Editing Tethys (moon) (section)

== Surface features ==
[[File:PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg|thumb|right|250px|Tethys viewed by ''[[Cassini-Huygens|Cassini]]'' (11 April 2015).]]

=== Color patterns ===
[[File:PIA19637-SaturnMoon-Tethys-RedArcs-Cassini-20150411.jpg|thumb|200px|left|Tethys – Red Arcs (11 April 2015)]]
The surface of Tethys has a number of large-scale features distinguished by their color and sometimes brightness. The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached. This darkening is responsible for the hemispheric albedo asymmetry mentioned above.{{sfn|Schenk Hamilton et al.|2011|pp=740–44}} The leading hemisphere also reddens slightly as the [[wiktionary:apex|apex]] of the motion is approached, although without any noticeable darkening.{{sfn|Schenk Hamilton et al.|2011|pp=740–44}} Such a bifurcated color pattern results in the existence of a bluish band between hemispheres following a great circle that runs through the poles. This coloration and darkening of the Tethyan surface is typical for Saturnian middle-sized satellites. Its origin may be related to a deposition of bright ice particles from the [[Rings of Saturn#E Ring|E-ring]] onto the leading hemispheres and dark particles coming from outer satellites on the trailing hemispheres. The darkening of the trailing hemispheres can also be caused by the impact of plasma from the [[magnetosphere of Saturn]], which co-rotates with the planet.{{sfn|Schenk Hamilton et al.|2011|pp=750–53}}

On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere. A comparable band exists only on Mimas.{{sfn|Schenk Hamilton et al.|2011|pp=745–46}} The band is almost certainly caused by the influence of energetic electrons from the Saturnian magnetosphere with energies greater than about 1&nbsp;[[MeV]]. These particles drift in the direction opposite to the rotation of the planet and preferentially impact areas on the leading hemisphere close to the equator.{{sfn|Schenk Hamilton et al.|2011|pp=751–53}}  Temperature maps of Tethys obtained by ''Cassini'' have shown this bluish region is cooler at midday than surrounding areas, giving the satellite a "Pac-man"-like appearance at mid-infrared wavelengths.<ref>{{cite web | url = http://solarsystem.nasa.gov/news/12745/cassini-finds-a-video-gamers-paradise-at-saturn/ | title = Cassini Finds a Video Gamers' Paradise at Saturn|date=26 November 2012|access-date=26 November 2012|publisher=NASA }}</ref>

=== 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}}

=== Impact craters and chronology ===

The majority of Tethyan impact craters are of a simple central peak type. Those more than 150&nbsp;km in diameter show more complex peak ring morphology. Only Odysseus crater has a central depression resembling a central pit. Older impact craters are somewhat shallower than young ones implying a degree of relaxation.{{sfn|Jaumann Clark et al.|2009|p=642}}

The density of impact craters varies across the surface of Tethys. The higher the crater density, the older the surface. This allows scientists to establish a relative chronology for Tethys. The cratered terrain is the oldest unit likely dating back to the [[Formation and evolution of the Solar System|Solar System formation]] 4.56&nbsp;billion years ago.{{sfn|Dones Chapman et al.|2009|pp=626–30}} The youngest unit lies within Odysseus crater with an estimated age from 3.76 to 1.06&nbsp;billion years, depending on the absolute chronology used.{{sfn|Dones Chapman et al.|2009|pp=626–30}} Ithaca Chasma is older than Odysseus.{{sfn|Giese|Wagner|Neukum|Helfenstein|2007}}