Tuesday, December 27, 2005
Moons
Some of last week's work from Cassini:
First, tiny Atlas caught with Saturn's rings nearly edge-on:
Here's a crescent Tethys. The sun is low on some cratered terrain, but also a valley. I think it's part of Ithaca Chasma. Surface features on this moon are named for characters from Homer.
This little beauty is Telesto, which shares an orbit with its larger sister Tethys.
Enceladus, the moon with ice geysers is caught from about 150,000 miles away. A few craters, but lots of grooves and smooth areas. The presumption is that ice melts in the interior of this small moon, and escapes through fountains at the south pole. Enceladus has a far weaker gravity than our own moon, so ice particles escape the body to form a faint ring around Saturn in Enceladus's neighborhood. We have the facts on this, but no solid theories why this happens on such a small moon. Jupiter's moon Europa, also fairly smooth and crater-free, is flexed internally by tides raised by its larger companions Io and Ganymede. The Earth has a warm liquid interior because of heat generated by radioactive elements. By conventional wisdom, 300-mile wide Enceladus should be a solid ice cube: nowhere near big enough to have radioactive heat, no moons nearby to raise slushy interior tides.
I like this shot of Mimas near the rings. They almost flew Pioneer 11 through that gray section of ring back in 1979. From Earth it appears dark and was named the Cassini division, for its seventeenth century discoverer.
A trip through the ring would be most hazardous to one's structural integrity.
