Surface of the stars

Iapetus is perplexing. Even from Earth, it shows two faces. One side is five times brighter than the other. No one really knows why, and Voyager 1 will not see it well. But Voyager 1 has begun seeing some startling features on the inner moons.

surface of earth

Mimas, the innermost, looks the most like the bland snowballs everyone expected, ex¬cept for an enormous impact crater. Its walls are five kilometers deep, its diameter a third that of the moon. It is among the largest craters, relative to the size of the body hit, ever seen. Mimas came very close to being blown apart.

Like the other moons, Mimas is so cold that ice on its surface is as rigid as rock. “It’s got about all the craters you can make,” says Gene Shoemaker. “If you make any new craters on Mimas, you’ll erase old ones.”

surface of planets

This cratering has “fluffed up” or “gar¬dened” the surface to a depth of at least several kilometers. So walking on Mimas would be a little like walking on a large snow cone, with many ice chunks, some larger than a house, sticking up from the rubble.

Farther out and much larger is Tethys. A great branching trench 65 kilometers wide stretches nearly from one end of this well cratered, 1,050-kilometer-diameter moon to the other. The valley looks too grand to be an impact fracture. It appears to have been created from within. Perhaps the stress of Tethys’s freezing and expanding in¬terior cracked the surface of the moon. Per¬haps internal geologic churning caused the trench. Yet Tethys has a density close to pure water ice. How could a body with so lit¬tle rock have been geologically active?

Tethys is extremely bright, yet not as bright as its astounding neighbor Encela¬dus. Enceladus reflects nearly 100 percent of the light striking it. Our moon, by contrast, reflects 11 percent.
Enceladus also seems to be strikingly smooth. Voyager observes no craters. Could some geologic process on Enceladus still be actively erasing or swallowing its craters? Enceladus could well be geologi¬cally alive.

Strange white wispy markings streak the next two moons out, Dione and Rhea, sug¬gesting that something once blew out of their interiors.