Saturn and its Moons

Saturn is notorious for its resplendent rings, which can even be seen in the telescopes of amateur astronomers. These rings consist of rock and/or ice debris, a few pieces of which are as large as a house, and most smaller. Image processing of the Voyager flyby images of the rings can make these stand out in beautiful (though artificial) colors which facilitate their separation into individual ring units (identified by letters). Saturn has since been visited by the Cassini spacecraft. Saturn also has a large number of satellites (34) of which seven are big enough to have evolved into spherical shapes. While sharing an icy composition, there are notable differences among the seven: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. Saturn and its Moons Now, on with the Tour! Both Voyager spacecraft observed the second largest planet, Saturn (diameter 84.3% that of Jupiter and 10% shorter at the poles). Voyager 1 flew past Saturn in November 1980; Voyager 2 passed by in August of 1981. When the first spacecraft was slightly above the rings, on the sunlit side, this planet presented a stunning image: Cassini, the largest spacecraft ever to be sent to planet, was placed in orbit around Saturn on July 1, 2004. The seven major rings begin about 7,000 km (4,350 miles) beyond the planet's discernible atmospheric edge and continue for another 74,000 km (45,984 miles), to the faint F ring (discovered by Voyager), a diameter of greater than 260,000 km (161,564 miles). But, the entire system is only about 1.5 km (about 1 mile) thick. The rings have been named using alphabet letters, arranged in the order of their sequence from closest to farthest from Saturn's gaseous surface (but the letters relate to the order of discovery): D, C, B, A, F, G, E. This can be shown diagramatically, in which the specific rings and the major satellites are shown in their relative positions: The E and G rings are usually not imaged. This composite mosaic made by the Cassini spacecraft when it was 3 million kilometers away shows these two and their relation to the inner rings: The rings are in fact very thin (less than a kilometer). This is suggested by this Cassini image, which also shows one of the saturnian satellites, which indicates how small these are relative to Saturn's size: Let's reexamine Saturn's inner rings. The Cassini Division (also referred to as a Gap) separates rings A and B, and the Encke Division lies within the outer part of A. This image made by the Cassini spacecraft is the closest to the true color of the rings: Note the details of the ring structure in this color-enhanced version, made by assigning red, green, and blue to clear, orange, and ultraviolet image frames, respectively. We chose it for its esthetic quality and to help further to distinguish its ring divisions: The inner blue region marks the C ring. Beyond it is the B ring, whose inner part is orange and outer is greenish-blue. There is a large dark area known as the Cassini division (a gap) that separates the B ring from the outer (purplish) A ring. A smaller gap, named the Encke gap, subdivides the A ring. Although not visible in this image, parts of rings (mainly in B) contain "spokes", which are narrow darker linear "shadows" that lie perpendicular to the bands. These may be density discontinuities or clusters of small particles levitated above the residual larger particles in the rings by electrostatic charging. Note the many dark bands, which are gaps with lower densities of particles. This Cassini image shows examples of the spokes: The Ultraviolet Spectrograph on Cassini has produced images whose colors indicate compositional differences. In this next image, of the B ring, the red bands have a higher proportion of "dirt" (rock/dust) whereas the blue bands are mostly just water ice. One model for the origin of the rings considers the band components with more dirt to be older than the blue bands. The visible rings consist of mostly ice and ice/rock fragments that (although spread apart) reflect sunlight. Most particles fall within the millimeter to several meter-size ranges. A few chunks attain sizes up to 100 meters. Here is an artist's conception of the objects within one of the rings: The actual ring particles were imaged just once, when the Cassini spacecraft actually passed through the gap between the F and G rings as it was about to enter its orbit around Saturn. This image shows that the particles in the F ring are mostly around meter-sized: The origin of Saturn's rings is still unsettled. The materials may be even now in the process of organizing into a satellite or could be residual materials around Saturn that never succeeded in building into a single large body. Or they might be debris from a large impact that shattered an earlier satellite. Or they could be the residue of a pre-existing satellite that moved so close to Saturn that the planet's tidal forces disrupted it (the distance at which such break up must occur is called the Roche Limit; for Saturn this is 1.5 times the planetary radius; for the Earth-Moon system it is 2.44 times Earth's radius). Recent discovery of small moonlets (typically 100 - 200 meters) has supported the idea of the rings being residuals from the destruction of a larger satellite moon. One generality: only the Giant planets seem capable of maintaining rings once formed, owing to the ability of their strong gravitational attraction to keep the rings from dissipating over time. One line of evidence considers the saturnian rings to be no older than 100 million years. At least the outer rings, A and F, are kept from drifting apart by the action of three small satellites orbiting close to them. Called "Shepherd Satellites", their gravitational focusing acts to push straying particles back in line. Their action also may cause the braiding observed in the A ring. The F ring is controlled by the small satellites Prometheus and Pandora. Prometheus appears in this Cassini image: As each satellite moves along, it deflects particles inward or outward depending on its position inside or beyond the ring. This diagram is relevant: Saturn has about the same composition as Jupiter, namely Hydrogen and some Helium, but lower internal pressures and temperatures (up to about 12,000° C) may limit the extent of the metallic Hydrogen zone. Helium makes up about 7% of Saturn's atmosphere (on Jupiter He was 11%). Saturn's density, ~ 0.7 gm/cm3, is the lowest of any planet. Saturn's thin surficial atmosphere, which is responsible for the visualization of an "apparent" surface, although moderately banded, lacks the reddish coloration typifying Jupiter's belts, but its yellowish color is similar to the Jovian zones. Circulation patterns, driven by jet streams, are also similar, including oval spots, cyclonic storms, eddies, and swirls. Winds can exceed 300 km/hr. The depth of atmospheric circulation may be as much as 2000 km (1200 miles). The atmospheric bands differ somewhat in composition (which has been investigated further when the spacecraft Cassini arrived there in 2002). The NICMOS camera on the Hubble Space Telescope (Section 20) has taken an interesting multispectral image of Saturn (including IR bands) in which the belts are assigned contrasting colors (like what was done on the rings above) based on spectral variations to help emphasize the significant differences that do not stand out in true color images. This trio of images compares Saturn's gaseous surface and rings as seen in three spectral regions: UV, Visible, and near Infrared. Again, color assignments are somewhat arbitrary: The various cloud bands and decks on Saturn leap out when viewed in the Infrared by the Cassini spacecraft. The white dots in this image are warm clouds arranged in a pattern described as a "string of pearls". When Voyager looked at parts of the cloud surface in detail, some interesting fine structures were evident: Compare these with the next two images of saturnian atmospheric bands obtained during the Cassini mission (next page): Similar to Venus, Saturn has a well developed vortex of swirling clouds at each pole. Here is the one formed over the south polar region: These clouds reach different heights, producing a topographic-like effect: Cassini's sensors caught remarkable images of clouds above the North Pole. In the image below, note the general circular pattern, but the inner clouds have formed into a hexagon shape; this feature is more than 40000 km across and 200 km high. Why this distinct shape has developed is without any explanation. This next image pair compares the two polar cyclonic systems side by side (North is on the left): Like Jupiter, Saturn can have huge storms in its atmosphere. This next image shows what may be the biggest "hurricane" (8000 km; 5000 miles across) ever found in the Solar System, as it developed on November 7, 2006 in Saturn's southern hemsphere. The white cloud heights reach to about 60 km (40 miles); note the conspicuous central "eye": Voyager observations also indicated the possibility that, like Jupiter, there are auroras at each of Saturn's poles. This was confirmed in the '90s by the ultraviolet measuring capability of the Space Telescope Imaging Spectrometer (STIG) on the Hubble Space Telescope. In the STIG image below the auroral torus is in orange tones, representing atomic hydrogen emission as the solar wind interacts with the saturnian magnetic field. As might be expected, active atmospheres such as on Saturn can have severe electrical storms. This next image captures a cascade of lightning as large in area as the United States: Saturn has a strong, albeit conventional magnetosphere, as shown in this diagram: Saturn has 34 known satellites. Eighteen of these (all small) are located beyond the rings. The two diagrams below name these but their location on the charts is not really their actual positions in terms of distance: All but one (Iapetus), orbit close to the planet's equatorial plane. Seven of the interior moons are spherical, ranging in diameter from about 400 km to 5,100 km (249 mi3,169 mi). Their names, in orbital-position order, starting at the innermost, are Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. The remainder are smaller and have irregular shapes (although Phoebe is nearly spherical), and are mixtures of ice and water. Of the larger ones, six appear to consist almost entirely of water ice, but interior compositions are unknown. The seventh and largest, Titan, seems completely different, as we describe below. Most of the satellites have synchronous, prograde (orbiting in the direction of the planets rotation) orbits, locked by Saturn's gravity that causes each satellite's rotational period and revolution (about Saturn) to coincide. That means the same hemisphere always faces the inner planet (as the Moon faces the Earth). We will look at one or two representative images of each of the big seven, describing the satellite briefly. Mimas is only 392 km (244 mi) in diameter. Its surface holds innumerable small craters. Because of the 136 km (84.5 mi)-wide crater, Herschel (approaching a size where disruption might have occurred), with its cyclopean eye-like visage, it has gained a special mystique because of its resemblance to the Death Star ship in the Star Wars movie series. The larger (501 km [311 mi] diameter) Enceledus is the brightest object, in terms of albedo, in the Solar System. This brightness suggests regions of fresh or uncontaminated ice. This Voyager 1 image has been contrast-stretched to enhance major features which do not show well in the high albedo versions. When examined from the multiple images returned, five distinctive terrains are visible on this satellite: 1) Smooth Plains (craters sparse) 2) Cratered Plains (relatively few craters); 3) Cratered Terrain (more craters but still less than the other ice satellites, implying a rather young surface); 4) Ridged Plains (subparallel ridges); 5) Grooved Terrain (including graben-like, straight to curvilinear depressions). Some of these features are indicative of tectonic stressing of a brittle crust underlain by liquid, with the energy to fracture it coming from bulging in response to tidal interaction with Dione, which orbits in resonance with Enceladus. Several linear scarps are faults that show offset of lines crossed by them. Tethys is a heavily cratered satellite (diameter = 1,060 km [659 mi]), on which appears a very large (about 400 km [249 mi] diameter) impact structure called Odysseus. A close look at Odysseus indicates both its rim and central peak have diminished in height under viscous flow. Because of this, its depth to diameter ratio is lower than normal for craters of this size. A huge valley, 2,000 km (1,245 mi) long, 100 km (62 mi) wide, and up to 5 km (3 mi) deep, known as Ithaca Chasma, suggests that the ice cracked at some stage, perhaps when rifting occurred, as water converted to contracting ice that redistributed tension. It is hard to see in this next image but look for the long, broad "slash" in the upper left quadrant. The slightly larger (1,120 km [696 mi]) Dione has two dissimilar hemispheres: on one side, the leading hemisphere (because of synchronous locking, where the same side always faces the direction of the advancing orbit), the surface is heavily cratered and fractured, with a uniformly medium bright tone. The other side, the trailing hemisphere, is darker, has a much lower crater density, and contains broad, very bright streaks in a diffuse network. Planetologists believe this strange pattern is frost and ice, expelled or extruded from fissures, and possibly from ice volcanoes, in which the water is the "lava" in an otherwise frozen structure. Rhea (1,530 km [951 miles] in diameter) is very similar to Dione, in having a leading hemisphere that has crater densities comparable to the Moon and Mercury, as is hinted at in this full disk view, and a trailing hemisphere with few craters, streaks (ejecta rays?), and fractures. This is the heavily cratered side of Rhea: This same theme repeats at Iapetus (1,460 km [907 miles] in diameter), which is further from Saturn. The leading hemisphere has a very low albedo (0.03-0.06), in sharp contrast to previously described satellites, and with abrupt boundaries, against the trailing edge terrain (albedo about 0.5), which is heavily cratered. The reason for this dichotomy within this satellite is still uncertain. This trailing-edge material may have emerged from within Iapetus during crustal foundering. But, the dark terrain is consistent with silicates that have high carbon content, such as those that comprise carbonaceous chondrite meteorites. Thus, this part of Iapetus could have formed as ejecta, which came from another satellite or more likely from infall of a similar, asteroid-like body onto Iapetus. Typical of the 23 smaller satellites is Phoebe (200 km ]134 miles] diameter), orbiting nearly 4x as far out as Iapetus. The first look at this tiny moon was taken by Voyager; later, much better views were obtained by Cassini. Slightly larger (286 km; 179 miles) than Phoebe is the more angular Hyperion: As a preview of the extent of improvement that Cassini has provided for the saturnian satellites, compare the above view with this view of Hyperion made by Cassini: Now move to the next page which covers Titan and the still incoming views of Saturn and its satellites being obtained by the Cassini-Huygens mission. Primary Author: N. M. Short, Sr. Adapted from NASA's "Remote Sensing Tutorial."