Swift as 2023 Person of the Year, and extending to her now-completed, 2-year long, record-breaking Eras Tour.

It was her recognition by TIME that landed her on my radar screen, and I was fascinated enough to begin a deep-dive into her life. (My previous one, which lasted about a year and ended only after I had read and seen on the internet about everything there was to read and see, was Elvis Presley.) My dive into Swift's life and music made me realize that I did not want to miss the opportunity to see her Eras Tour concert.

So, at great expense, I purchased a $2700 ticket, dragged along with me the 23-year old daughter of a friend of mine, and we both headed off to see Swift's next-to-the-last show in Vancouver, Canada.

Caption: Image from Taylor Swift’s Eras Tour show, Vancouver, CA, Dec 7, 2024 (Image Credit: Carolyn Porco)

Swift is singular in every way, and her Eras Tour was no exception. Much press has been devoted to her and her tour, and analyses are easy to find. Here are some basic facts about her phenomenal success, gleaned from various sources: the Wikipedia entries on both Swift and the Eras Tour, an NPR piece devoted to the Tour’s statistics, a Swift interview with TIME, and articles on the websites of Q30TV, People magazine, Billboard magazine and elsewhere. [At the end of this essay, I include a link to my YouTube channel where I've posted 8 brief videos that I personally took during the Vancouver show. In this way, you can see for yourself what all the fuss has been about.]

• "The tour recorded unprecedented public demand, ticket sales and attendances, bolstering economies, businesses, and tourism worldwide, dominating social media and the 24-hour news cycle, and garnering tributes and accolades from governments and organizations."

• "Critics praised the Eras Tour for its concept, production, and immersive ambience, as well as Swift's vocals, stage presence, and versatile showmanship."

• The Wall Street Journal called the tour one of the most expensive and "technically ambitious" productions of the 21st century.

• “The Eras Tour staging is expansive, made of digital displays. It consists of three separate stages connected by a broad ramp. ... the stages feature various visuals and effects throughout the show. The tour's main artistic concept is world-building, featuring a diverse set of props and performing styles to convey the varying moods and aesthetics of Swift's albums.”

• “The tour's massive production is heavily inspired by Broadway theatre. It was described as a 4D cognitive experience, featuring pyrotechnics, laser lights, smoke machines, fire cannons, indoor fireworks, PixMob LED bracelets, and image projection technology, such as projection mapping.”

• Swift performed 149 shows in 51 cities across 21 countries on 5 continents between March 2023 and December 2024.

• By its completion in Vancouver, Canada, on December 8, 2024 (the day after the concert I attended), the Eras Tour had sold a total of $2,077,618,725 in tickets to 10,168,008 people, breaking the previous record by a wide margin. And those numbers do not include a secondary market of ticket sellers, sales of merchandise ($200M in 2023 alone), earnings of $262M from the highest-grossing concert film of all time, and more. No other concert tour has come close.

• She headlined a record 8 shows at Wembley Stadium, London ... a venue that holds 90,000 people.

• A typical Eras show featured 44-46 songs and ran for 3 hours and 15 minutes. Swift sang every song, with no lip-synching.

• In an interview with TIME, Swift revealed that, prior to the tour, she trained by singing the entire 40+ songlist every day for 6 months, while running on a treadmill. [I wouldn't wanna meet this woman down a dark alley!]

• In July 2023, Seattle fans danced so hard that they created the seismic equivalent of a 2.3 magnitude earthquake.

• Swift's biggest crowd (of both the tour and her entire career) was 96,000 people at the Melbourne Cricket Ground in Australia, at which she performed 3 shows in February 2024.

• She released 3 albums while on tour: She re-recorded "Taylor's Version" of Speak Now and 1989 in 2023, and wrote new songs for, and released, The Tortured Poets Department in April 2024 (which also yielded 4 music videos). That's all in addition to her concert film and book.

• Swift was Spotify's most-streamed artist for 2 years in a row, driving 26.6 billion global streams in 2024 alone.

• She has also excelled in video and film production, winning numerous awards and critical acclaim for music videos and short films that she has written and/or directed.

• She holds 118 Guinness World Records.

• Swift's storytelling, lyricism, cultural impact, and entrepreneurship have been the subject of entire courses at various colleges across the US. Some examples are: Harvard University, University of Texas at Austin, Stanford University, University of California - Berkeley, New York University, Arizona State University, Northeastern University, University of South Carolina, University of Florida, Colorado State University, University of Miami, University of Tennessee, American University, Pennsylvania State University, Brigham Young University, Duke University.

• She is the first musician to achieve billionaire status predominantly through her musical achievements.

• She is extremely generous. People magazine reports that over the past 2 years, Swift gave out $197 million in bonuses to everyone working on her Eras Tour, including truck drivers, caterers, dancers and musicians. And the Billboard magazine website documented on December 12, 2024 some of her notable gifts between October 2011 and December 2024 to charitable organizations as well as individuals whose pleas for help she had seen on social media.

Attending the Eras Tour and seeing it live was an experience I will not forget. It was a 60,000 person lovefest and a glorious, unmistakable display of bad-ass girl power. Despite it being her next-to-last, 148th show in a nearly 2-year tour, she was in top spirited form, completely engaged with her audience, obviously enjoying every minute of it, as were we.

I feel blessed to have lived long enough to see a woman single-handedly achieve the highest rank in pop culture and musical influence ... up there with Elvis, the Beatles, and Michael Jackson. How did she do it? Through sheer talent, resolve, confidence, a stoic work ethic, sufficient self-respect to ignore her critics, and the brains to outsmart those with plans to exploit her.

And she did it through love. Paul McCartney has remarked admiringly on the intimate relationship that Swift has developed and nurtured with her fans. She is famous for her generosity towards them and actively interacting with them through social media, concerts, and even surprise visits to their homes. And she consistently credits and thanks them for her success. One of my concert videos captures her speech to all of us describing her gratitude. It’s obvious she meant it.

Of course, one must give huge kudos to her parents, who showered this girl with love all her life, and recognized and nurtured her young talent from the start. They've been active in promoting her career since she was 11 years old, even leaving their home in Pennsylvania when she was just 14 and moving to Nashville, Tennessee in an effort to assist her entrance into the country music scene. It worked! They have been literally by her side every step of the way, and often with her on tour. It is clear that their devotion to her and the extraordinary grounding she received from them, and still does, have been big contributors to the mental and emotional stamina she has displayed in the face of all that very famous people have to endure.

One cannot deny that she comes from a wealthy, privileged family. But love, kindness and comfort in difficult times can be freely given by anyone. The Taylor Swift story exemplifies what a loving and supportive family can do, at least in kind if not to the same degree, for any child. I would love to read a future book written by her parents, perhaps called Raising a Star, on how they guided their daughter through the various stages of her extraordinary life. I bet it would be a best-seller.

Maybe not surprisingly, by every account, Swift has remained through it all remarkably down to Earth ... authentic, free of artifice, full of good will, kindness, and compassion, with no scandalous, self-destructive behaviors or abusive relationships in her background. She is the opposite of a tragic figure, examples of which abound throughout the history of entertainment. She’s simply a well-adjusted, big-hearted, admirable individual … an excellent role-model for young women. If I had a daughter, I'd be deliriously happy to know she was a Swiftie, and I’d be sure to be there with her and her friends, making friendship bracelets and loudly singing “I’d be the man!”.

In the end, this is why Swift and her accomplishments indeed deserve an entry in the tale of our coming of age in the solar system ...

It is the 21st century in this very developed country we call ‘the land of the free’ and we have yet to reach the point where the notion that women should be accorded the same rights and societal status as men would not encounter axe-wielding opposition by a not insignificant segment of the voting public. (See the US election, November 5, 2024.) When examined against today's political backdrop of unrepentant misogyny and ugly, bold rejection of age-old, legitimate concerns about sexual violence, reproductive and maternal health, economic and legal inequality, all of which now make women in America and many places around the world more vulnerable than we have been for decades, the global phenomenon that is Taylor Swift, in stark contrast, makes patently clear that women are in fact more than capable of serving as the architects of our own spectacular success. Taylor Swift has become girl power incarnate and, even in this particularly dark time, her rise signals the coming of age of women … a moment of glory and peak pride in our evolution.

So, do I consider myself a Swiftie? You bet! And I am here to proclaim ... pass around the friendship bracelets and Long Live Taylor Swift!

Now, take a look at my videos and glimpse the Eras Tour for yourself: The Great Taylor Swift, Eras Tour, Vancouver, Canada, December 7, 2024

I wish you all a splendid, holiday season and a peaceful, happy start to the New Year.

Carolyn Porco

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Dear Friends,

Today is Carl Sagan's 90th birthday. The first post I ever made to this site was his birthday last year.

In light of recent events, I’ve been thinking about him a lot. I find myself acutely grateful for that brief period of time when Carl's lifeline and mine crossed, and I called him friend and colleague. I was blessed and didn't then fully appreciate it. I certainly do today. I would imagine there are many of his colleagues and his students who also miss him today. Especially today.

I'm also enormously thankful for that time when all of us could hear from the man himself, and take freely his insights and wisdom as guides for evaluating the world, both natural and human. We were all blessed. His words, even truer today, are of course still there for us to savor and contemplate. I do from time to time. Do you?

Happy 90th, Carl! Wish you were here. And thank you for everything.

Carolyn

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Hello to all!

It’s been a long while since I’ve communicated with you. That’s because it’s been one of those harrowing times when several obligations and promises, and the deadlines that haunt them, have all matured at the same time and I had little time for much else.

But I’m writing to you now to tell you some good news … at least if you’ll be in or near Scotland at the end of this month.

Richard Dawkins, renowned evolutionary biologist and fierce defender of science and reason, is on his very last book tour for his latest book 'Genetic Book Of The Dead'.

He'll be at the Royal Concert Hall in Glasgow on October 31, and I am delighted to inform you that I will be his host and interviewer for the evening!

This means that I am now busy with another deadline … reading his book and composing my list of questions and talking points. It’s another insightful Dawkins’ look at the natural world, it’s been a real joy to read, and I have even planned one very funny question which I’m certain he won’t expect.

It will be educational, fun, and of course sentimental to hear Richard riff in public one last time. If it’s at all possible to get yourself to Glasgow on Halloween, don't miss it!

Tickets are running low, so get yours now at richarddawkinstour.com

Hope to see you there!

Carolyn

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Dear Friends,

If you are like me, you are aghast at recent developments within America’s political life. In my post on this website last New Year’s Eve, I remarked:

“There is much to be concerned about in the near future.  The US, and indeed the world, today are being buffeted by political and social winds that were last in play nearly a century ago.  And the disastrous undoing of the world order at that time, its horrifying evolution, and the clear object lesson those past events serve today in how to ensure it doesn't all happen again are, to the amazement of many of us, being unheeded by too many.”

But, despite the ominous signs, I found reasons to remain optimistic and hoped that you all would join me.

“This time of year presses us to look forward at the 'second chance' a new year always brings, so that's what I'm doing one more time this year, with feeling, and the hope that you are too.”

That was only 6 months ago. Back then, never did I imagine that the Supreme Court would rule, according to the Associated Press website, that “former presidents have absolute immunity from prosecution for official acts that fall within their ‘exclusive sphere of constitutional authority’ and are presumptively entitled to immunity for all official acts.” This ruling knocked the teeth out of Special Prosecutor Jack Smith’s “use of allegations that Trump tried to use the investigative power of the Justice Department to undo the election results, holding that his communications with agency officials is plainly protected from prosecution.”

And never did I imagine that Biden’s apparent but not completely unexpected cognitive decline would engender animus and blow back from his own party, a response especially dreadful when one considers the far more serious cognitive impairment of Trump and his amply demonstrated lack of fitness to lead, and the very long list of impressive achievements of Biden’s administration … a list that proves he has surrounded himself with an effective, very well-functioning governing body that Democrats should be desperate to keep running.

These developments have made it seem not at all impossible that Trump could escape prosecution and, should he win, install himself as the first American dictator.

Of course, the rest of the world is undergoing this same tilt away from democratic forms of government towards authoritarianism. There’s certainly no comfort to be had in that. If anything, it forces one to look at what the underlying causes for such a global phenomenon might be.

In doing just that, I find myself from time to time stepping back from the details and the noise in order to gain some perspective. And I have come to regard with great respect the writings of Robert Reich on this matter. The man served in government under 3 US presidents — 1 Republican, 2 Democrats — and has wisdom to spare as well as the long view that comes from deep experience. If you don’t already subscribe to Reich’s free Substack newsletter, do. He has a 10-part series of posts on his website entitled The Roots of Trumpism. He examines the intersections of various political and economic threads in American government, going as far back as McCarthyism in the 1940s, to explain today’s state of political affairs. It makes for fascinating reading.

I was also in need of perspective at the time of the 2016 election, when I was on the Board of Advisors of Scientific American. The day after the election, each of us advisors was asked to write a statement about the outcome and our thoughts about it. The other advisors -- about 8 others, all very accomplished people — were playing it carefully, even being cautiously optimistic that it was a one-off event and things would soon return to normal. I was the only one who saw in it a nightmarish, possibly irreversible transition that “could signal the end of the American republic”.

My piece was called A Sense of the Inevitable and, in looking for the causes of Trump’s win, I looked far deeper into the past than even Reich has done. While I thought after reading all the other essays that mine might have been a tad bit melodramatic, it is clear now that what I wrote was, instead, horribly on target.

Please read it, and let me know in the comments below what you think. I’d love to hear from you.

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Dear Friends,

Twenty years ago today and a billion miles from here, the Cassini spacecraft came to the end of its seven-year interplanetary journey and flawlessly glided into orbit around the planet Saturn. Flying over Saturn's rings at a swift 25 kilometers per second, we finally arrived. It was momentous!

As one might expect, there was a lot of press activity surrounding this event, and as the leader of the Cassini imaging team, I was tasked with reporting to the world what we saw when we got there. At the bottom of this post is a video clip of my presentation at the press briefing that was held at JPL on the morning of July 1, 2004, after the orbit insertion maneuver was completed …

… and we had successfully imaged the rings as we flew closer to them than we had ever been, returning shocking details that had never before been seen. It was, after all, why we spent all those many years getting to Saturn … to see what hadn’t already been seen and to learn what we didn’t already know.

I had been asked by The Planetary Society (TPS), a space advocacy organization founded in 1980 by Carl Sagan, Bruce Murray, the former director of JPL and one of my favorite Caltech grad school professors, and JPL aeronautics engineer Louis Friedman, to write an article for their bi-monthly publication, the Planetary Report, about Cassini’s entry into Saturn orbit. I’ve decided that instead of writing a new article to commemorate this special anniversary, I would simply post here the Planetary Report article from 20 years ago.

I called it Captain’s Log: 2004.184 and you will see as you read it that I was very much rocked by the magnitude and significance of the moment, and groping to put into words the impressions and feelings left in me by the whole surreal event. Over the years, re-reading this chronicle, which I’ve done many times, has given me great pleasure. It describes an occasion of great historical significance, for sure. But, it also captures a pivotal and triumphant moment in the life of a planetary explorer who had spent her professional career, first in training and then on assignment, learning her trade and working like mad to meet the challenges that lay before her. It is me at my most authentic, deeply affected, contemplative self.

I describe in this piece the discoveries we had made in the month or so before Saturn arrival. We had found 2 small moons orbiting among the inner major moons, and had an extraordinary, mind-blowing close flyby of Phoebe … the largest of Saturn’s irregular satellites. [Saturn’s cloud of irregulars was discussed in my Remembering Cassini - Entering the House of Saturn entry from April 14, 2024.] Phoebe was an unforgettable overture to the great adventure up ahead and made it strikingly clear that our time at Saturn would be spectacular.

We also had been keenly and frequently observing Titan as we approached Saturn, using a filter combination designed to minimize the effects of scattering by the atmosphere and allow views of the surface. By early June, we had turned those observations into a movie that showed the entirety of Titan’s surface.

It was still rather low resolution, in places 55 miles/pixel at best. But we were rapidly closing in on this alien world — the last, single and greatest expanse of unexplored terrain we had remaining in our solar system — and we were hopeful that in time, the secrets that Titan had hitherto held from human view would be revealed. [Pssst … we were not disappointed!]

Not surprisingly, I spent considerable column-inches in the piece describing the elegant, precisely executed features that dominated the startling images we took of Saturn’s rings during orbit insertion. I spent the time allotted to me in the press briefing (below) doing the same. At this stage in my career, my focus was the study of planetary rings. I knew Saturn’s rings intimately and this was that moment when I was seeing them again for the first time in 23 years. It was, for me, a rapturous homecoming.

The essay ends with our first, Voyager-class flyby of Titan about a day after we entered orbit. Though it wasn’t an exquisitely close encounter like the many we would see over the ensuing 13 years, we were already seeing features on the surface that looked like sinuous channels, and wondering out loud if these might be riverbeds or canyons carved by the liquid hydrocarbons that had been predicted to be there. At this point, we didn’t know exactly what was in store for us, but we were growing in confidence that it would turn out to be fantastic.

Today, we all know how the story of Cassini’s time at Saturn turned out.

Twenty years ago, this is how it began.

Captain’s Log: 2004.184

There are times when human language is inadequate, when emotions choke the mind, when the magnitude of events cannot properly be conveyed by the same syllables we use to navigate everyday life. Last night, a balmy and ordinary summer evening in Pasadena, California, was such a time.

In a history-making maneuver so flawless, so perfect it seemed dreamt, one comparatively tiny machine, along with its builders, its operators, its scientists, its well-wishers—indeed, all of humankind—fell into the embrace of giant Saturn, a place that had been a distant destination, in the mind and in the future, for nearly a decade and a half.

After seven years of design and development, Cassini was launched in October of 1997. Its initial years were spent cruising the inner solar system, with flights by Venus and Earth, and then a distant flight by Jupiter in late 2000, just to gather sufficient momentum to place it on its final, southerly approach to Saturn.  Even at speeds approaching 6 kilometers (4 miles) per second, Cassini required more than three years to cross the empty abyss between the two giant planets, a distance as great as that separating Jupiter from the Sun. Then, last night, June 30, 2004, following instructions that had been uploaded into its memory two weeks earlier, the craft rose through a wide berth in Saturn’s rings, glided swiftly up and over their northern unilluminated expanse, ignited its counter-thrusting main engine, and quietly took up orbital residence around Saturn. At that moment, Cassini became the farthest robotic outpost humanity had ever established around the Sun, a tiny but glistening newcomer to the skies of the ringed planet. We had arrived.

It was difficult to know what to say, what to feel. Fourteen years of thinking, designing, building, testing, planning, dreaming . . . fourteen years of anxious anticipation . . . had been insufficient to prepare us. Perhaps nothing could.

Not even Phoebe.

Prelude at Phoebe

Phoebe, a moon 220 kilometers (130 miles) wide, plies the outer reaches of Saturn's gravitational influence in a retrograde orbit, a clear indication that this body is an interloper, an intruder to this particular planetary system, captured into orbit probably billions of years ago as Saturn and its moons were forming. We had buzzed this small world only three weeks earlier, coming within 2,000 kilometers (about 1200 miles) of its surface, in a daring flyby that already seemed far away and long ago. At that range, the relative motion between a spacecraft and the surface of a body can be so large that it impedes the acquisition of sharp images. However, Cassini had been designed for such maneuvers: a staggering 53 of them have been planned over the course of its nominal four-year investigation of the Saturnian environment. It had been programmed to pivot while it flies, keeping the sights of the cameras and other remote-sensing scientific instruments staring at a particular point on the surface of a body, despite the rapid movement between the two.

Planning and programming notwithstanding, we can never fully predict what such an encounter will produce. If we could, there would be no point in conducting one.

Ground-based observers had told of the presence of water on Phoebe’s surface, an observation which strongly suggested that Phoebe had its origins not, for example, in the rocky waterless asteroid belt whence the moons of Mars arose, but most likely in the hinterlands of the solar system, out in the dark, cold regions patrolled by Uranus and Neptune.

However, in science, suggestions are one thing; direct, detailed evidence—the kind that speaks a thousand words—is quite another. As is often the case in the study of planetary bodies, what we found at Phoebe was similar in some respects to what had been seen on equivalently sized bodies, making what was different about Phoebe all the more intriguing.

Phoebe's landscape is cratered all over, indicating an ancient surface, one not reprocessed, as on larger moons, by internal mechanisms. This alone was not unusual. Phoebe also was grooved in places, with grooves similar to those seen on the Martian moons, including Phobos. The origin of the grooves on both bodies is a mystery, but their forms suggest cracks in the “bedrock” of the moon, filled in by loose overlying rubble. The closest images, in which details as small as 15 meters can be seen, reveal blocks 50–300 meters across—again, a circumstance similar to that observed on the Moon and asteroids such as Eros.

However, the variations of brightness across Phoebe’s surface were larger than had been seen on a small body, with very bright areas exposed by large landslides and even very small craters. Furthermore, the excavation of some of the larger craters revealed a layering of bright and dark material at the surface. In places, the upper layers were clearly several hundred meters thick. This was unusual. These and other imaging observations (such as a measure of Phoebe’s volume) and readings from other instruments on board Cassini—in particular, the reading of the body’s mass by Doppler tracking of the spacecraft—told of a body rich in ice though probably more rich in rock, with a mean density that could be explained best if Phoebe formed in the outer solar system. Its outer layers were clearly compositionally and spatially variable, and its surface was coated in a very thin, dark blanket of something. The Visual and Infrared Mapping Spectrometer, capable of discerning composition, found that “something” to be, in places, partly organic, also suggestive of an origin in the outer solar system.

We concluded, as Phoebe receded behind us, that we had just paid visit to an ancient, primitive relic of the early solar system, one member of the population that formed Uranus and Neptune and a cousin to the dark bodies inhabiting the outer reaches of the solar system beyond Pluto, the famed Kuiper belt. If that conclusion is correct, we had just stared a Kuiper belt object look-alike in the eye. In our first historic episode at Saturn, we had just put a face to a name. Not bad for a prelude.

The Moons at Saturn

The superiority of our cameras over anything previously carried to Saturn would lead to other findings, both surprising and beautiful.

Even prior to Phoebe, we imaged the Saturn atmosphere and rings with exacting deliberation to search for temporal variations. We made distant approach observations of Titan and the other known moons. We searched for new moons that had escaped detection from the Earth and from Voyager. If Cassini did not make it into orbit, these data collected on approach would be all that we could show for years worth of effort and funding; our tour of the Saturnian system would devolve to a flyby. Deciding which images to take and which to leave out was a serious matter.

Our search for new moons was particularly gratifying. We looked among the main Saturnian moons, searched just outside the rings, and peered within the rings themselves, especially within the vacant gaps, where we surmised they would be lurking. In doing so, we uncovered, once again, Atlas and Pan—two small bodies, about 32 and 20 kilometers (20 and 12 miles) across, respectively,—orbiting just outside and just inside the main rings. These moons had not been seen by human eyes since the days of Voyager.

After a good deal of analysis of images taken in early June, we eventually discovered two very small objects, only several kilometers across—S/2004 S 1 and S/2004 S 2—between the orbits of Mimas and Enceladus. They had skipped around Saturn, presumably for billions of years, unnoticed until now. How lovely it was to know that our efforts to coax previously unseen bodies into view were successful and that our legacy would now include the discovery of new real estate within the Saturn system.

These small bodies should have been broken apart easily by the great number of small, icy comets (only about 30 meters, or 100 feet, across) believed to have originated in the Kuiper belt and to have rained down on the bodies throughout the outer solar system over the course of billions of years. The very existence of these two bodies tells us either that the flux of the small comets is not as large as previously assumed or perhaps that our new finds have had a violent history of multiple cycles of collisional disruption, re-accretion, followed again by collisional breakup and re-accretion.  The answer to this riddle will be sought in images we hope to collect of the surfaces of moons such as Enceladus. These images should record, and therefore provide measure of, the flux of small comets within the Saturnian system.

Viewing Saturn’s Rings

A  week after the Phoebe flyby, the planet’s rings appeared to our cameras as if painted in watercolor, with subtle shades of salmon, blue, gray, and brown. They are predominantly water ice, and their colors presumably reflect varying degrees and kinds of contamination by non-water components. …

Future color imaging of the rings, along with data collected by the spectrometers on board Cassini, will readily tell us the composition of the rings and its variation across them. Disentangling the composition of the original ring progenitors—those bodies whose disruption and subsequent dispersal created the rings in the first place—from the composition of the tons of meteoritic material that daily falls onto them will be a more difficult challenge.

Finally, on June 30, we found ourselves flying over the rings at a swift 25 kilometers (15 miles) per second. We were closer to Saturn and closer to the rings now than we had ever been or ever would  be again during the Cassini tour, and maybe even during our lifetimes. This was an opportunity that could not be missed, and mission scientists had been clamoring for years to be permitted to utilize it to full advantage.

The speed of the spacecraft and the minute-long shoot-and-readout image acquisition cycle meant that consecutive images, one taken immediately after the other, did not overlap in their coverage of the rings: they were separated by about 1,000 kilometers (600 miles), several times the size of one field of view. We were not going to be able to make a complete scan of the rings. It would be hit and miss, but no one cared. Whatever we got would be the best we would ever get, and we were eager for it.

The ringscape, enormous and almost supernaturally flat, spread out beneath us; we were riveted by what we saw. For the 24 years since Voyager 1’s first passage by Saturn, I have carried around in my mind the geography of Saturn’s rings. Their structure, their shape, and their behavior have been my profession and my life. Now, I looked upon familiar sights with unfamiliar clarity and saw something wholly new. I found myself almost without words to describe and explain.

Forty-three images, with spatial resolution some 25 times greater than we had seen with Voyager, were taken while the spacecraft sped in darkness over the rings, starting in the C ring, covering the inner B ring, and then beginning again at the outer B ring, continuing over the Cassini division and the A ring until we were once again staring down at empty space. We had chosen the shortest possible exposure time—5 milliseconds—to avoid any chance of smearing the images. After descending through the plane of the rings again and emerging onto the illuminated side, 18 more images—9 narrow angle and 9 wide angle images—were taken, from the F ring inward, terminating just interior to the Encke gap. These had a level of detail several times coarser than the dark side views but still five times as good as anything we had obtained with Voyager and comparable to the closest images we will take of the rings during the remainder of our tour. They were spectacular.

It was immediately evident, upon inspection of these 61 images, that we had discovered a collection of phenomena never seen before, and some not even predicted.

We saw features without any sensible order at the limit of resolution in one dark side image. In other images, we saw very narrow ringlets standing apart from broader bands carved up in a profusion of seemingly incoherent structure. But mostly, we saw waves, and more waves.

These are the handiwork of orbiting Saturnian moons which perturb the orbits of ring particles at the sites of gravitational resonances and force the particles into spiral patterns. Where the ring particles are perturbed into eccentric orbits, we found a density wave in which the enhanced concentration of particles takes the form of a spiral pattern wrapped around the planet, as in the three waves in image above and the lower left hand wave in the image below. Where the orbits are forced to be inclined by the action of an inclined moon, we found a bending wave, where the warp in the ring plane takes the form of a spiral pattern, wrapped round and round the planet, as in the right hand wave in the image below. This latter structure is akin to a traditional wave created by a pebble thrown into a pond, only the crests are spiral and not circular.

Waves are precise but curious structures, and we don’t have completely satisfactory explanations for why they look the way they do. Theories stumbled years ago, when sufficiently accurate data were lacking to guide them. Now, back at Saturn, we have collected a bounty of information we didn’t have before, a circumstance that surely will advance the study of wave formation and propagation. We can expect deeper insights into the behavior of icy particles arrayed into the giant sheet that is Saturn’s rings, and even into the behavior of icy particles that ultimately coalesced to form the outer planets billions of years ago. Saturn’s rings are our closest analog to the great flattened structures we see throughout the cosmos: from the disks of material, being discovered as we speak, surrounding protostars in our galaxy in which planets are forming even today, to the vast pinwheels of stars and gas we call the spiral galaxies. The scientific reach of Cassini and the investigations we hope to conduct with it are truly universal.

Our most artful image is surely one of the F ring, in which periodic drape-like scallops are carved by the action of Prometheus in the delicate, diaphanous material interior to the ring. But the image that kills at a thousand paces is our dayside view of the Encke Gap, 300-kilometer (180-mile) division in the outer A ring kept open by the perturbations of Pan, the gap’s only known inhabitant.

In this one image (above), we come face to face with the order and mathematical precision of countless icy particles influenced simultaneously by Saturn, Pan, and each other. The effects of Pan on the inner edge of the gap are particularly stunning and regular: scallops in the edge caused by the periodic and precisely phased in-and-out motion of many small orbiting bodies acting in unison; white streamers, marking the enhanced concentration of these bodies resulting from this phased motion, that spiral away from the edge and fade as they go; and ringlets within the gap, kept narrow by Pan’s shepherding action.  What a feast!

Last night, the world applauded us. We were exhausted and relieved. But our work had only begun.  Titan loomed up ahead.

Imaging Titan

We are now thirty hours passed the rings and we are staring down Titan, making our first Voyager-class flyby in the Saturn system. This is the telling event. The hazy Titan atmosphere had prevented the Voyager cameras, which could only `see’ at the visible wavelengths of light, from penetrating to the surface.  However, ground-based observers had learned to see the Titan surface by looking in specific and narrow spectral regions in the near infrared, where the transparency of the atmosphere actually begins to increase in the `windows’ between those spectral regions where Titanian methane is absorbing.  We had this `peering through the window’ trick in mind in outfitting the Cassini cameras, ensuring that they had the proper, narrow near-infrared filters, as well as haze-reducing polarizers, needed to see to the surface. But how far would this go? We were fairly confident we could see the large-scale, 300-kilometer (180-mile) features seen by the ground observers and that we would have greater visibility of these structures, whatever they might be. But would we be able to see features as fine as the cameras and closest approach distances of the more than 40 Titan flybys could serve up—in some cases, a few tens of meters per pixel? Even in the first Titan flyby, the image scale would be 2 kilometers (1 mile) per pixel. Would we see such detail?

Available models of the Titan atmosphere covered the full range of possibilities between two extremes: on one hand, the total abundance of haze is large, in which case we would do no better than seeing features about 100 kilometers (60 miles) in size, or the total haze abundance is modest, like a smoggy Los Angeles day, in which case we might see the finest features, assuming they exist (and are of sufficiently high contrast) in the first place.

Surface contrast observed through an atmosphere will be reduced by the scattering of light from the air-borne haze, and the amount of scattering, and therefore the reduction in observed contrast, depends on the abundance of overlying haze and its vertical distribution.  Thus, features on the surface with greater contrast have greater chance of being seen by a spacecraft cruising overhead.

What we see on Titan certainly lacks the clarity of the Phoebe and Saturn ring images, but is perfectly within the range of expectations. Between light and dark regions, we see nebulous boundaries so diffuse that we all find ourselves fighting the impression that we are looking at clouds and low-lying fog. 

But we are not: many of the features seen in the south polar region looked similar in their light and dark variations and boundaries to the motionless, presumably surface features seen in the mid-latitude regions, first by ground-based observers and now by us. 

One feature immediately grabs all of us—an obvious cloud complex hovering near the south pole and as big as the state of Arizona. In a series of images, taken one after the other, we can see the evolution of its structure. It is likely to be a cloud field, similar to that seen with much less detail by ground-based Titan observers. Its study over the Saturn orbital tour should provide us with a sensitive probe into the thermal inertia of the Titan surface and convective character of the atmosphere.

With a bit of work, we can also make out a few sinuous features, only 10 kilometers (6 miles) wide. It is seductive to imagine riverbeds and streams or deep canyons and channels, features perhaps caused by the rain of liquid methane and ethane that Titan theorists have long predicted. At this stage, however, there is no further evidence to develop this line of imagining into fact. Scientists pride themselves on their discipline and restraint.  And so we remain restrained, unsure of what we are observing, awaiting future opportunities to view Titan in greater detail and with greater coverage, and awaiting the results of the investigations of Titan by other Cassini instruments, before offering any conclusions about the character of the markings we have seen on its surface.

However, one thing we can readily offer is the hope that greater detail will be available to us if indeed it exists on the surface at all. The putative visibility limit of 100 kilometers (60 miles) for a large-haze-abundance scenario clearly does not apply to Titan. If we see features 10 kilometers (6 miles) across with a 2-kilometer image scale, then we should be able to see features 100 meters across with the 20-meter image scale that we will have on future Titan flybys. That assumes again, of course, that such features exist. Thus, our first flyby of Titan leaves us more excited than ever that our investigations of Titan over the next four years will bear the fruit that we have come to enjoy. We may yet learn what this deeply mystifying body has kept hidden for so long. It’s only a matter of time.

And time we have. We are now in orbit around Saturn. Like Phoebe, we have come to dwell permanently in the house of this great patriarch of a planet. Now, it is we who are the interlopers … looking, probing, gathering, reading, measuring … silently, methodically, without disturbance.   Now, it is we who are the new Saturnians.

Carolyn Porco is the Cassini Imaging Team leader and the director of the Cassini Imaging Central Laboratory for Operations (CICLOPS) within the Space Science Institute in Boulder, Colorado. She is the creator/editor of the CICLOPS website (ciclops.org) where Cassini images are posted, and writes the site's home page opening greeting to the public.  

July 1, 2004 … The morning-after press briefing at JPL in Pasadena, CA to report the scientific findings … on the planet’s magnetosphere and its rings … obtained on approach to, and immediately after, orbit insertion around Saturn. From left to right: Ed Weiler, NASA Associate Administrator for Space Science; Bob Mitchell, JPL Cassini Project Manager; Carolyn Porco, Imaging Team Leader; Tom Krimigis, Principal Investigator, Magnetospheric Imaging Instrument, and Bill Kurth, Deputy Principal Investigator, Radio and Plasma Wave Science Instrument. (Video clip by JPL.)

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Dear Friends,

Twenty years ago today was a day that we on Cassini, and especially those of us on the imaging team, had eagerly awaited ever since our mission to Saturn had been officially started, the science teams had been formed, and we had all set about designing and building the scientific instruments we would fly to Saturn. 

Though Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, it wasn’t until 1944 that it was found by another Dutch astronomer, Gerard Kuiper, working in the US, that Titan had an atmosphere. Kuiper deduced this after he recognized a feature in the visible spectrum of the moon’s reflected sunlight at a wavelength of 619nm where the molecule methane had a strong absorption.  This was the first detection of an atmosphere on a satellite, and the presence in Titan’s atmosphere of methane – a molecule with one carbon and four hydrogen atoms –came as a great surprise. Upon his discovery, Kuiper said, “It is of special interest that this atmosphere contains gases that are rich in hydrogen atoms; such gases had previously been associated with bodies having a large surface gravity." He was referring to the detection of methane on Jupiter in the 1930s and his expectation that a small body like Titan would be incapable of retaining such a light molecule.

Methane is one of the simplest organic materials and its presence on Titan eventually gave rise to some public speculation that there might be creatures living on the moon’s surface. Scientists may not have taken such notions seriously, but nonetheless, for those of us who were present at the Jet Propulsion Laboratory the day in late 1980 that Voyager 1 flew by Titan, the suspense in waiting for those first images of Titan was intense and exciting.

Sadly, what Voyager found was a moon enshrouded in a thick globe-enveloping orange haze that scattered visible light so strongly the cameras were unable to see the surface. However, other instrument science teams were successful in revealing the near-surface environmental conditions and their reports were astonishing:  A pressure 50% greater than that on Earth and a temperature around 300 degrees Fahrenheit below zero.  Under such conditions, methane would be near its triple point, as water is here on Earth. That meant it could take the form of a gas, liquid, or a solid, depending on how far the conditions at any locale, on the surface or in the atmosphere, deviated from the triple point.  Here was a moon with an atmosphere hosting a substance that could in principle do on Titan everything that water can do here on the Earth:  form bedrock, rain from the skies, collect to become rivers and cataracts, carve channels, fill low lying basins, evaporate into clouds, and so on.  Voyager may have left behind no tremendous images of Titan’s surface, but what it did leave was solid justification to return to Saturn in the future to try again.

In the years following the Voyager Saturn encounters, our understanding of the Titan environment took two big leaps.  Based on the Voyager results, it was predicted that the near-surface temperature was cold enough and there was likely sufficient methane on Titan to cover the entire surface with liquid hydrocarbons. A global methane ocean. Wow! Straight out of science fiction.

It had also been found by ground-based astronomers that it was indeed possible to see down to the surface of Titan if you knew where to look.  The spectrum of Titan, from the orange region to the near-infrared wavelength limit of our cameras – ie, outside the region where Voyager was able to see -- was dominated by strong methane absorption bands. If you looked in these narrow bands of the spectrum only, the surface would be invisible because methane would absorb any sunlight in those bands that fell on the moon and no signal would ever make it out.  But instead, if you looked at wavelengths in between the methane bands, where methane wouldn’t be a problem, in principle you could see the surface. In these spectral windows, the only thing that might produce poor visibility would be the haze in the atmosphere, and the ground-based astronomers examining Titan had found if you looked in those windows falling in the red and near-infrared, haze scattering was reduced and the surface could in fact be seen.  This was proven decisively when the near-infrared camera on the Hubble Space Telescope was used to produce a map of features on Titan’s surface looking in such a window. (See figure caption below.)

It became obvious to my team and me that we needed to carry narrow-band filters that could accept light from each of the 3 inter-methane band windows through which our cameras were able to see. And for good measure, we would also carry polarizing filters that could work in tandem with the inter-methane filters to decrease scattering even more.

Also, as part of our Titan observing campaign, we sought out those times in the mission when we would be looking at Titan under optimum viewing conditions: ie, close to full illumination, with the sun at our backs, and looking as close as we could to ‘directly down’ on the surface in order to decrease the path length and, therefore, scattering through the atmosphere. 

By the time launch day, October 15, 1997, had arrived and we bid our cameras farewell, we knew we were sending to Saturn an imaging system as ideally equipped as anyone could possibly build to investigate, at close range, the largest, continuous piece of real estate in our solar system that still had not been seen by human eyes. 

And when the day finally came, 7 years later, that we learned our filter strategizing had worked and we were seeing the surface of Titan for the first time, it was like being called to report for duty on the greatest exploration of all time.  We knew then there would be extraordinary sights and victorious moments in our future.

On that day, I recorded in my Captain’s Log how the moment struck me. This is what I wrote.

The Veils of Titan

Today, we have reached a turning point in our travels on approach to the ringed planet.

We have at last glimpsed the surface of the fabled world Titan, Saturn's largest moon and the greatest single expanse of unexplored territory remaining in the Solar System today. What wondrous sights now await us on this remarkable journey we can only imagine.

Titan has long intrigued those who watch the planets. It is a Mercury-sized icy body whose surface environment may be, in some respects, more like Earth's than any other in the Solar System. Like Earth's, its atmosphere is thick and largely molecular nitrogen. Unlike Earth's, it is lacking free oxygen and is suffused with small but …

Caption: Two images of Titan taken with the narrow angle camera through a filter designed to look through the inter-methane-band window at 938nm. The image on the left was taken 4 days after the image on the right. Between the 2 views, Titan had rotated about 95 degrees. The images have been magnified 10x using a procedure which smoothly interpolates between pixels to create intermediate pixel values. They have been enhanced in contrast to bring out details. The original image scale is 143 miles/pixel. It was noteworthy that the surface was visible to Cassini from this approach geometry which was not the most favorable for surface viewing. The map shown was constructed from images taken by the Near Infrared Camera (NICMOS) on HST (Meier et al., Icarus 145, 462, 2000). The blue and green are the darkest areas; yellow and red are the brightest. The brightest, red region in the map was named Xanadu. We can see Xanadu in the Cassini image on the right. But at this point in the mission, we did not know what Xanadu or any other feature on Titan actually was.

… significant amounts of gaseous methane, ethane, propane, and other simple and not-so-simple organic materials containing hydrogen and carbon. Some of these compounds, methane and ethane, may be liquid at the surface, despite the unimaginable cold of -300 degrees Fahrenheit. And though there is no liquid water, what water does on Earth, methane does on Titan. The presence of this simple hydrocarbon as a liquid on the surface and a gas in the atmosphere gives Titan a terrestrial-like greenhouse cycle and a boost in temperature, warming its lower atmosphere. If present-day Titan could be warmed enough to melt its icy exterior, its atmosphere would bear a striking resemblance to that of early Earth, billions of years ago, prior to the emergence of life. Might Titan be a frozen, pre-biotic Earth, telling a tale littered with clues to the origins of terrestrial life long ago?

Despite the Voyager explorations of the early 1980s, the details of Titan's story remain unknown, hidden beneath an atmosphere impenetrable to the Voyager cameras. At the moment, what lies on its surface exists only in the mind's eye.

And in the mind's eye, it is a strange place indeed.

Patchy methane clouds float several miles above the icy ground. In places, large, slow-moving droplets of methane mixed with other liquid organics fall to the surface in cold but gentle rains, cutting gullies, forming rivers and cataracts, carving canyons, and filling basins, craters and other surface depressions. Imagine Lake Michigan brimming with paint thinner.

Above the methane clouds and rain lies two hundred kilometers worth of globe-enveloping red smog, making the Titan nights starless and the days eerie dark, where high noon is as dim as deep Earth twilight. Over eons, smog particles have drifted downwards, growing as they fell, to coat the surface in a blanket of organic matter. On high, steep slopes, methane rains have washed away this sludge, revealing the bright bedrock of ice. Could Xanadu, the brightest feature on Titan, be a high, methane-washed, mountain range of ice?

Occasional bolts of lightning momentarily brighten the gloomy landscape, and wind-blown waves lap the shores of hydrocarbon lakes and seas dotting the scene.

This is a rich and complex environment, where oddly familiar terrain is carved by odd and unfamiliar substances ... a fascinating, virgin world whose only rival may be the Earth itself, with sights still unseen by human eyes.

Anticipation is at its greatest. The pulse quickens, the mind races, the soul is grateful. It is a singular privilege to be standing on the threshold separating ignorance and knowing.

And that's exactly where we are.

This is exploration at its finest and is precisely why we have come to this strange and far-away place.

Step aside, Captain Kirk. This one belongs to us.  

— Carolyn Porco, Cassini Imaging Team Leader

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Dear Friends,

On this day, 20 years ago and a billion miles from here, Cassini slipped into the large torus of space inhabited by many dozens of small icy bodies, all moving on highly irregular orbits around Saturn. The largest of this flock, Phoebe, at ~133 miles across, was first noticed in 1899. But the remainder weren’t discovered until the eve of the 21^st century when astronomers finally had at their disposal very large and properly equipped Earth-based telescopes to conduct deep searches for distant Saturnian satellites.

At this moment, we know of 122 such bodies, referred to sometimes as the ‘irregulars’, that compose a distinct population of Saturnian moons. The smallest known irregulars are only ~2 miles across; the vast majority are smaller than 5 miles. Some of their orbits are prograde, some are retrograde, all are highly eccentric and steeply inclined to the plane in which Saturn’s rings and the other known moons reside, and all are found at great distances from the planet. There are those whose …

Caption: A planar view of all of Saturn’s 146 satellites. The irregular nature of the shapes and orientations of the outer 122 irregulars, and the distinct group that they form, is obvious from this point of view high above the pole of Saturn. The two separate, pink orbits interior to the irregulars indicate the locations of Iapetus (most distant) and Titan. [Source: https://sites.google.com/carnegiescience.edu/sheppard/moons/saturnmoons ]

Caption: This graphical representation of the orbits of the 59 Saturnian irregulars known as of 2022, viewed from the plane of Saturn’s rings, shows the extreme inclinations of this group. At present, there are 122 irregulars. Saturn and the orbits of its major moons lie along the red line which is, in this diagram, coincident with the rings. The positions of the outermost ‘main moons’, Hyperion and Iapetus, are indicated near the center of the diagram. [Source: https://tilmanndenk.de/outersaturnianmoons/ ]

… eccentric orbits take them as close to Saturn as 4 million miles and very near the realm of the planet’s well-known interior moons, and some that travel as far from Saturn as 24 million miles.

In its descent into the planet’s gravitational well, Cassini stepped across this outer threshold 20 years ago today and, in doing so, entered the house of Saturn.  And if our insertion into Saturn orbit three months later worked out the way we had planned, we would never leave.

Soon after I arrived at the California Institute of Technology in Pasadena, California for graduate school in late 1974, I began a research project with a post-doctoral fellow in the Division of Geological and Planetary Sciences named Thomas Heppenheimer. We wanted to understand how these outer irregular satellites of Jupiter, at that time known to be only 9 in number, came to be.

It made no sense that they formed when the planet and its four Galilean satellites did. The standard concept for the formation of gaseous giant planets like Jupiter begins with a nebula of gas and solids, like the solar nebula that eventually birthed the Sun and its planets, only smaller.  In the final stage, the newly formed central body rotates in the same direction as the newly formed moons move in their (essentially) co-planar, circular orbits … all the result of universal laws governing the motions of bodies throughout the cosmos. This conceptual model has succeeded in explaining the major moons of Jupiter, other large planetary systems in our solar system, other solar systems, and even the spiral disk galaxies. But not the irregulars.

Our research began with the notion that Jupiter’s irregulars might be bodies, once in orbit around the Sun, that approached close enough to Jupiter to be permanently caught in its grasp. This scenario required certain conditions.  Either Jupiter had to be gaining mass as the body entered the planet’s sphere of influence, or there had to be sufficient nebula material still present either around the planet, or around the Sun but in the vicinity of Jupiter, to slow, through friction, the motions of the captured bodies and cause them to lose energy, thereby preventing their escape. Though very specific, neither condition seemed at the time implausible and, by analogy, the same mechanisms could apply to the other giant planets.

Our work was published in 1977.  Since then, an enormous number of irregulars have been discovered around all the giant planets, and there have been more than a few efforts to divine the mechanisms by which these objects end up moving from an orbit around the Sun to an orbit around a planet … efforts enabled by far more powerful computers and far more sophisticated techniques than were available in 1977. One recent concept, consistent with our modern understanding of the evolution of early solar system architecture, posits that young Jupiter and Saturn, in migrating outward through the last remnants of the solar nebula as planetary bodies in a nebula are now known to do, passed through a strong mutual gravitational resonance that made their orbits eccentric enough to cross the orbits of the smaller giant planets, Uranus and Neptune. The close planetary encounters that ensued, within a nebula environment populated by a great many, very much smaller bodies, distorted the gravity fields of all four giant planets and, at times, came close enough to cause their spheres of influence to overlap. These events made possible the permanent capture of some of the nearby smaller bodies into orbits around each of the giant planets.

Conceived by dynamicist David Nesvorny at Southwest Research Institute in Boulder, Colorado, and his collaborators, this model has so far been remarkably successful in reproducing the numbers and distributions of irregulars seen around the four giant planets, and even in explaining other groups of small bodies in the outer solar system, like the Trojan asteroids of Jupiter. It is consequently regarded today as the most likely scenario to be correct.

As unaltered remains of the solar nebula, the irregulars are fascinating in what they can tell us of events that happened here, in our little corner of the universe, 4.5 billions of years ago. With this in mind, it might be surprising that the study of Saturn’s irregular satellites, with the exception of Phoebe, was not on our Cassini list of fan-favorite scientific objectives … until you take a look at the dates. None of the small irregulars was even known to exist until 3 years after Cassini’s launch!

Nonetheless, once they were discovered and orbits were in hand, one of our imaging team associates from Germany, Tilmann Denk, recognized that Cassini’s unique vantage point from inside the Saturn system looking out at the irregulars permitted a class of observations not possible from the Earth, and he made it his quest to use our highest magnification camera as one would use an astronomical observatory and perform a systematic study of this ancient swarm of interlopers.

The variation in the Sun’s illumination on any of these moons would appear to any onlooker more or less like the different phases of our Moon look to us:  new moon, waxing crescent, half-moon, waxing gibbous, full moon, waning gibbous, half moon, waning crescent, new moon.  But neither Cassini nor Earth-based observers could see what partially illuminated irregulars looked like because in both cases, the satellites were sufficiently far away that neither big telescopes on the Earth looking a billion miles across the solar system to Saturn, nor our comparatively not-very-powerful high resolution camera looking across a very much smaller distance, could see anything other than points of light.

Caption: A Cassini image of the ~6-mile wide Saturnian irregular moon, Erriapus, taken on Dec 24, 2016 with an exposure time of 220 seconds at a distance of 6.7 Million miles. The moon is the point of light in the cross-hairs at the center. The other bright dots, blotches and streaks are background stars, galaxies, and imperfections in, and cosmic ray strikes on, the CCD. It was serendipity that caught, in the same image, the spiral galaxy NGC 300, ~6 million light years in the distance. [Source: https://tilmanndenk.de/outersaturnianmoons/ ]

So, collecting the light reflected by those points at instances that spanned their orbital periods around Saturn (which ranged among them from 1.3 - 4.5 years) and therefore covered a large range in solar illumination geometries, and doing so for long continuous observing periods spanning sometimes multiple rotations of the body, became the main features of Tilmann’s observing campaign. Undertaking a campaign of this nature is challenging from the Earth where getting adequate observing time on large telescopes is next to impossible, and where weather and the day/night cycle …

Caption: An animation of the first 7.4 hours of the 24 December 2016 Cassini observation sequence of the Saturnian irregular Erriapus. [Source: https://tilmanndenk.de/outersaturnianmoons/ ]

… can cause long interruptions. But from Cassini, frequent, long-duration observations made from significantly different viewing directions, including both above and below the satellite, and unhindered by the stray light from Saturn that occasionally affects ground-based observations, were very doable and yielded a rich collection of brightness measurements, more or less evenly sampled in space and time, from which the body’s light curve and hence physical properties could be readily extracted.

Caption: The light curves of the 25 irregular Saturnian moons observed by Cassini. Rotational phase is the fraction of the moon’s rotational period. The moons’ rotation periods ranged from 6 hours to 3 days. [Source: https://tilmanndenk.de/outersaturnianmoons/ ]

These advantages ultimately allowed Tilmann and his collaborator Stefano Mottola to constrain the moons’ shapes, sizes, light-scattering properties, colors, rotation periods, and spin axis orientations. And his Cassini measurements, along with observations of the irregulars from Earth-based telescopes, were used by master ‘orbit guy’ at the Jet Propulsion Laboratory and an honorary member of our Cassini imaging team, Bob Jacobson, to improve the orbits — especially the orbit shapes and orientations — of 20 of the Cassini-observed irregulars to a precision better than could be obtained from Earth-based data alone. In this work, Bob had sufficient information to uncover groupings and gravitational resonances among these bodies that were previously unknown, and in the process reveal relationships that hold clues to how this flock of interlopers at Saturn came to look the way it does.

All told, Tilmann’s creative idea became a brilliant and wonderfully productive use of an asset already orbiting Saturn.  As someone who studied irregulars when she was a young student and to this day retains a sentimental attachment to the subject, and as the leader of the team on Cassini responsible for addressing, with the mission’s prime imaging instrument, the major scientific questions of the day about Saturn and its environs, I could not be prouder or more delighted with the outcome.

Bellissimo! Magnifico! Bravo!

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Dear Friends,

On this day, 20 years ago and a billion miles from here, our Cassini cameras acquired a series of images that nicely demonstrates what knowledge can be gained by viewing only a narrow slice of the electromagnetic spectrum.

Each of the Cassini cameras can be simply described as a telescope that focuses the light it gathers and passes it through spectral filters and finally onto a CCD detector sitting behind the optics and in the camera’s focal plane. Alongside and behind the CCD — similar to the one in your cell phone — are all the electronics that control the operation of the CCD and the 2 filter wheels in front of it, the pre-processing of the data coming from the CCD, data compression, internal system temperature, synchronization with the main spacecraft clock, etc.   

The human eye can typically detect light with wavelengths between 380 and 700 nanometers (nm), where a nanometer is a billionth of a meter. These wavelengths form the ‘visible’ part of the spectrum.  The spectral range of the cameras aboard the Voyager spacecraft, 300nm to 640nm, was close to this.  

But there were good scientific reasons to push the limits of vision of the Cassini cameras into the ultraviolet at short wavelengths and into the near-infrared at long wavelengths.  Determining the composition of Saturn’s icy moons, for example, and how their compositions relate to the geology and modification processes acting on their surfaces would come down to determining their mass densities, by measuring how much each moon’s gravity altered the spacecraft’s trajectory during close flybys, and establishing the composition of their surfaces through global and high resolution imaging at different wavelengths.  Strong diagnostic compositional signatures exist at both ultraviolet and infrared regions of the spectrum, and together these aid in discrimination of surface materials of special interest, such as carbonaceous materials, on the moons and within Saturn’s rings.

For the atmospheres of Saturn and Saturn’s largest moon Titan, imaging in the ultraviolet would provide the best visibility of aerosols high in the atmosphere and of aurorae in the polar regions if there were any.  And red/near-infrared imaging would allow searches for lightning, access to spectral bands where methane, a constituent of the atmospheres of both bodies, strongly absorbs light, as well as access to the clear windows in between the methane bands on Titan through which we hoped to see down to the surface for the first time.  In combination with what we would see in the visible, the new capabilities would allow us to sound the atmosphere: ie, see down to different levels by looking in different wavelengths.  By sounding, we could work out the cloud and aerosol vertical structure and energy balance in the atmosphere, discover previously unseen atmospheric phenomena, map the three-dimensional general atmospheric circulation, including indirect inferences about conditions below the visible cloud tops, and determine the spatial, spectral, and time-varying properties of Saturn’s auroral and lightning emissions, the latter being especially important as tracers of atmospheric storms.

Of course, the visible wavelengths in between the extreme end members could be used to image in the different colors that our eyes can see. 

Consequently, the Cassini cameras were designed, and filters were chosen, to reach all the way up to the near-infrared wavelength of 1000nm.  And by using a special UV-sensitive coating on the CCD detector, we pushed the response of the narrow angle camera (NAC) at short wavelengths down to 200nm.  (The wide angle camera (WAC), a refracting telescope built around a lens and not a mirror, was UV-blind and could only go as low as the violet wavelengths at 380nm.)

In the end, the imaging system on Cassini was tremendously more capable than that on Voyager and, with its new ultraviolet and near-IR eyesight, would reveal to us vistas that had never been seen before.  We were stoked.

The images we took 20 years ago today on approach to Saturn proved there was more to the ringed planet than meets the eye. These four images were acquired over a period of 20 minutes when the spacecraft was 27.7 million miles from the planet. All four show the same face of Saturn with an image scale of 166 miles per pixel.

Caption: Four Cassini images taken on approach to Saturn, April 3, 2004. They were imaged at … in upper left, ultraviolet wavelengths centered at 298nm; in upper right, blue wavelengths centered at 440nm; in lower left, far red wavelengths centered at 727nm; and in lower right, near-infrared wavelengths centered 930nm.

What is revealed are the effects of absorption and scattering of light at different wavelengths by both atmospheric gas and clouds of differing altitudes and thicknesses. They also show absorption of light by colored particles mixed with white ammonia clouds in the planet's atmosphere. The contrast in the images has been enhanced to aid visibility of the atmospheric features.

In the upper left image, Saturn is seen in ultraviolet wavelengths centered at 298nm;  at upper right, in visible blue wavelengths at 440nm; at lower left, in far red wavelengths just beyond the visible at 727nm; and at lower right, in near-infrared wavelengths at 930nm.

All gases scatter sunlight efficiently at short wavelengths. That's why the sky on Earth is blue. This effect is even more pronounced in the ultraviolet than in the visible. On Saturn, molecular hydrogen and helium gases, the main atmospheric constituents, scatter ultraviolet light strongly, making the atmosphere appear bright in the upper left image.  In this image, only high altitude cloud particles, which tend to absorb ultraviolet light, appear dark against the bright background.  This explains the dark equatorial band in the upper left image.

The contrast is reversed in the lower left image taken in a methane-absorption band at 727nm where light is absorbed by methane gas but scattered by higher clouds. The equatorial zone in this image is bright because the high clouds there reflect this long wavelength light back to space before much of it can be absorbed by methane.

Scattering by atmospheric gases is less pronounced at visible blue wavelengths than it is in the ultraviolet. Hence, in the top right blue image at 440nm, the sunlight can make its way down to deeper cloud layers and back to the observer before it get scattered, so that both deep clouds and the high equatorial cloud particles, which are reflective at visible wavelengths, are apparent. This view is closest to what the human eye would see.

At bottom right, in the near-infrared region at 930nm, some methane absorption is present but to a much lesser degree than at 727 nanometers. At this point in the mission, we were not certain whether the contrasts here are produced mainly by colored particles or by latitude differences in altitude and cloud thickness.

The sliver of light seen in the northern hemisphere appears bright in the ultraviolet and blue wavelengths (upper images) and is nearly invisible at longer wavelengths (bottom images). The clouds in this part of the northern hemisphere are deep, and sunlight is illuminating only the cloud-free upper atmosphere. The shorter wavelengths are consequently scattered by the gas and make the illuminated atmosphere bright at these wavelengths, while the longer wavelengths (lower images) are absorbed by methane, the light never is returned to the observer, and that region of the atmosphere is dark.

[Saturn's rings also appear noticeably different from image to image, whose exposure times range from 2 to 46 seconds. The rings appear dark in the 46-second ultraviolet image because they inherently reflect little light at these wavelengths. The differences at other wavelengths are mostly due to the differences in exposure times.]

Throughout the rest of the mission, this technique of sounding was used in concert with information from Cassini’s other instruments to yield an unprecedented view of the 3-dimensional structure and motion of the Saturn atmosphere.  At this point in the mission, 20 years ago, we were just getting started.

— Carolyn Porco

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Dear Friends,

On this day, 20 years ago and a billion miles from here, the Cassini spacecraft had reached a point in its approach to Saturn where the planet and its rings just exactly filled the field of view of our highest resolution telescope, the narrow angle camera (NAC). From that point onward, imaging the rings end-to-end with the NAC would require a mosaic of multiple images.  So, I thought back then that it would be nice to mark this special ‘last eyeful’ turning point with the release to the public of a color image. 

Producing natural color was not, on any previous planetary mission, ‘the thing’ that I made it on Cassini. Here’s the backstory.

Between the years 1979 and 1989, the Voyager spacecraft paid visit to each of the giant planets in the outer solar system … Jupiter, Saturn, Uranus and Neptune.  And on each morning of the week straddling each planetary encounter, there would be a news briefing to report to the gathered press corps and to the world, the scientific findings of the previous 24 hours and our interpretations of them.  It was always a mad, chaotic rush, often with scientists and JPL graphics personnel pulling all-nighters, to get the materials ready for presentation, followed early the next morning by a pre-press-briefing review and selection of the most presentation-worthy items.

Caption: Voyager Imaging Team leader Brad Smith, myself, and personnel from both the JPL public affairs office and image processing lab selecting the best images for presentation at a Neptune encounter press briefing, August 1989.

The era had not yet arrived when digital files could be readily shipped from one person’s computer to another in the next room, and images could be routinely attached to emails and shipped across the country or posted on a social media platform.  Whatever processing was needed to make the Voyager raw images look good – either contrast enhanced, cleaned of noise, mosaicked together, and/or turned into color – it all had to happen very quickly for the next morning’s press conference. 

Given the urgency, no one was especially concerned with making colors accurate. It had never in the history of the planetary space program been a concern and I’m pretty sure the thought never occurred to anyone. All processing was done simply to make some scientific point as clear as possible to the press the following morning.

And when finally the encounter was over and everyone went home, if the press or even we Voyager scientists later wanted images from JPL, they were shipped out as either hard-copy or as a package of slides.

But there was one member of the press corps who took the Voyager imaging team to task at every encounter for not making the effort to present the images in natural color: ie, the color that we would see if we were there.  His name was Andrew Young, and he was also an amateur astronomer and therefore familiar with what planetary objects, especially colorful ones like Jupiter and Saturn, should look like.  I think the older team members were irritated with his criticisms. But I happened to agree with him.

So when the day eventually came that I learned that I had been chosen as the leader of the imaging team on the Cassini mission, I began constructing my bucket list of goals and wishes for our forthcoming adventure.  One was to re-do the Voyager 1 Pale Blue Dot image of Earth that we took from beyond the orbit of Neptune. Another was to process images, when we could, in natural color. I took it as my responsibility to release to the public image products that would convey what the scene would look like, in color and configuration, if we were actually there. It was extra work for sure, but I gladly committed myself to it.

And that’s how Cassini became the first planetary mission on which processing images in colors as true to natural as possible, and creating a color-consistent body of work, became ‘a thing’.

To produce natural color requires an accurately calibrated image production pipeline.  We had insufficient funding prior to launch to do a complete job at this important task.  Even if we had the funds, we still needed to do calibration in flight so that we would know how the imaging system’s response to in-flight conditions changed over time. And it did change. Calibration of our cameras, as it turned out, would be ongoing throughout, and up to the end of, the mission.

Our calibration software wasn’t ready yet during Jupiter flyby so I sought the advice of amateur astronomers as to what the color of Jupiter should be. The most prominent color was a brick orange/red. Cassini’s first color image, created at CICLOPS, was of Jupiter and its moon Europa, taken on October 4, 2000 as we started our official approach to Jupiter.

Caption: Cassini’s first color image, of Jupiter and its moon Europa, was released on October 9, 2000, John Lennon’s 60th birthday. Europa’s shadow is seen on the planet.

I very much wanted to publicly release the image on October 9, John Lennon’s 60^th birthday. It would be my tribute to him. I had to sweet talk the public affairs people at JPL and NASA Headquarters in Washington D.C. into letting me release it on that day. I was delighted that they agreed.

I don’t remember now why, in the early days of our time at Saturn, the ring colors and some regions of the Saturn atmosphere turned out pinkish. I know our calibration procedure was not yet very accurate, and towards the end of the mission, the colors became more beige and somewhat muddied.  They also could change depending on the illumination and viewing conditions, which was expected.  That the uncertainties in our calibration process early on were large enough to accommodate color differences that would be perceptible to the human eye was another complication.  In summary, achieving exact true color, though certainly a worthy goal, can be difficult even in the best of circumstances, and at this early stage in the mission, as we approached Saturn, we were somewhat winging it.

Caption: This color NAC image we took on ‘the last eyeful’ day of March 27, 2004 from a distance of 29.7 million miles from Saturn, is a composite of three exposures, one each in red, green, and blue spectral filters.  The image scale is 178 miles per pixel.

However, color variations within our images were reliable. And variations in the colors of different atmospheric bands and features in the southern hemisphere of the planet, as well as subtle color differences across Saturn's middle B ring, were becoming more distinct the closer we got. Such variations generally imply different compositions, but under certain viewing conditions can indicate particles of different sizes. The nature and causes of the color differences in both the atmosphere and the rings were major questions to be investigated by Cassini scientists during the mission and each objective was pursued in depth as the mission progressed.

About the image itself … the bright blue sliver of light in the northern hemisphere is sunlight passing through the Cassini Division, the noticeable gap in Saturn's rings, and scattered by the cloud-free upper atmosphere.  We hadn’t seen any blue on Saturn during the two Voyager encounters and it caught the imaging team scientists completely off guard. I will leave the mysterious blue on Saturn to a future episode of this newsletter.  Or maybe my book.  We’ll see.

The moons visible in the image are (clockwise from top right): Enceladus (313 miles across), Mimas (246 miles across), Tethys (660 miles across), and Epimetheus (70 miles across). Epimetheus is dim and appears just above the left edge of the rings. Brightnesses have been exaggerated to aid visibility.

Also in the image above, two faint dark spots are visible in the southern Saturn hemisphere. These spots are close to the latitude where Cassini had earlier seen two different storms merging in mid-March 2004. 

Caption: A month and a half into its long approach to Saturn, the Cassini spacecraft captured two storms, different than the ones in the color image above, each a swirling mass of clouds and gas, in the act of merging.

There is a fascinating and very scientifically gratifying story regarding the provenance of such storms, and how they relate to the underlying structure of Saturn’s interior. That story too will be left for the future. Stay tuned!

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Dear Friends,

Recently, I was interviewed by Isabelle Boemeke, a Brazilian fashion model known also as the world’s first nuclear influencer, I_sodope … that weird and sassy character on Twitter/X who made nuclear energy cool among the younger generations. The interview was motivated by a book she is writing on the subject of nuclear energy that will be titled ‘Rad Future’, to be published by Penguin Random on Earth Day 2025.

(By the way, did you catch that name … i_sodope?  That’s a play on the word ‘isotope’ while subtly messaging how dope it really is. If you don’t get it, nevermind!)

As Isabelle tells it, she began her explorations into the subject of civilian nuclear energy after reading a Tweet of mine.  Proud? Who, me?  After a couple of years of I_sodope and her catchy, clever videos (here’s an example) …

… Isabelle was invited to present her work on the TED stage. You can check out her beautiful 2022 TED talk right here, especially the first minute! :-)

Caption: Isabelle Boemeke makes her case for civilian nuclear power on the TED stage. She received a standing ovation. (Credit: TED)

And here is a picture of Isabelle and me on a tour of the Diablo Canyon Nuclear Power Plant near San Luis Obispo, USA, December 2021 …

Caption: Isabelle Boemeke and I at the Diablo Canyon nuclear power plant, San Luis Obispo County, December 2021. (Credit: Carolyn Porco)

… when she was in town to lead a rally in downtown SLO to keep Diablo Canyon open.

Caption: That’s me speaking at the Save Diablo Canyon rally in downtown San Luis Obispo, December, 2021. Isabelle is at my side. (Credit: Unknown)

Operations of Diablo Canyon NPP have since, thankfully, been extended at least until 2030. There was lots of celebrating over that!

Caption: Image of Diablo Canyon Nuclear Power Plant on the California Central Coast. Even the wildlife celebrated its extension into the future. (Credit: John Lindsey)

Back to the interview … I thought you all might be interested in what I had to say in response to Isabelle’s questions so I’ve posted the interview below. I hope it’s obvious that I wouldn’t mind one bit, and would really appreciate it, if you encouraged your friends, family, colleagues, and anyone you think might also be interested, to subscribe to this newsletter. The more, the merrier. FInally, if you are so moved, leave your own responses and comments at the bottom of the page. I’d love to read them.

Enjoy!

Carolyn Porco

Carolyn, you lived through the birth of the anti-nuclear movement in the US and attended anti-proliferation protests. How was that experience?

I can't recall civilian nuclear power appearing on my radar screen, either during my school years or even into my career.  I was too consumed with my education and later my professional life to take much notice of political things, like the anti-nuclear movement.

But I did attend a 1987 protest at which I knew Carl Sagan would be speaking. Carl, who was a colleague, friend, and mentor of mine, had been against nuclear weapons proliferation since his student years at the University of Chicago where the first nuclear reactor had been built and where he first became aware of the seriousness of nuclear proliferation.  And from that grew his crusade later in life against nuclear weapons.

I was a research associate in the Lunar and Planetary Lab at the University of Arizona in Tucson in the 1980s.  When I heard that Carl was going to be speaking at the Nevada Test Site, near Las Vegas, on February 5, 1987, I joined a caravan of UofA students, post-docs and others headed for Nevada to protest along with Carl.  There were 2000 demonstrators.  Carl was one of about 400 people …

Caption: Images from the February 5, 1987 protest against nuclear weapons proliferation at the Nevada Test Site near Las Vegas. Sagan is speaking in the bottom-most image. (Credit: Carolyn Porco)

… who were arrested trying to enter the site.  He was putting himself on the line to draw attention to the need to stop the proliferation of nuclear weapons.

The experience was for me a good introduction into the effort that is often required if you want to bring about policy changes at the highest levels.  I began taking mental notes.

How did you come to be an advocate of nuclear energy?

In January 2011, during a visit to Pasadena, California for a Cassini meeting at the Jet Propulsion Laboratory (JPL), the organization that managed the Cassini project for NASA, I learned of a lecture to be given at Caltech by a renowned, highly accomplished astrophysicist whose reputation I knew well.  His name was Frank Shu, and along with my thesis advisor and my advisor’s collaborator, was one of the architects of the scientific discipline in which I worked for my dissertation … the study of planetary rings. He was gifted, one of only nine distinguished University Professors in the University of California system, a former president of Taiwan’s top research university, and a recipient of the Shaw Prize, considered the Nobel Prize of the East. You don't pass up the chance to hear someone like that speak. 

I went to his lecture and was pleasantly surprised to learn that he was now using his intellect and time trying to figure out what to do about the climate crisis. His lecture was on nuclear power. He was professionally an expert in the structure, composition, and behavior of stars so he understood radioactivity and the structure of the atomic nucleus at a very deep level.  He was pushing a reactor design that he felt was superior to the common designs that had been in use for decades.  His design used molten salt as a coolant and fuel and radioactive thorium as a breeder fuel.  In such a reactor, if there's a problem and the fuel gets too hot, it expands, the density goes down and the reactions cease until it cools. In other words, meltdowns are impossible because the fuel is already molten.  One statement Frank made that day really stuck with me: 'Such a reactor behaves like the Sun.'  I thought that was brilliant.

Frank’s lecture was such an eye-opener and his proposal sounded so promising that I became very interested in the subject and started studying up on it.  I eventually became convinced that nuclear power is the best source of energy there is and, at large scale, better than wind, better than solar. I decided that the most effective way I could make a contribution to solving the climate crisis was to advertise my own support for it on social media.  That's when I started Tweeting about it.  One of those Tweets is the one you saw, and the rest is history!

Why do you think so many people conflated nuclear weapons and nuclear energy?

Can you think of an industry that had a worse rollout than nuclear?!  The first demonstration of the kind of energy that could come out of the nucleus of an atom were events of unprecedented, horrific, grotesque mass destruction.  (I'm of course referring to the bombings of Hiroshima and Nagasaki in 1945.)  Even if it's simply being a TV witness, that’s the kind of experience that never leaves you and is not the kind of first impression you would want to make on others if you're hoping to convince them that something can be used for good. And to top it off, you tell them, 'Oh, and we want to put this in your backyard'.  Not a sensible strategy.  But that was indeed the backdrop to the first suggestion of civilian nuclear power.  It's been a long, steep uphill battle for nuclear power plants ever since. 

In addition, the technical distinction between bombs and nuclear reactors is lost on most of the listening public. I think fear prevents them from absorbing the notion that the amount and kind of nuclear material and what is done with it are the core distinctions between bombs and reactors.

Most people, I suspect, have the same ignorance about the way their automobiles work.  You don't need to understand the mechanics of a car to drive one, so we happily, thoughtlessly get in and out of them every day.  But people have also never seen an automobile used to kill 140,000 people in one go!  If we had, maybe we'd still be using horse-drawn carriages.

I'm seeing the current surge, especially among the young, in the acceptance of nuclear power as the result of several facts.  First, we have many reactors around the world that have been up and running for decades with only 2 really notable accidents ... Chernobyl and Fukushima. It's been 79 years since the end of WWII and we've never in that time seen a nuclear bomb used in warfare.  So, young people have neither the memory nor the fear of a nuclear bomb detonation, nor do they have the long-standing, entrenched, knee-jerk, negative reaction that their elders have.  There is an old expression, 'Science progresses one funeral at a time.'  I think in this case, the appropriate expression might be, 'Human societies progress one generation of funerals at a time'. Fears like this take a long time to uproot.

Did you ever have the same issue?

No. I was a physics major and so I was familiar with the basic concepts like critical mass density.

You were a part of the Cassini mission, which used a nuclear battery. You’ve mentioned that people were very apprehensive about that. What were some common misconceptions they had about the dangers of Cassini?

Some months before the Cassini launch on October 15, 1997, I was chosen by the public affairs office at JPL to be one of the spokespersons for the Cassini project on the matter of the radioisotope thermoelectric generators (RTG) that Cassini carried for power. The radioactive material in them is plutonium-238 which, by the way, is not fissile and cannot sustain a chain reaction as in a bomb. In an RTG, the plutonium is bonded with oxygen in the form of a ceramic, PuO2 or plutonium dioxide. The natural radioactive decay of the plutonium heats the ceramic to tremendous temperatures and that heat is converted into electricity. Cassini would carry the largest amount of PuO2 ever taken into space. 

At the time of launch, a movement grew, headed by Michio Kaku, a well-known physics professor at City College of New York, opposing the launch because of the dangers they saw.  They worried that a rocket explosion at launch would spread radioactive plutonium-dioxide dust all over southern Florida, and that an accidental Earth atmospheric re-entry and incineration of Cassini during the 1999 Earth flyby would spread enough material around the world to have dire global consequences.  'The most toxic substance known to man' was the tagline used at the time to describe plutonium.  That, in fact, was a statement made by Ralph Nader in 1975 in a debate about nuclear power, in which he also said, 'One pound of Plutonium could kill every human being on Earth.'  Both statements were repeated regarding Cassini.

Neither statement is true in the real world.  Plutonium is not the most toxic substance by far; Botulinum toxin apparently is.  Also, to kill every human on Earth, a bit of PuO2 dust would have to be specially delivered into the lungs of every human on Earth and would have to remain there for a while and not be expelled by coughing. That would not have been at all likely during Cassini's flyby of the Earth.

I participated in interviews and debates and I wrote an OpEd to explain the facts.  It's a never-ending job and responsibility, really, to educate the public.  We who have worked with scientific concepts all our lives can easily forget how complex and confusing things can appear to the non-scientist.  But we must continue with it, in this case especially, because the very health of the planet and the creatures living on it are at stake. 

You have been a vocal supporter of nuclear energy throughout your career. Have you ever faced criticism for this from within the scientific community?

Well, actually only for the last 13 years have I been vocal about it.  In that time, sure, I’ve gotten into arguments on social media about it and some of those people, I suppose, have been scientists.  But no one in my community of scientists has been critical, at least as far as I know.

You worked very closely with Carl Sagan, who was a very vocal anti-nuclear weapons activist. Did he ever share his views on nuclear energy with you?

No, we never spoke about it and I never heard Carl say anything in person or otherwise about nuclear reactors until just recently, in the last few years, when this video began circulating on social media.  It is Sagan's 1985 Congressional testimony on climate change. 

At timestamp 12:35, he asks ‘What can be done about it?’ He answers i) fewer government subsidies [to the oil industry] and ii) alternative sources.  He mentions solar power and then "safe fission power plants, which are in principle possible, and on a longer timescale the prospect of fusion power.  Fission and fusion power plants vent no infrared active gases"

This was said before the Chernobyl accident which occurred in April 1986.  I know he spoke against nuclear weapons after the Chernobyl accident but I don't know if he went so far as to unequivocally oppose nuclear reactors after the accident. Carl was a very well-reasoned individual.  He would surely see the benefit of safe nuclear power and would, like myself, see the distinction between the Chernobyl accident — a reactor of poor design operated by poorly trained operators — and what we build and how we operate in the US.

Do you think accidents like Chernobyl are a warning to us that we should consider nuclear energy too dangerous and reject it?

The accident at Chernobyl has no bearing on our civilian nuclear activities here in the US.  The Chernobyl reactors, which utilized a faulty design unique to the Eastern Bloc of the Soviet Union, went on line in the early 1970s. That means they were being built during the 1960s.  At that time, the US was in a race with the Soviet Union to get to the Moon.  As we all know, the US won handily and for a very good reason.  When Gorbachev became the Soviet leader in 1985 and the openness of glasnost allowed freer exchange of information between the US and the Soviet Union, we learned that the Russians never stood a chance against us in getting to the Moon.  Our engineering standards, technological knowledge and safety culture were superior to theirs.  And the same applies to the Chernobyl nuclear power plant. So, in my mind, it is a false equivalence to point to the Chernobyl accident and say that nuclear power of all kinds, therefore, is dangerous everywhere. Do we point to electrical power and say, when someone is electrocuted or a transformer on the electrical grid explodes and starts a deadly fire, that we must quit using electricity?  Of course not. Instead, we make efforts to make it safer. The record shows, nuclear power can be made very safe.

How do you think public opinion on using nuclear reactors for electricity has changed?

It's getting very much better because we've seen for decades now the proof of concept all over the world.  Many years have passed since the 1970s and we have seen the superiority of nuclear power — its lack of greenhouse gases, its persistence and dispatchability — over the usual renewable sources during that time.  As I stated above, the younger generations look at all the evidence without prejudice.  The older generations are still saddled with those old fears and ideas. I'm hopeful youth will prevail.

What gives you hope for a nuclear-powered future?

The internet and social media have each changed a great many things, for good and bad, but the best in my mind is the access to information.  As a result, I think now people in general, myself included, are very much more aware of the detrimental changes human civilization is bringing to the climate and the biosphere, and many of us want to know what's going to be done about it, and what we can do to stop it.  For the younger generations, who have most of their lives before them, it is a matter of acute significance. If we don’t set this ship right, it is they and their progeny who will suffer.  The future is theirs and they will make it so.

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Dear Friends,

Twenty-years ago this month, the Cassini spacecraft began its official approach to the Saturnian system.  For those of us who had been assigned by NASA 14 years earlier to undertake the long voyage across the solar system to Saturn, it was a time of heightened vigilance, nervous jitters, and great exhilaration at what lay ahead.

Once Cassini leapt off Launch Pad 40 at Cape Canaveral on October 15, 1997, cleared the launch tower, climbed skyward and veered eastward, it began a circuitous, 7-year, 2.2-billion-mile journey around and across the Solar System to reach its destination, 10 astronomical units away from the Sun.  Those seven years were neither idle nor easy.  A great many tasks needed to be completed to ready us for our exploration of the Saturn system, and the less pressing ones were postponed until the spacecraft and its scientific payload were safely on their way.  With that assured, all of us on the Project began to settle into a new rhythm as we adjusted to a life in flight and a whole new set of activities.

I had been tasked in late 1990 with leading the 14-person Cassini Imaging Team, a group of scientists whom a gauntlet of scientific review panels and senior NASA officials had selected to conduct a scientific exploration of Saturn and its environs with what would be the mission’s prime imaging instrument.  The Imaging Science Subsystem (ISS) consisted of both a high-resolution narrow-angle and a low-resolution wide-angle telescope.  It was tradition within the space program to refer to these as cameras.  Together with assorted electronics and the software to operate them, the Cassini cameras composed the most sophisticated, highest resolution, 2-dimensional imaging system ever carried into the outer solar system.  The whole package had been specifically designed to image the bodies in orbit around Saturn, and in its sensitivity, optical resolving power, stray-light rejection, the variety of data collection and compression modes, the range of the electromagnetic spectrum that it could see, from the ultraviolet into the near-infrared, and its many spectral filters that would slice that spectrum into narrow bands, and more, it represented a significant advance over its predecessor carried on the Voyager spacecraft.  We on the imaging team, and indeed the rest of the world, had every good reason to expect that we'd all be mind-blown once we got to Saturn.  And the record now shows … we were not disappointed!

Cassini was the first planetary mission in which mission operations would be distributed among the science teams and not centralized at the Jet Propulsion Laboratory as had been done in years past.  This was a genius move, as it was (certainly in my case) less costly and put the job of developing sequences of observational commands, destined for uplink to the spacecraft from the Deep Space Network around the globe, in the hands of those who cared about the outcome the most ... the scientists.  Accordingly, all the science team leaders like myself were responsible for establishing the uplink and downlink ground operations for their instruments at their home institutions.  In my case, this entailed designing, assembling, and staffing the laboratory in which ground operations for the ISS would take place. I called this organization CICLOPS, an acronym that took me an entire month to conjure.  It stands for Cassini Imaging Central Laboratory for Operations. (Wildly brilliant, don’t you think!?)  With a staff of 12 personnel at our peak, we wrote the computer code that enabled my science team members to choose camera parameters and thereby build their own imaging sequences, and developed software, procedures, and protocols for image handling, pointing reconstruction, annotating, cataloguing, scientific analysis, and archiving once the images made their way back to Earth and to our lab. Unique to CICLOPS was the responsibility of processing and captioning for public consumption select images of either scientific interest or outstanding beauty, and we built the tools to do that, too.

Of course, there was also the monstrous job in which all the science teams had to engage … working with the mission designers to determine what the nominal 4-year tour of the Saturn system would look like, and, once the tour was set, planning in great detail every single minute of it, something that amazes me even more in hindsight.  Imagine planning every minute of the next 4 years of your life and committing every detail of it to paper. That would be onerous enough, but on Cassini it involved participating for many years in negotiations that took place via grinding weekly telecons, sometimes multiple telecons a week, attended by the mission's scientists and sometimes engineers too, in the service of those two objectives. It was an extremely wearying, contentious, and political part of the mission and not my favorite, though not nearly as political as the Project would later become as we got closer to Saturn.  I was asked once, during the Q&A of a public lecture I had given, what this many-years-long activity was like.  I answered, 'It works like the American Congress'.  The audience laughed.  They got it immediately.

Another necessary but more pleasant post-launch activity for Cassini's engineers and science teams was the in-flight instrument checkouts during which the spacecraft subsystems and all the scientific instruments were tested.  In our case, images were taken at infrequent intervals with both cameras, through various spectral filters and camera states, of stars that had been well-studied from the Earth and whose brightnesses were stable and well-known — eg, Spica, Fomalhaut, the Pleiades — for calibration and for assessing instrument performance. Calibration is the process by which we determine how much signal, and the uncertainty on that signal, our camera system — optics, electronics, and software — would yield for a particular input intensity of light.  Knowing precisely this translation from one form of information to another is required if we are to make scientific use of an image and, say, determine the physical or chemical processes that might be producing the amount of light recorded in the image. Bob West, an atmospheric scientist on my team who later became my Deputy Team Leader, was the uncontested king of calibration, taking great care to plan and acquire the images we needed during the entire mission. With the exception of one scare, when it seemed that our high-resolution camera, critical for our science as well as the optical navigation of the spacecraft, had badly lost focus — thankfully, it had not and the problem was reasonably easy to fix — our cameras performed faithfully throughout the rest of the mission.

Our first two years of flight were spent close to home, marking time in the inner solar system, encircling the Sun, patiently awaiting an infrequent planetary alignment between Jupiter and Saturn that would power the final leg of our journey.  These two giant outer planets find themselves about every 20 years in a configuration that permits the flight of a Saturn-bound spacecraft to sail by Jupiter sufficiently close to steal a modicum of Jovian orbital energy and thereby significantly shorten its voyage. This maneuver, well known to all space aficionados as a 'gravity assist', was used to exquisite advantage in a very rare, every-176-years alignment of Jupiter, Saturn, Uranus and Neptune in Voyager's Grand Tour of the outer planets from 1979 to 1989.  Similarly, Cassini's launch date and solar system trajectory had been specifically designed to allow passage through the Jupiter-Saturn “planet gate” that opened near the turn of the millennium. 

During that two year wait, we paid visit to Venus twice (in April 1998 and again in June 1999) and the Earth once (August 1999).  All three flybys provided some measure of gravity assistance, but also presented opportunities for the Project’s scientists and engineering flight teams to test, and collect data on, the spacecraft and science instruments during rapidly changing flyby conditions. As temperatures at Venus were too hot for our cameras to work well, and the spacecraft orientation during Earth flyby geometry did not favor imaging of Earth, imaging with our cameras was limited to the Moon, which served exceptionally well as a calibration test.

Caption: Cassini’s Earth-Moon Flyby

Our moon is not the most visually exciting object in the solar system but nonetheless, I was delighted with the results. Here was our first real 2-dimensional planetary body imaged with our cameras and the images looked great! 

Caption: Cassini Images the Moon

With the Earth flyby successfully completed, we set our sails for the high seas.  First up?  Passage through the asteroid belt.  

Now, the asteroid belt at first blush might sound like a bad dream to a passing spacecraft, but it's not the danger it seems.  One of the main objectives of the Pioneer mission and its two craft, launched in 1972 and 1973 to Jupiter and Saturn, was to verify safe passage through the belt for the following outbound spacecraft that would truly investigate, in detail, all the outer giant planetary systems … the Voyagers launched in 1977.  Pioneers 10 and 11 sped through the asteroid belt unscathed as did Voyagers 1 and 2.  We have since learned that despite 100,000's of known asteroids, and a total number likely in the millions, the belt is mostly empty space. You'd have to be very lucky to enjoy a close, unplanned encounter with an asteroid.

It took us about 7 months to pass through the belt.  We entered sometime during fall 1999 and exited around May 2000, without a hitch. But how remarkable it was that, with no intention whatsoever, on January 23, 2000, about mid-way through, we came within 1 million miles of Asteroid 2685 Masursky. That’s not a close encounter but it

was, in fact, an amazing and poignant coincidence for those of us on my team who had also been Voyager imaging team members. Harold Masursky was a renowned geologist, one of those early planetary explorers chosen to participate in the historic Mercury and Apollo programs, the Viking mission to Mars, and the Voyager mission. It was on Voyager that I came to know Hal.  He was an expert punster, a joy to be around, and has become, like the other early planetary explorers who populated the Voyager imaging team, an integral part of my cherished memories of the time I spent on that iconic mission so long ago. I was privileged to serve with him and very happy that we were able to capture an image of his asteroid. 

A single pixel in an image taken with our narrow-angle camera at a range of 1 million miles was ~6 miles across.  In our highest resolution image, the body was only just resolved.  Still, we were able to eke out some scientific tidbits.  Asteroid 2685 Masursky was likely ~19 - 25 miles wide and seemed not to be of the asteroid category to which it had been assigned … not too shabby for a serendipitous rendezvous.  [I would love to know if anyone has since confirmed or refuted our findings, or has done any further analysis. If you know, please pass the information on to me in the comments below.]

By June 2000, the asteroid belt was behind us and glorious Jupiter, our next port of call, was up ahead.  Arrival at Jupiter is always significant because it means you've officially entered the outer solar system. So, our encounter with Jupiter was not only a great boost to our spirits, but critical to our preparations for Saturn. Closest approach would be a distant 6 million miles from the planet's cloud decks. We wouldn't be buzzing the planet the way Voyager 1 and 2 did, with their much closer flybys of 174,000 and 401,000 miles, respectively. That meant we wouldn't be taking high resolution images of anything on the planet or its single narrow ring. But, because of Cassini’s leisurely flyby, the large data storage capacity and high data-downlink rate of the spacecraft, and the superior imaging characteristics of our cameras, what we could do that even the Galileo orbiter at Jupiter, with its impaired antenna, could not was acquire over the several months preceding closest approach very high-quality, data-voluminous movies of Jupiter's changing atmosphere that were better than those returned by the two Voyagers during their Jupiter flybys in 1979.  We could even capture time-varying processes within the jovian satellite and ring systems, if there were any to be seen.   In other words, our distant flyby of Jupiter would be Voyager class and, in some regards, even better.

Such a major planetary flyby requires a commensurate level of planning.  Trouble was, we were woefully unprepared!  The science teams had neither the funding nor time to hire software developers and sequencing personnel to assist in developing Jupiter observational plans and instrument commands.  The Project personnel at JPL hadn't yet figured out how the remote ground systems supporting each science team would efficiently interface with JPL.  And I don't think anyone was even thinking about information traveling in the opposite direction: what do we do when all that data hits the ground?  Sufficient funding for such developments wouldn’t appear for another 2 years.

Consequently, Jupiter flyby became a high-stress, chaotic, making-it-up-as-we-go-along affair.  With no assistance, I ended up doing all the sequence designing for the atmospheric and ring sequences myself, participating in telecons to negotiate how observing time and downlinked data volume would be allocated across the science teams, and using low-brow spreadsheets and text files (would you believe?) to send image designs and camera commands to JPL over email.

Despite the makeshift accommodations and frenetic pace, in the end Jupiter encounter turned out to be a blessing ... a detailed, full dress rehearsal that all of us on Cassini sorely needed before getting to Saturn.  For me, the sleeves-rolled-up, in-the-trenches perspective made it crystal clear just exactly what capabilities, software, and experience CICLOPS would need to conduct operations at Saturn. It also gave me the insights and know-how to hire and train appropriately skilled support personnel and to efficiently lead both science team and staff members in preparing for the biggest task of all ... a 4-year expedition through the Saturnian system. 

We did some fabulous science at Jupiter.  Were it not for the fact that Jupiter was not our main gig and our findings there were understandably eclipsed by what we found at Saturn 3.5 years later, our encounter with Jupiter would have been far more celebrated than it was.  Data collection began on October 1, 2000 and ended on March 22, 2001.  We acquired about 26,000 images, well exceeding the clutch of the Voyager spacecraft at Jupiter of ~18,000 images each.  The glorious, planet-wide movies we captured, rendered by team member Andrew Ingersoll and his associates, are still the best made thus far of Jupiter's kaleidoscopic atmospheric motions. Here's one of our most remarkable … smoothly interpolated to remove noticeable jumps between frames and, note well, rendered from an unrealistic reference frame … in 2D:

… and in 3D:

Caption: Interpolated Movies of Jupiter … in 2D and in 3D

Movies and still atmospheric images revealed features of Jupiter's atmosphere that had not been seen before. Among them was the presence of coherent zonal jets in Jupiter's polar region up to very high latitude; the presence of small vortices, both in the mottled polar region and at low latitude, that get carried around by the regions' zonal winds, and the relatively long-lives of both;  the obvious lack in Jupiter's polar

Caption: Jupiter’s Polar Winds

jets of the large-amplitude excursions in latitude that are seen in the polar jets of both the Earth and Saturn; the detection of vigorous convective storms that are restricted to Jupiter's dark belts and not its white zones, as had been traditionally believed; and more.

We even managed to make a movie, planned by team member Alfred McEwen, of Jupiter's volcanic moon, Io, passing through the shadow of Jupiter … a movie that

made very clear that faint, diffuse emissions arising from Io are due to atmospheric aurorae caused, as they are on Earth, by the collisions of charged particles with gases in the atmosphere, and that the gradually shifting positions of the glows are due to the changing orientation of Jupiter's magnetic field. These results were visual confirmation that the aurorae are caused by the electrical currents flowing between Io and Jupiter along the planet’s field lines. We even processed the movie in color, where the emissions we colored red (and would look red to your eye if you were there) likely arise from the atomic oxygen in the tenuous Io atmosphere, and the emissions colored blue, which are visually deeper down in the atmosphere and thus closer to the surface, likely

correspond to near-ultraviolet emissions (which we can’t actually see with our eyes) from molecular sulfur dioxide, which itself would be nearer the surface than oxygen because it is heavier.

How cool is that?! 

And I'm sure those who were watching us at the time remember our last Jupiter image release, about three years after the images were on the ground, after an enormous amount of work on the part of one of the team's associates Ashwin Vasavada. I called it “The Greatest Jupiter Portrait” because at the time it was and, to me, still is. 

Caption: Cassini’s Greatest Jupiter Portrait

Do read the caption and go to the link there to bring up the full-sized image.

Once passed Jupiter, we returned to our in-flight calibration observations, stealing longing glimpses of Saturn from time to time … one from a distance of 177 million miles, or about 2 astronomical units, on October 21, 2002, and another from 69.2

Caption: Cassini first sights Saturn

million miles, on November 9, 2003, when it had obviously grown in size and greater detail could be seen, including 5 of its satellites:

Caption: Portal View

And the day did finally come, 20 years ago this month, when we arrived at Saturn's doorstep. On February 6, 2004, our official approach to Saturn began. The pace of activities within the Project picked up and data collection on Saturn and everything around it started in earnest … rapid and steady.  It felt like we had stepped onto a conveyor belt and strapped ourselves in, knowing it wouldn’t stop for the next four years. (Little did we know then that our years at Saturn would extend to 13!)  

At this time 20 years ago, my team members and I were not yet seasoned Saturn explorers — no one was — and I, for one, was anxious about the magnitude of the task before us.  We did not yet have our imaging ground system fully up and running and we were still using spreadsheets!  But the images that we were seeing on our computer screens were breathtaking.  Habituated as I was to impressions of Saturn, its rings, and moons gleaned from Voyager images of 23 years earlier, I was stunned at the clarity of Cassini's images in comparison.  It was like opening your eyes after LASIK surgery and suddenly there before you is the world as it really is. 

Our first image sequences taken on February 6 were atmospheric movies of the type we had designed at Jupiter for the very same purpose: to take advantage of the large distance from Saturn, which would never be so large again once we were in orbit, and make long duration, planet-wide, multi-wavelength movies of the planet's atmosphere to study its meteorology and sound its atmosphere. We eventually made movies of the rings, too … to search for the famous spokes, discovered on approach to Saturn by Voyager, and any new moons within or near the rings.  We also executed special imaging searches for new satellites throughout the inner satellite system, from the rings out to the orbit of the moon Iapetus, and planned routine images of the known moons, big and small, to improve their orbits. 

Of course, everyone on the mission was terrifically interested in seeing the hitherto unseen surface of Saturn’s largest, hazy moon Titan, the largest single expanse of unexplored terrain remaining in our solar system before Cassini got there, and it was from the beginning a top scientific objective for us imaging folks and indeed the entire Cassini mission. Routine imaging of Titan would begin when our resolution became comparable to that seen from the Earth. 

And June would deliver a planned, very close flyby of the retrograde moon, Phoebe ... a body 125 miles wide, believed to have been captured into Saturn orbit billions of years ago, as Saturn and its inner moons were forming. We didn’t want to miss the chance to see a Saturnian interloper up close!

Though we anticipated being in orbit for at least four years, and taking 100s of thousands of images while we were there, we also knew that if we didn't make it into Saturn orbit, if something went wrong ... say, a valve in the main engine, intended to slow Cassini's acceleration so it could be inserted into orbit, didn't work, or the spacecraft was fatally damaged by a ring particle while it flew through the ring plane during the insertion maneuver ... and the mission devolved to a simple flyby, the information we collected during those 5 months on approach would be all we’d have for 14 years of effort.  It became both scientifically and emotionally important that we did the approach sequence right.

On February 27, 20 years ago today, we released our first image of Saturn as an

Caption: Approach to Saturn Begins

announcement to the world that after toiling 7 years to get to the launch pad, and another 7 years journeying to Saturn, the show had finally begun.  On that day, I advised visitors to the CICLOPS website, “Prepare to be amazed!”              

Over the next year or so, I hope to remind you just how amazing it all turned out to be, and hope that you follow along with me here and encourage others to do the same, as we look back at those privileged years of wide-eyed wonder traveling Saturn.

- Carolyn Porco

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Dear Friends,

About two weeks ago, I made an honest attempt to write a substantial and meaningful end-of-year newsletter to send to you and, in fact, did so, producing a first draft that I had planned to finish this weekend.  In it, I wrote of reflections on the year just passed -- which for me personally was momentous in both good and bad ways -- and my hopes for the new year.

Unfortunately, I spent the last few hours searching for that first draft and can't find it.  I obviously saved it somewhere on my devices so secure that I can't myself locate it.  As it's already nighttime here, this brief note will have to do.

There is much to be concerned about in the near future.  The US, and indeed the world, today are being buffetted by political and social winds that were last in play nearly a century ago.  And the disastrous undoing of the world order at that time, its horrifying evolution, and the clear object lesson those past events serve today in how to ensure it doesn't all happen again are, to the amazement of many of us, being unheeded by too many.  This past year, I've taken to asking friends, 'What are you going to do if the US turns fascist in the next election?  Are you going to leave the country?'  I can say that a not insignificant number of those friends are working on their list of possible destinations. 

This time of year presses us to look forward at the 'second chance' a new year always brings, so that's what I'm doing one more time this year, with feeling, and the hope that you are too.  Remember: it's not over til it's over.  There are some impressive countervailing forces at work that could win in the end: eg, Jack Smith, kick-ass special prosecutor if there ever was one; states removing the orange one from their election ballots; successful lawsuits challenging seriously gerrymandered election maps; etc.  And people are coming to realize that traditional pre-election signposts, like polls, that have indicated in the past the direction the country is likely to go, are no longer as accurate as they once were.  

These are, in fact, just some of the good reasons to hope for the best.

We now face a new beginning and another chance to get it right.  I've vowed to myself to work hard in the coming year to stay positive about the possibilities, despite the struggles, and do what I can do to ensure that good things happen in the coming year.   Will you be joining me.

Happy New Year to All!
Carolyn Porco

Dear Friends,

Today is the 89^th birthday of my colleague and friend, and one of my biggest heroes, Carl Sagan … a man still very much loved and still very much missed.

Happy Birthday to Carl!

It is also a very special day in the life of the exploration of our own planetary system.

It was 20 years ago today that we on Cassini sighted the planet Saturn, alluring, mysterious, beckoning, 111 million kilometers (69 million miles) in the distance … about three-fourths the distance between the Sun and the Earth.  https://ciclops.org/view.php%3Fid=74.html

After 14 long years designing, building, and launching the spacecraft and all its many systems and scientific instruments, and after enduring 7 years to cross the solar system, we were now less than 8 months away from entrance into Saturn orbit.       

"Portal View"

It wasn’t the first time we sighted Saturn on our journey there.  We had done so a year earlier.  There is a rather embarrassing story behind the public release of that first 2002 image that I won’t tell here. I am saving it, and many other backstories of our imaging adventures to and around Saturn, for my first book.  How’s that for a teaser?

But what made today’s anniversary image different from its predecessor a year earlier was the cache of five of Saturn’s seven main satellites -- Mimas, Enceladus, Dione, Tethys, Dione and Rhea – and the details we could now see on the planet and in the rings.  We could not resolve any of these moons at this point but divisions in the planet’s rings, like the 4800-km (2980-mi) wide Cassini Division between the outer A ring and the bright B ring, and the much narrower, 325-km (200-mi) wide Encke gap near the outer edge of the A ring, were clearly visible, as was the fainter C ring interior to the B ring.  The multi-banded structure and the delicate hues of  yellow, brown and pink in the southern atmosphere were also becoming more apparent.

And then there was that blue in the northern hemisphere.  I thought at the time: “Blue?  Where did that come from?!” There was no blue in the Saturn atmosphere when the Voyagers flew by the planet in late 1980 and mid-1981.  I was mystified.  The Saturn experts on my team guessed that it was produced by molecular hydrogen scattering at altitudes above the haze and clouds, where the atmosphere is clear … in other words, the same process (though a different compound) that turns our sky blue. That guess, turns out, is likely only part of the answer -- see my forthcoming book for the full answer -- but I was overjoyed at the presence of this new color in the Saturn atmosphere. It meant that our images were going to be gloriously colorful – watch out, Jupiter! -- and would no doubt dazzle us as well as our followers. I was not wrong.

With this distant Saturn image in hand, our excitement and anticipation of what lay ahead began reaching acute levels. 

One of my team members, Gerhard Neukum, a professor at Free University in Berlin, Germany, said of the moons, "Soon we will be in orbit around Saturn to investigate these worlds in detail and to decipher their geologic history from close-up images - an exciting prospect."

Another imaging team member, Anthony DelGenio, a specialist in atmospheric studies from NASA Goddard Institute for Space Studies in New York, said, “For all of us who have worked for more than a decade preparing for this mission, seeing Saturn grow larger and larger in the eyes of the Cassini cameras is a bit like the feelings children have as they come downstairs on Christmas morning to see what gifts are waiting for them under the tree. But this Christmas will last for four years."   What we didn’t know then was that Cassini’s Christmas would last 13 years!

And Wesley Huntress, the director of NASA’s Solar System Exploration Division in 1990 and the individual who made the final selections at that time of all of us principal investigators and team leaders, exclaimed, “Wow! So far away, so long to travel, so much effort to make it happen, and so worth it".

Of course, how ‘worth it’ it would be depended on us getting successfully into orbit on July 1, 2004.  But at this point in the voyage, it was looking very good. 

It has seemed to me for a while now that today’s 20-year anniversary of Cassini’s ‘Portal View’ might provide a lovely opportunity to begin a chronological retrospective of what we found at Saturn during Cassini’s time there.  At a time when so many aspects of modern life seem to be unraveling, it would remind us of how wonder-filled and reassuring it felt to know that our magnificent, golden emissary to Saturn was faithfully conducting its work, revealing to us one marvel after another, and that for so many years, we could look forward almost daily to the possibility of a new and spectacular vision of a far-flung alien world.  

For those members of the generation who missed it the first time around, it would be their chance to learn the full story and witness it as it unfolded.

And note well …. all of you would receive from me something you cannot get from the countless individuals who post our Cassini images on social media …  the real stories behind the images.  Why they were taken? What did we learn from them that we didn’t know before? Were they scientifically successful?  Did the taking of the image pose any particular challenges?  Were they processed to bring out any particular aspect of the body being imaged?  Did it require any additional information or data gathered from other Cassini instruments to arrive at the full scientific value?   All this and more … from the person who not only led the team that planned all those images and made them happen, but from the person who also set the guidelines and procedures for the careful and artful processing, captioning, and posting of those images for public consumption.  It all required visual balance, attention to natural color reproduction, and a degree of meticulousness that had never been done before in the planetary program.  As a result, our images were and still are much  beloved the world over.  Call me proud!

I have yet to work out how long this retrospective will take.  To take a full 13-years might be too much, you think?  I would be 84 years old by the time Cassini re-crashes into Saturn!  Probably not going to happen.

I’ll certainly be thinking about this matter before the next installment in this re-enactment. And if you have any suggestions, send them along!

Know that the Cassini Retrospective will not be the only subject matter I will discuss in this newsletter. But it will be the main communication vehicle for it.

So, consider this newsletter the beginning of our sentimental look back at Cassini’s time at Saturn.  And if you know anyone else who would like to join this adventure, invite them to subscribe to this website: 

With that, I bid you farewell, and as I said long ago as we were approaching Saturn …  Prepare to be amazed!

Wishing you all the best!

Carolyn Porco