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Friday, September 28th, 2012

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Saturn's Coolest #7: Tethys
Third world out from the gas giant, Tethys is an ice ball, cratered but slightly smoothed from old ice melts. Like Mimas, it has an enormous crater - Odysseus - that, while it's more smoothed out and eroded than Herschel, is as big - proportionally - as Herschel is. Since Tethys is twice the size of Mimas, that makes Odysseus twice the size of Herschel.

To explain what I find so interesting about this impact on Tethys, go look at a map of Mars for a minute. In the southern hemisphere of Mars, there's a ginormous ancient impact basin - Hellas. Same relative size - Hellas is about as big compared to Mars as Odysseus is compared to Tethys. Now, if you turn Mars over and look opposite Hellas, you see the largest dry-land chasm in the Solar System: Valles Marineris (three thousand miles long). When the Hellas impact happened on Mars, part of its crust split, like the skin of an orange, opposite the impact.

The same thing happened on Tethys. Ithaca Chasma is the second-largest dry-land chasm in the Solar System: it's about half the length of Valles Marineris. Except it's on a moon with a diameter of less than seven hundred miles... which means the fourteen hundred mile long chasm goes three quarters of the way around the world. It's sixty miles wide and three miles deep, and pretty much splits the entire moon in half. When something hit Tethys and made Odysseus Crater, it hit hard.

Tethys also has resonances with several other moons in the Saturn system. For every orbit of Tethys, Mimas orbits exactly twice. (This isn't so special; Enceladus and Dione have the same relationship, and around Jupiter, Ganymede-Europa-Io are all bound together the same way.) But Tethys also has two Trojan companions - that is, rocks a couple miles in diameter that share the same orbit as Tethys, one sixty degrees ahead, one sixty degrees behind. This is all in addition to the relationships Mimas has with Pandora and with Saturn's rings.

The impact basin and chasm, and the complicated gravitational effects combine to put Tethys on my list.

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