On July 1, 2004, after traveling for seven years across interplanetary space, the American-built Cassini spacecraft and its European-built Huygens probe glided flawlessly into orbit around the planet Saturn. At that point, at a distance 10 times farther from the Sun than is the Earth, the six-metric-ton, bus-sized craft became the first artificial moon of Saturn and the farthest robotic outpost humanity had ever established around the Sun.
Six months later, the Huygens probe drifted on a piece of fabric through the atmosphere of Saturn's largest moon, capturing panoramic images as it fell, and after two-and-a-half hours came to permanent rest on the dark equatorial plains of Titan. In another spectacular first, a device of our making had made landfall on a moon in the outer solar system. It was a Jules Verne adventure come true.
In the early 1980s the Pioneer and Voyager spacecraft passed through the Saturnian system, but Cassini's expedition has been different. Our journey back to Saturn has afforded us the kind of insight and scientific perspective available only through prolonged, systematic investigation. In my mind, our journey has been, and continues to be, both part of and a metaphor for a much larger human quest: to discover the universal interconnectedness of all that surrounds us, and to come to understand a bit of our own origins. And Cassini's offerings in all these regards have not disappointed. The images that it has returned have been an explorer's dream—breathtaking, dazzling and informative—and mission scientists expect even greater things to come.
Saturn's Rings at Center Stage
Why go to Saturn? Besides being the most iconic of "otherworldly" planets, the same processes ongoing in Saturn's rings are believed to have occurred in the disk of material that ultimately became our solar system, in similar disks around other stars, and—at a trillion times the size—in the disks of stars and gas that make up spiral galaxies. So, in studying Saturn's rings, scientists gain a better understanding of disk systems throughout the cosmos.
This mosaic is made from 36 Cassini images taken from about 1.23 million kilometers away. In order to capture the dark side of the rings, the sunlit half of the planet was deliberately overexposed. The stripe of shadow across the lit surface is cast by the rings, themselves consisting of countless bodies, ranging from the size of fine, powdered snow to that of a large house. Despite their enormous speeds around Saturn—up to 64,000 kilometers per hour—the particles nudge one another only gently. Violent collisions in this system died down long ago; as a result, the rings are very thin—only about two to three stories thick—and very mathematically precise, tracing out the plane of gravitational equilibrium around the planet.
Since its release of the Huygens probe, Cassini has made about 45 loops along different trajectories. During its first thousand days in orbit, the craft discovered four new Saturnian moons—Methone, Pallene, Polydeuces and Daphnis—and several new rings. A few are coincident with the orbits of the moons, such as Pallene and the "co-orbitals" Janus and Epimetheus. Other rings are embedded within the main rings. Saturn's rings are lettered in the order of discovery; distances are all from Saturn's center (table). The Saturnian rings and major moons, out to Iapetus, span some 3.5 million kilometers (top); Phoebe is a member of the much more distant outer-irregular-satellite system. The rings and moons are shown at relative sizes to one another, except for the smallest moons, but their sizes do not correspond to the length scale, which has been broken to accommodate the system's full breadth.
Grooves and Waves
Ring particles consist mainly of water ice, but subtle colorations across the rings indicate that the surfaces of particles in each region are contaminated by small amounts of material of different compositions (below). Each ring has its own characteristics. The middle-region B ring is the most massive, and the outer A ring is punctuated by lots of narrow moon-driven features, whereas the inner C ring is very transparent. It's possible that each ring could have a different origin and age, and all could be much younger than the planet itself, perhaps no more than a few hundred million years old. Moons embedded within or near the rings can affect the surroundings through their gravitational perturbations (see the section "Moons Small and Large" below).
The largest of these ringmoons, such as Pan and Daphnis, are responsible for maintaining the gaps in which they lie and for creating waves and other structures in the nearby rings. The smallest ringmoons, only a few times bigger than the biggest ring particles, create incipient, incomplete gaps in the rings. The particles that comprise the main rings likely come from the catastrophic destruction of one or more pre-existing moons by incoming material, followed by the inevitable grinding and erosion of the remnants into smaller and smaller sizes by the process of collision. The small ringmoons may house within them the remnants of these events. The outer F ring (left) is the most contorted—Cassini has shown that it consists of a core ring that is knotted and kinked by the actions of one of its shepherd moons, Prometheus, and perhaps hundreds of other unseen moonlets within it.
An Active Atmosphere
Cassini arrived at Saturn in its northern hemisphere's winter and, surprisingly, revealed that although the usual buttery color of the planet's clouds remained in the southern climes, it was gone in the north, replaced with a clear sky-blue (below). The cause of this change is still unknown, but mission scientists suspect that the cold winter temperatures drive the formation of clouds deeper in the atmosphere where they are harder to see.
Saturn's atmosphere has provided even more mysteries. Cassini's infrared mapping spectrometer captured the planet's north pole and revealed a bizarre hexagonal feature encircling the site (top image, below). The red color, indicating heat, implies that the area is largely clear. At the south pole, Cassini picked up a swirling hurricane-like vortex (bottom image, below), a phenomenon characterized by a well-developed eye ringed by towering clouds, that has never before been seen on another planet. The monster storm, about 8,000 kilometers across, has winds blowing at 550 kilometers per hour and clouds rising 30 to 75 kilometers above those in the center.
Around the middle southern latitudes of the gas giant, a false-color view shows a massive dragon-shaped electrical storm, with red indicating clouds deeper in the atmosphere and gray and white indicating higher ones (below).
Cassini found this storm to be a strong source of radio waves, much like the short bursts of static generated by lightning on Earth, and identical to electrostatic discharges that were first detected by Voyager but not understood at that time. This is one mystery that Cassini's prolonged campaign of observations of the Saturnian environment has finally solved.
Moons Small and Large
Within and close to Saturn's rings is a set of small, non-spherical moons; shown below are (from upper left) Pan, Daphnis, Atlas and Pandora. Daphnis resides in the Keeler Gap in the A ring, and like other ringmoons affects the ring around it. In Daphnis's case, it induces the orbits of the ring particles along the gap's edges to become eccentric, creating waves both upstream and downstream of the moon. These dynamical systems of small moons embedded within gaps within a larger disk of material are the best nearby analogues we have for disks of dust and gas around other stars in our galaxy in which growing planets are embedded. The shapes and densities of all these ringmoons imply that they too were built up by the accretion of material around a denser core, very likely itself a remnant of the collision that destroyed the progenitor ring body, which finally grew large enough to open a gap in the rings.
Saturn's larger moons, such as 1,123-kilometer-wide Dione (below, top image), are pocked by billions of years of impacts, perhaps the most dramatic being the 130-kilometer-wide Herschel crater on 396-kilometer-wide Mimas (middle row, left image). These moons are largely water ice, in some cases surrounding a rocky core. Rhea, 1,529 kilometers across, here hiding behind Saturn's rings (middle row, center image), like Dione has wispy, bright fractures. Iapetus (middle row, right image), a moon 1,471 kilometers across, sports a landslide at the base of a 15-kilometer-high cliff along the side of one of its several very large basins. Tethys, 1,066 kilometers across (bottom row, left image) exhibits a fissure 65 kilometers wide that girds three-fourths of its circumference, possibly a result of crustal fracturing after the interior of the moon solidified. Hyperion is surely the oddest looking moon of Saturn (bottom row, right image). It's the largest irregularly shaped satellite ever observed, with a strange sponge-like appearance across its 270-kilometer width that can be attributed to a surface thoroughly saturated with unusually well-preserved craters.
Taking in Titan
Saturn's largest moon is 50 percent larger that our own Moon and has long intrigued planetary scientists. Before Cassini's arrival, its surface environment was believed to be in some respects more like Earth's than any other body in our Solar System, despite temperatures of 300 degrees below zero Fahrenheit. Its thick atmosphere, like Earth's, contains molecular nitrogen, but it also contains simple organic materials that form a ubiquitous orange, globe-enveloping haze, which over eons, it was believed, might have come to coat the surface in a thick organic sludge. The simplest compound, methane, can take on solid, liquid and vapor states in the Titanian atmosphere and so can do on Titan what water does on Earth. It was consequently suspected that on arrival Cassini would find clouds and bodies of liquid methane.
When observing Titan in the right light from Saturn orbit, Cassini's cameras can peer through its obscuring atmosphere and see its surface (above). Light, continent-sized regions can be seen, as well as darker features. What are they? From a distance, it was hard to tell.
Huygens Through the Haze
As Huygens plowed through Titan's atmosphere, from 16 kilometers above the surface a landscape veined with signs of liquid drainage and features that looked like islands (below, top image) gradually came into view. Tracts of channels and gullies and what looked like a shoreline became more distinct at eight kilometers (below, lower image).
Huygens eventually came to rest on a mud flat, an unconsolidated ground wet with liquid methane, on the edge of a large dark expanse. Right away, imaging scientists knew the dark bodies seen from orbit were not liquid at all. In fact, despite clear evidence of a dendritic pattern of channels that were almost certainly, as on Earth, carved by liquid, no open bodies of liquid were evident anywhere.
Cassini's radar instrument found vast areas of dark equatorial plains covered with 100-meter-high dunes (above), a geological feature implying steady bi-directional winds and sufficient dryness to loft tiny particles. Where were the fabled open bodies of liquid? Cassini finally looked poleward.
Lakes at Titan's Poles
A view of the south pole came first, in June 2005. Our cameras spied a lake-like feature (below, top row, left image) with all the right morphological and environmental markings of a body of liquid—a cloud complex, presumably made of methane, can be seen in the lower right portion of the image—but it was still uncertain. When scaled relative to the surface area of Titan, this feature is as large as the Black Sea. A year and a half later, Cassini's cameras and radar instrument captured images of the north pole, and details began to emerge. The radar strip (below, bottom row) from a near-polar flyby, captured in early 2007, shows exceptionally dark areas closest to the north pole (below, far right side of strip), which the radar team interpreted to be liquid hydrocarbons. These same bodies were also seen in the images captured by the cameras; the region of overlapping radar/imaging coverage is outlined in blue (below, top row, right image). The largest body seen, when scaled relative to Titan, is about the size of the Mediterranean.
Scientists do not yet know why open liquids seem to be preferentially located near the poles, but it no doubt says something about the meteorology of Titan's atmosphere that we have yet to understand. Our close-up explorations have shown us a moon that, although different in detail than expected, is every bit as fascinating.
The Fresh Face of Enceladus
A relatively small but very bright white moon, Enceladus has yielded one of Cassini's most startling and significant findings. Although part of its water-ice surface is pocked with craters (below), very large regions are essentially crater-free, indicating major surface-altering events in the course of its history.
One of these altered regions is at the moon's south pole, crossed by a systematic pattern of fissures that, as shown in false color, are compositionally distinct, coated with simple organic materials (below, left image). The south polar terrain was found to be surprisingly warm, with the fissures being warmest of all. And when positioned to look back in the direction of the sun over the south pole, Cassini captured a most incredible sight: a dozen or so jets of fine, icy particles spraying from these large fractures and extending tens of kilometers into space. These in turn feed a huge plume of material that towers above the south pole (below, upper right image); north is rotated to the right in this image), and extends itself into the E ring, which, it's now obvious, Enceladus creates (below, lower right image).
Other instruments on Cassini found that this spray of icy particles is accompanied by water vapor laced with simple organic compounds. Some scientists have proposed that the jets are caused by simple evaporation of warm ice and the subsequent freezing of vapor as it hits the cold of space. Others have proposed the explosive release of gases trapped in cages made by the crystalline structure of water ice as the source of the vapor. Our group of imaging scientists on the Cassini Imaging Team has suggested that the jets may in fact be geysers erupting from subsurface pockets of liquid on this moon. And if we are correct, then Cassini has indeed made a heart-stopping discovery: a combination of excess warmth, liquid water and organic materials, or in other words, an environment potentially suitable for living organisms. For planetary explorers, it doesn't get more exciting than this.
Why Enceladus is warm, how long it's been this way or how long such conditions will last on the moon is not obvious. Neither heating by radioactive materials within the moon nor by present-day tidal flexing can produce the amount of heat that has been observed. The answer is not readily forthcoming and may lie buried in the moon's distant geological past. But one thing is manifestly clear: Our first one thousand days traveling Saturnian highways have opened our eyes to the splendor and discovery inherent in exploring a faraway land that for all of human history had been unreachable and now is sitting quietly, without disturbance, under our watchful gaze. Our next one thousand days spent exploring this enchanting planetary system should be no less remarkable.