Remembering Cassini: The Mystery of Iapetus
December 31, 2024
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 17th 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 6th 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
This is a great story of the wonders of planetary research as well as the perseverance and dedication of scientists that wanted to obtain knowledge for the benefit of all humanity. Thank you, Carolyn. This is a great way to begin 2025.
What a great story and an insight into how theories are validated by closer inspection. I am always amazed and impressed into how knowledge is advanced by remote exploration. The prioritisation of missions due to time and fuel constraints makes me wonder what other research had to be dropped from the mission.