Tethys is a cool little moon. With all the excitement (and water) generated from Enceladus, plus the mysteries of Titan, it doesn’t get much attention. It’s sort of the Jan Brady of the Saturnian system. Of the planet’s nine major moons, it’s third out from Saturn. It’s dead middle, fifth, in size of these major moons. It was also, with Dione, the 4th/5th to be discovered back in 1684.
The Cassini web page recently posted an image (above, left) showing the three main features of Tethys: the giant impact crater Odysseus (nearly on edge at upper left), the valley named Ithaca Chasma (lower right), and the dark band in the middle.
Here’s another image of the band, which also features some of the Ithaca Chasma at the bottom:
The Cassini site desribes it as follows:
Around the equator on its leading side, Tethys wears a band of slightly darker surface material. Cassini imaging scientists suspect that the darkened region may represent an area of less contaminated ice with differently sized grains than the material at higher latitudes on either side of the band.
Different-sized grains okay, but why? What if the expulsion of water from ice geysers is the reason? And what if the dark band is really an older surface? The brighter regions extending from the poles would be resurfaced with fine ice grains falling to the moon from those geysers. Not nearly as much as Enceladus, which is mostly smooth around its underground lake. But enough to spew particles into Saturn’s magnetic field, and enough to coat much of the moon in a blanket of purer ice.
Here’s an image showing the dark band running south of the impact basin Odysseus.
Tethys deserves a closer look, but unfortunately, there are no fly-by plans for Cassini through June 2010.