Icy Tethys: the Second-brightest Moon of Saturn

Tethys

Tethys seen 900,000 miles from Saturn

Credit: NASA/JPL/SSI

The Cassini spacecraft sent home images of Tethys starting in 2004, that made this mid-sized moon of Saturn an international celebrity. Odysseus, the largest crater on Tethys, bears striking resemblance the to the laser array on Darth Vader's malevolent Death Star station. (Another ice moon of Saturn, Mimas, looks even more convincing). The resemblance is coincidental, as high definition images were not obtained until after George Lucas and his team were already at work on the Star Wars film.




Italian astronomer Giovanni Cassini discovered Tethys in 1684, along with three other moons of Saturn: Iapetus, Rhea, and Dione. The Cassini spacecraft, named in his honor, sent back phenomenal images of Tethys, along with other detailed information. Prior to the Cassini flyby, our modern understanding of Tethys came primarily from three earlier space probe missions to the outer Solar System.

Tethys, seen by Cassini in 2005
Credit: NASA/JPL/SSI

On September 1, 1979,  Pioneer 11 made its closest approach to Tethys, at a distance of 330,000 km. Next, Voyager 1 flew past in 1980, sending back detailed images including that of a giant rift valley, the Ithaca Chasm.  The chasm reaches three-quarters of the way around Tethys, making it one of the longest rifts in the Solar System.  Tethys welcomed another visitor the following year when Voyager 2 flew past. The Voyager 2 spacecraft sent back images of Tethys from a distance of 594,000 km.  These images revealed the surface of Tethys as a contrasting terrain of densely cratered ancient surfaces among fresher, more recent plains. Tethys measures 1,060 km (660 mi) in diameter, orbiting deep within the magnetosphere of Saturn.

Voyager 2 image of Tethys 

Tethys is the least dense of all major moons in the Solar System. In simple terms, Tethys is not very heavy for its size. We now know from spectroscopic studies that water ice is the main constituent of the Tethysian surface. Water ice is generally less dense than other materials. That is why ice tends to float. Only a small portion of Tethys is rock, along with some as yet unidentified darker material. Tethys reflects back a high proportion of the light striking its surface. A rain of ice from the E ring of Saturn may explain why the surface of Tethys has such high albedo. Interestingly, the ice deposits of the E Ring may be a product of famous ice geysers from Enceladus.  Enceladus is even brighter than Tethys: the brightest moon in the Solar System.

Ice Moon Station will publish more on Tethys in future posts.  
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