12.4: Saturn's Moons

The Saturn System

Saturn has at least 82 known moons in addition to a magnificent set of rings. The largest of the moons, Titan, is almost as big as Ganymede in Jupiter’s system, and it is the only moon with a substantial atmosphere and lakes or seas of liquid hydrocarbons (such as methane and ethane) on the surface. Saturn has six other large regular moons with diameters between 400 and 1600 kilometers, a collection of small moons orbiting in or near the rings, and many captured strays similar to those of Jupiter. Mysteriously, one of Saturn’s smaller moons, Enceladus, has active geysers of water being expelled into space.

The rings of Saturn, one of the most impressive sights in the solar system, are broad and flat, with a few major and many minor gaps. They are not solid, but rather a huge collection of icy fragments, all orbiting the equator of Saturn in a traffic pattern that makes rush hour in a big city look simple by comparison. Individual ring particles are composed primarily of water ice and are typically the size of ping-pong balls, tennis balls, and basketballs.

Saturn has 62 known moons (as of August 2014); a large number like Jupiter. Titan is the second largest moon in the solar system (to Jupiter’s Ganymede) and is also larger than the planet Mercury. Christian Huygens discovered Titan in 1655. Most of Saturn’s moons are very small; many are smaller than 50 kilometers (30 miles) across. As noted when discussing Saturn’s ring, there are several Shepard moons and Gap moons.

 

Saturn Satellites of Distinction

Brief Description

Enceladus

• Mimas
  • Prometheus
    • Titan
      • Thick atmosphere of mostly Nitrogen, liquid methane on the surface

Enceladus via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Geysers erupting from Enceladus, there are 101 geysers in what is called the Enceladus Geyser Basin via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Mimas via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Prometheus via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

F ring (left), Prometheus (center oblong), and Saturn’s rings to the right – note the F ring twists via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Titan and its thick atmosphere via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Surface of Titan via NASA Huygens SpacecraftPublic Domain | Image courtesy of NASA.

Titan

Titan, first seen in 1655 by the Dutch astronomer Christiaan Huygens, was the first moon discovered after Galileo saw the four large moons of Jupiter. Titan has roughly the same diameter, mass, and density as Callisto or Ganymede. Presumably it also has a similar composition—about half ice and half rock. However, Titan is unique among moons, with a thick atmosphere and lakes and rivers and falling rain (although these are not composed of water but of hydrocarbons such as ethane and methane, which can stay liquid at the frigid temperatures on Titan). Titan is the only moon or planet other than Earth where we have found evidence of bodies of surface liquids.

The 1980 Voyager flyby of Titan determined that the surface density of its atmosphere is four times greater than that on Earth. The atmospheric pressure on this moon is 1.6 bars, higher than that on any other moon and, remarkably, even higher than that of the terrestrial planets Mars and Earth. The atmospheric composition is primarily nitrogen, an important way in which Titan’s atmosphere resembles Earth’s.

Also detected in Titan’s atmosphere were carbon monoxide (CO), hydrocarbons (compounds of hydrogen and carbon) such as methane (CH₄), ethane (C₂H₆), and propane (C₃H₈), and nitrogen compounds such as hydrogen cyanide (HCN), cyanogen (C₂N₂), and cyanoacetylene (HC₃N). Their presence indicates an active chemistry in which sunlight interacts with atmospheric nitrogen and methane to create a rich mix of organic molecules. There are also multiple layers of hydrocarbon haze and clouds in the atmosphere, as illustrated in Figure 12.12.

Figure 12.12 : Structure of Titan’s Atmosphere. Some characteristics of Titan’s atmosphere resemble those of Earth’s atmosphere, although it is much colder than our planet. The red line indicates the temperature of Titan’s atmosphere at different altitudes.

These Voyager discoveries motivated a much more ambitious exploration program using the NASA Cassini Saturn orbiter and a probe to land on Titan called Huygens, built by the European Space Agency. The orbiter, which included several cameras, spectrometers, and a radar imaging system, made dozens of close flybys of Titan between 2004 and 2015, each yielding radar and infrared images of portions of the surface (see Exploring the Outer Planets). The Huygens probe successfully descended by parachute through the atmosphere, photographing the surface from below the clouds, and landing on January 14, 2005. This was the first (and so far the only) spacecraft landing on a moon in the outer solar system.

At the end of its parachute descent, the 319-kilogram Huygens probe safely touched down, slid a short distance, and began sending data back to Earth, including photos and analyses of the atmosphere. It appeared to have landed on a flat, boulder-strewn plain, but both the surface and the boulders were composed of water ice, which is as hard as rock at the temperature of Titan (see Figure 12.13).

The photos taken during descent showed a variety of features, including drainage channels, suggesting that Huygens had landed on the shore of an ancient hydrocarbon lake. The sky was deep orange, and the brightness of the Sun was a thousand times less than sunlight on Earth (but still more than a hundred times brighter than under the full moon on Earth). Titan’s surface temperature was 94 K (−179 °C). The warmer spacecraft heated enough of the ice where it landed for its instruments to measure released hydrocarbon gas. Measurements on the surface continued for more than an hour before the probe succumbed to the frigid temperature.

Figure 12.13 : Views of the Surface of Titan. The left image shows the views of Titan from the descent camera, in a flattened projection, at different altitudes. The right image, taken after landing, shows a boulder-strewn surface illuminated by faint reddish sunlight. The boulders are composed of water ice. (credit left: modification of work by ESA/NASA/JPL/University of Arizona; credit right: modification of work by ESA/NASA/JPL/University of Arizona; processed by Andrey Pivovarov)

Radar and infrared imaging of Titan from the Cassini orbiter gradually built up a picture of a remarkably active surface on this moon, complex and geologically young (Figure 12.14). There are large methane lakes near the polar regions that interact with the methane in the atmosphere, much as Earth’s water oceans interact with the water vapor in our atmosphere. The presence of many erosional features indicates that atmospheric methane can condense and fall as rain, then flow down valleys to the big lakes. Thus, Titan has a low-temperature equivalent of the water cycle on Earth, with liquid on the surface that evaporates, forms clouds, and then condenses to fall as rain—but on Titan the liquid is a combination of methane, ethane, and a trace of other hydrocarbons. It is a weirdly familiar and yet utterly alien landscape.

Figure 12.14 : Titan’s Lakes. (a) This Cassini image from a September 2006 flyby shows the liquid lakes on Titan. Their composition is most likely a combination of methane and ethane. (Since this is a radar image, the colors are artificially added. The dark blue areas are the smooth surfaces of the liquid lakes, and yellow is the rougher solid terrain around them.) (b) This mosaic of Titan’s surface from the Cassini-Huygens mission shows in detail a high ridge area and many narrow, sinuous erosion channels that appear to be part of a widespread network of “rivers” carved by flowing hydrocarbons. (credit a: modification of work by NASA/JPL-Caltech/USGS; credit b; modification of work by NASA/JPL/ESA/University of Arizona)

These discoveries raise the question of whether there could be life on Titan. Hydrocarbons are fundamental for the formation of the large carbon molecules that are essential to life on our planet. However, the temperature on Titan is far too low for liquid water or for many of the chemical processes that are essential to life as we know it. There remains, though, an intriguing possibility that Titan might have developed a different form of low-temperature carbon-based life that could operate with liquid hydrocarbons playing the role of water. The discovery of such “life as we don’t know it” could be even more exciting than finding life like ours on Mars. If such a truly alien life is present on Titan, its existence would greatly expand our understanding of the nature of life and of habitable environments.

NASA has selected a new mission to Titan for launch in 2027. Called Dragonfly, this mission is a drone that will fly in Titan’s atmosphere, with emphasis on study of pre-biotic chemistry. Other future proposed missions include a balloon operating in the atmosphere and a “boat” floating in one of the Titan lakes.

The Cassini mission scientists and the visual presentation specialists at NASA’s Jet Propulsion Laboratory have put together some nice films from the images taken by Cassini and Huygens. See, for example, the Titan approach and the flyover of the Northern lakes district.

Enceladus

Saturn's Rings

Saturn’s rings are made up of particles that range from dust-size to meters in diameter. The rings are mostly water and ice in composition, with some rocky silicate materials. Galileo Galilei first observed them in 1610; he could not discern them as rings and referred to them as the ears of Saturn . In 1655, astronomer Christian Huygens hypothesized that Saturn had a ring, based on his telescopic observations. Giovanni Cassini suggested in 1655 that Saturn’s ring was actually made up of numerous smaller rings with gaps or divisions between these smaller rings. Saturn exhibits seven major rings with multiple divisions and gaps in the rings. The outer F ring is twisted and knotted like a piece of taffy; this is due to interactions of the F ring with Saturn’s moon Prometheus. There are small moons, called shepherd or sheepherder moons, which orbit near the outer edges of a ring or within gaps in the rings.

These sheepherder moons maintain a sharply defined edge to the ring. Radial features in the B ring, called spokes, were suggested by Steve O’Meara (Earth observations) and confirmed by spacecraft. These spokes, seasonal in nature, appear to be caused by electrostatic forces. A quite-distant ring of dust was discovered in 2009, called the Phoebe ring, and the Saturnian moon Phoebe that is not visible in Earth-based telescopes.

Saturn’s rings via NASA Hubble Space TelescopePublic Domain | Image courtesy of NASA.

Saturn’s rings, with Saturn blocking the Sun via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Saturn’s F ring, with the moon Prometheus pulling material from the ring via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Spokes in Saturn’s B ring via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Shepherd Moons

Attributions

https://phys.libretexts.org/Bookshel..._Saturns_Rings

https://phys.libretexts.org/Bookshel...rns_Satellites

https://phys.libretexts.org/Bookshel...ems_Introduced

https://phys.libretexts.org/Bookshel...tan_and_Triton