Updated and bumped:
Saturn's moon Enceladus is turning out to be full of surprises. The first surprise was that Enceladus is apparently the source for Saturn's E-ring. Microscopic particles impact Enceladus at a few kilometers per second, which kicks up a bunch more microscopic particles. These particles escape the tenuous gravity of Enceladus and form a donut of such particles in the approximate orbit of Enceladus. As it continues to orbit within this donut, it causes more particles to impact, which kicks up more dust, which keeps the ring supplied with particles.
Enceladus is odd in that it displays several different types of terrain. The moon is made of water ice and it is unique in being one of the brightest moons in the solar system. It reflects about as much light as white snow, but still has a very varied terrain:
Parts of the moon show craters no larger than 35 kilometers (about 22 miles) in diameter. Other areas show regions with no craters indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. All of this indicates that the interior of the moon may be liquid today, even though it should have been frozen eons ago. Enceladus' surface is believed to be geologically "young," possibly less than 100 million years old.
The big surprise, however, came a few days ago when Cassini did a closer flyby of the moon (it came within 109 miles of the surface). It looks like the reason that the terrain is so diverse is that it is being reshaped: Enceladus is host to active volcanoes of water. Something (possibly tidal forces) is warming the interior of the moon, which causes warm water to well up onto the surface. The water also evaporates and gives Enceladus a tenuous atmosphere composed mostly of water vapor. As the picture above shows, Cassini found that the south pole of Enceladus is much warmer than expected. No one knows yet exactly where all of the heat is coming from or why it is concentrated at the south pole.
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It's interesting how hospitable Saturn's moon system is. There is ample water, organic material and even minerals such as iron. Titan has all of the hydrocarbons you could ever need and about 1/7th of the gravity of Earth. It wouldn't be that hard to colonize and it has the very best scenery in the entire solar system.
Update: And now, only five years later, NASA thinks they have a model that explains many of the bizarre features of Enceladus: it's a lava lamp. According to this latest thinking, tidal stresses generate heat deep within Enceladus, down near its rocky core. This heat builds up until chunks of warm ice float upward (just like the wax in a lava lamp) and displace colder ice, which sinks back down. This keeps some parts of the surface "young." Interestingly, these episodes of warm ice drifting upward are about 10 million years long, while quiet periods – times when no ice is drifting up – last 100 million to 2 billion years. We were quite lucky to catch Enceladus at just the right time with the Cassini probe.
