You can get a little more Rhea info here if you care to do so. This image was enhanced with filters gathering light from the infrared and ultraviolet, so is a little more colorful than it would be through normal human eyesight.
Here’s a close-in view from earlier in 2005:
It looks like the moon, but don’t be fooled (those of you with IR or UV vision probably weren’t). This is mostly all ice with just a dash of dust and dirt. By observing how much a moon will bend the path of a passing spacecraft, scientists can determine its mass. Figuring in a known diameter, it’s easy to arrive at a density.
Once you know the density, you can classify the body quickly. Earth, with a heavy core of iron and nickel comes out with a value of 5.5 grams per cc. The moon is about 3.3, so we deduce it has a significantly reduced iron core content. (In fact, the difference in densities was something of a clue as to the moon’s origin.)
Rhea is 1.33, putting it a bit above water (1.0). The deduction is that this celestial body (“sidus” in Latin) is ice laced with some impurities. It looks like the moon–mostly featureless gray–but from a distance dry dust and dirty ice look somewhat the same.
Some of Saturn’s moons have densities so low that ice cannot account for their lightweight quality. The moon Hyperion, for example, has a substantial portion of empty space. G. David Nordley’s excellent short story “Into the Miranda Rift” (available on 1994’s The Year’s Best Science Fiction: Eleventh Annual Collection) explores the notion of probing the interior of such bodies by future spelunkers. Unlike Han and Princess Leia, they probably won’t find monsters waiting to gorge themselves on the Millennium Falcon, but I imagine the delights of geological discovery might surpass what earthbound cave explorers enjoy today.