Titan and Tethys

[n2doc]

First things first, Tethys didn't really get swallowed by Titan's atmosphere, it simply passed behind Saturn's largest moon from the perspective of the Cassini Equinox spacecraft currently in orbit around the ringed gas giant. Although they may look fairly close to one another, the distance between Titan and Tethys is twice the distance between Titan and Cassini. Tethys is 1.4 million miles (2.2 million km) from Cassini; Titan is 621,000 miles (1 million km) from Cassini.

Cassini captured the rare event of one moon passing behind another in the Saturnian system on Nov. 26, 2009. Tethys is much smaller than Titan, measuring only 660 miles (1062 km) across (whereas Titan is 3200 miles or 5150 km in diameter). An obvious difference in the two bodies isn't their size, however.

Titan has a very thick nitrogen-rich atmosphere, and the moon is one of only two bodies (the other being Earth) in the solar system thought to have stable masses of water. Tethys is a completely frozen moon, composed primarily of water ice and no measurable atmosphere.

And you might be wondering why Tethys bears a striking similarity to a certain classic sci-fi moon weapon, well that's because the icy moon sports a huge crater called Odysseus, measuring 277 miles (445 km) across.

Taken with Cassini's narrow-angle camera, this 18 minute passage of Tethys behind Titan will ultimately help planetary scientist refine the details of the orbits of Saturn's moons.
http://news.discovery.com/space/titan-tethys-saturn-cassini-equinox.html