10. Saturn: lord of the rings

The moons of Saturn

Titan - moon of mystery

Titan is the largest of Saturnís satellites, much larger than the planetís other moons. It is the second largest satellite in the solar system, and the only satellite possessing an extensive, dense atmosphere with a surface pressure comparable to that of the Earthís air.

Fig. .. 

Visible light cannot penetrate the veil of orange smog that coverís Titanís surface. In the satelliteís dry, cold atmosphere, the smog builds up to an impenetrable haze that extends to altitudes as high as 300 kilometers. On Earth, smog similarly forms by the action of sunlight on hydrocarbon molecules in the air. The urban smog usually forms within a kilometer of the Earthís surface. Titanís atmosphere extends far above its surface because of the high atmospheric pressure and the relatively low mass and gravitational pull of Titan.

Instruments aboard the Voyager spacecraft determined the composition of Titanís atmosphere. The dominant gas surrounding the satellite is molecular nitrogen, between 82 and 99 percent. Methane, the one gas identified with certainty before the Voyagers arrived, turned out to be a minor constituent, with an abundance of 1 to 6 percent. So, nitrogen molecules account for the bulk of Titanís atmosphere as they do on Earth Ė 77 percent of our air is molecular nitrogen.

Fig. .. 

High above Titanís surface, abundant nitrogen and methane molecules are being broken apart continuously by the Sunís energetic ultraviolet light and by the bombardment of electrons trapped in Saturnís magnetic environment. Some of the fragments then recombine to form more complex molecules that have been detected in small amounts by Voyagerís infrared spectrometers. In addition to methane, CH4, the list includes hydrocarbons like ethane, C2H6, acetylene, C2H2, and propane, C3H8, and nitrogen compounds such as hydrogen cyanide, HCN. Many of these molecules can join together in chainlike, polymer structures that contribute to Titanís dark, smoggy haze.

Fig. .. 

Although you cannot see beneath the smog-covered globe, Voyagerís radio signals have been used to infer the pressure and temperature down to the surface. The surface pressure is an ear popping 1.5 bars. That is one and half times the 1-bar air pressure at sea level on Earth, and equivalent to the pressure experienced by a deep-sea diver at about 6 meters under the oceanís surface.

Although liquid water cannot now lap the shores of Titan, it might contain shallow hydrocarbon seas. In fact, ethane or methane could play the role of water on Earth. The methane can condense in Titanís cold atmosphere to produce thick clouds that lie beneath the haze. Infrared observations that penetrate the smog suggest the presence of short-lived methane clouds in Titanís lower atmosphere, which form briefly and irregularly. Since the atmosphere is not fully saturated with methane, there cannot be extensive oceans of pure methane on the surface, but both ethane and methane can rain out of the atmosphere. They can exist as a liquid rather than a solid at the surface temperature of 94 degrees kelvin. Evaporation of the liquid seas can resupply the hydrocarbons to the atmosphere, completing the cycle.

Fig. .. 

We now know that Titan is not completely covered by a global hydrocarbon sea. Radar signals that penetrate the haze indicate that different regions of the surface reflect radar by varying amounts, so any liquid would have to be pooled in lakes or small seas rather than in a homogeneous covering. Titanís thick orange smog is also transparent enough at infrared wavelengths to allow mapping of the surface. Images obtained with the Hubble Space Telescope indicate an inhomogeneous landscape with bright and dark features that reflect infrared radiation by different amounts. They could be continents and oceans, but no one knows for sure. We might find out when the Cassini spacecraft arrives at Saturn in July 2004, and parachutes the Huygens Probe down to Titanís surface.

Saturnís medium-sized icy moons

Saturn has only one large satellite, Titan, comparable in size to Jupiterís four Galilean satellites, but it has an extensive family of smaller moons, including six mid-sized icy bodies that range from 196 to 765 kilometers in radius. The mean mass densities of these moons are low, between 1100 and 1400 kilograms per cubic meter, which suggests that they are mainly composed of pure water ice. This is consistent with their highly reflective surfaces. With the exception of Iapetus, they all reflect more than 50 percent of the incident sunlight, and one of them, Enceladus, reflects almost 100 percent of the sunlight that strikes it. Water ice was also identified by infrared spectroscopy in the years prior to the Voyager missions.

Fig. .. 

Saturnís innermost, medium-sized moon, Mimas, has a surface that is saturated with overlapping impact craters, including one crater that is about one-third the diameter of the moon itself. The impact that made this crater was nearly powerful enough to completely shatter Mimas.

Though of comparable size, Enceladus is a very different world from Mimas. Parts of the smooth, nearly crater-free surface of Enceladus have been coated with fresh icy material that rose from the warm interior of the satellite. Other parts of the surface contain cracks and grooves, suggesting that internal stresses may have discharged liquid water that froze into smooth ice. As the satellite moves around its eccentric orbit, produced by Dioneís gravitational tugs, tidal flexing by Saturn probably heats the interior of Enceladus, melting the water ice and permitting its eruption. Active ice volcanoes may be even now be erupting on Enceladus.

Tethys is about twice the size of Enceladus with nearly the same mean mass density, but Tethys is more akin to Mimas, with a large number of impact craters and one enormous impact that nearly broke the moon apart. A gigantic fracture covers three-fourths of the moonís circumference, suggesting internal activity early in its history. But the satellite shows no evidence for current activity.

Fig. .. 

Dione is nearly the same size as Tethys but denser, and shows a wide variety of surface features. Next to Enceladus among Saturnís moons, it has the most extensive evidence for an active interior. It has enough rocky material in its makeup to produce internal heat from natural radioactivity. Most of the surface is heavily cratered, but differences in the number of craters within various regions indicate that several periods of resurfacing occurred during the first billion years of Dioneís existence. Bright, wispy streaks, which stand out against an already-bright surface, are believed to be the result of internal heat and subsequent flows of erupting material.

Fig. .. 

The surface of Rhea is completely saturated with impact craters. It appears to be a dead world, geologically inert and without signs of internal heat. Yet, it is the largest of Saturnís icy moons. Compression resulting from its greater size and mass may have closed any volcanic vents, shutting off the outward flow of warm material.

Fig. .. 

Curiously, early astronomers could only observe the icy moon, Iapetus, on one side of Saturn. The satellite seemed to disappear when its orbit carried it to the other side of the planet. The reason for this strange behavior is that Iapetus is a divided world; half its surface is as bright as ice, and the other is as dark as asphalt or coal and is thought to contain complex organic compounds. Like the Earthís Moon, the satellite Iapetus keeps one side toward its planet, and as it revolves around Saturn the bright and dark parts are successively turned toward the Earth. When the dark half is pointed at the Earth, the moon becomes very difficult to observe.

Small, irregularly shaped satellites of Saturn

Instruments on the Voyager 1 and 2 spacecraft have discovered a host of small, irregularly shaped satellites that reside within the inner parts of Saturnís satellite system. They are all bright objects, probably composed of ice, and many of them have orbits that are remarkable in one way or another. Six of these tiny moons are associated with the rings: Pan, Atlas, Pandora, Prometheus, Janus and Epimetheus. Pan disturbs particles in the A ring to form the Encke division. Pandora and Prometheus shepherd the F ring; Atlas shepherds the outer margin of the A ring. Saturnís two co-orbital satellites, Janus and Epimetheus, are even more bizarre. Janus and Epimetheus move in almost identical orbits. The satellite on the inner orbit that is closest to Saturn moves slightly faster, overtaking the outer satellite every four years. But the bodiesí diameters are greater than the distance between their orbital paths, so they cannot pass without some fancy pirouetting. They avoid a collision at the last moment by gravitationally exchanging energy and switching orbits. The inner one is pulled by the outer one and raised into the outer orbit, and vice versa. They then move apart, only to repeat this pas-de-deux four years later, and exchange again. Three so-called Lagrangian satellites move along the orbits of Saturnís larger satellites Tethys and Dione.

Fig. .. 

This concludes our survey of Saturn, the most distant planet known to the ancient. We will now travel out beyond this enchanting world to the next wanderer, Uranus.

Fig. .. 

(Summary Diagram.)

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Copyright 2010, Professor Kenneth R. Lang, Tufts University