Remembering Cassini - Entering the House of Saturn

April 14, 2024

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.

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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.

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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!