Editing Tethys (moon) (section)

=== Geology ===
The surface of Tethys mostly consists of hilly cratered terrain dominated by craters more than 40&nbsp;km in diameter. A smaller portion of the surface is represented by the smooth plains on the trailing hemisphere. There are also a number of tectonic features such as [[chasmata]] and [[trough (geology)|trough]]s.{{sfn|Moore Schenk et al.|2004|pp=424–30}}
[[File:PIA07738 Tethys mosaic contrast-enhanced.jpg|thumb|right|''[[Cassini–Huygens|Cassini]]'' view of Tethys's Saturn-facing hemisphere, showing the giant [[rift]] [[Ithaca Chasma]], crater Telemachus at top, and smooth plains at right]]
The western part of the leading hemisphere of Tethys is dominated by [[Odysseus (crater)|Odysseus]], a large impact basin whose 450&nbsp;km diameter is nearly 2/5 of that of Tethys itself. The crater is now quite flat – more precisely, its floor conforms to Tethys's spherical shape. This is most likely due to the viscous relaxation of the Tethyan icy crust over geologic time. Nevertheless, the [[rim (craters)|rim]] crest of Odysseus is elevated by approximately 5&nbsp;km above the mean satellite radius. The central complex of Odysseus features a central pit 2–4&nbsp;km deep surrounded by massifs elevated by 6–9&nbsp;km above the crater floor, which itself is about 3&nbsp;km below the average radius.{{sfn|Moore Schenk et al.|2004|pp=424–30}}

The second major feature seen on Tethys is a huge valley called [[Ithaca Chasma]], about 100&nbsp;km wide and 3&nbsp;km deep. It is more than 2,000&nbsp;km in length, approximately 3/4 of the way around Tethys's circumference.{{sfn|Moore Schenk et al.|2004|pp=424–30}} Ithaca Chasma occupies about 10% of the surface of Tethys. It is approximately concentric with Odysseus—a pole of Ithaca Chasma lies only approximately 20° from the crater.{{sfn|Jaumann Clark et al.|2009|pp=645–46, 669}}
[[File:Tethys near true.jpg|thumb|left|Huge, shallow [[Complex crater|crater]] [[Odysseus (crater)|Odysseus]], with its uplifted central complex, the Scheria Montes, is at the top of this image.]]
It is thought that Ithaca Chasma formed as Tethys's internal liquid water solidified, causing the moon to expand and cracking the surface to accommodate the extra volume within. The subsurface ocean may have resulted from a 2:3 [[orbital resonance]] between Dione and Tethys early in the Solar System's history that led to [[orbital eccentricity]] and [[Tidal acceleration#Tidal heating|tidal heating]] of Tethys's interior. The ocean would have frozen after the moons escaped from the resonance.{{sfn|Chen|Nimmo|2008}} There is another theory about the formation of Ithaca Chasma: when the impact that caused the great crater Odysseus occurred, the shock wave traveled through Tethys and fractured the icy, brittle surface. In this case Ithaca Chasma would be the outermost ring graben of Odysseus.{{sfn|Moore Schenk et al.|2004|pp=424–30}} However, age determination based on crater counts in high-resolution Cassini images showed that Ithaca Chasma is older than Odysseus making the impact hypothesis unlikely.{{sfn|Jaumann Clark et al.|2009|pp=645–46, 669}}

The smooth plains on the trailing hemisphere are approximately antipodal to Odysseus, although they extend about 60° to the northeast from the exact antipode. The plains have a relatively sharp boundary with the surrounding cratered terrain. The location of this unit near Odysseus's antipode argues for a connection between the crater and plains. The latter can be a result of focusing the [[seismic wave]]s produced by the impact in the center of the opposite hemisphere. However the smooth appearance of the plains together with their sharp boundaries (impact shaking would have produced a wide transitional zone) indicates that they formed by endogenic intrusion, possibly along the lines of weakness in the Tethyan lithosphere created by Odysseus impact.{{sfn|Moore Schenk et al.|2004|pp=424–30}}{{sfn|Jaumann Clark et al.|2009|pp=650–51}}