View source for Tethys (moon)

{{Short description|Moon of Saturn}}
{{Redirect|Saturn III|the 1980 film|Saturn 3|the rocket|Saturn C-3|titan in Greek mythology|Tethys (mythology)}}
{{Use dmy dates|date=August 2024}}
{{Infobox planet
| name = Tethys
| pronounced = {{IPAc-en|ˈ|t|ɛ|θ|ə|s}}<ref name=OED>{{OED|Tethys}}</ref> or {{IPAc-en|ˈ|t|iː|θ|ə|s}}<ref name=MW>{{MW|Tethys}}</ref>
| mpc_name = Saturn III
| adjectives = Tethyan<ref>[https://web.archive.org/web/20090327170130/http://saturn.jpl.nasa.gov:80/science/moons/tethys JPL (2009) ''Cassini Equinox Mission: Tethys'']</ref> {{IPAc-en|ˈ|t|ɛ|θ|i|ə|n|,_|'|t|iː|-}}<ref name=OED/><ref name=MW/>
| named_after = [[Tethys (mythology)|Τηθύς]] ''Tēthys''
| image = PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
| caption = Tethys imaged by the ''[[Cassini–Huygens|Cassini]]'' orbiter, April 2015
| discoverer = [[Giovanni Domenico Cassini|G. D. Cassini]]
| discovered = 11 March 1684
| semimajor = {{val|294619|u=km}} <!-- Computed using http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html μ value -->
| eccentricity = {{val|0.0001}}{{sfn|Jacobson 2010 SAT339}}
| period = {{val|1.887802|u=[[day|d]]}}<ref name=saturnian_sat_fact/>
| avg_speed = 11.35 km/s<!---{{efn|name=calculated}} - undefined note? --->
| inclination = 1.12° (to Saturn's equator)
| satellite_of = [[Saturn]]
| dimensions = 1076.8&thinsp;×&thinsp;1057.4&thinsp;×&thinsp;1052.6 km{{sfn|Roatsch Jaumann et al.|2009|p=765|loc=Tables 24.1–2}}
| mean_radius = {{val|531.1|0.6|u=km}}{{sfn|Roatsch Jaumann et al.|2009|p=765|loc=Tables 24.1–2}}<ref name="Jacobson2022"/>
| mass = {{val|6.1749|e=20|u=kg}}<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |pages=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref> (1.03{{e|-4}} Earths)
| density = {{val|0.9840|0.0033|u=g/cm<sup>3</sup>}}<ref name="Jacobson2022"/>
| surface_grav = {{Gr|0.6174|531.1|3}} [[Acceleration|m/s<sup>2</sup>]] {{efn|name=surface gravity}}
| escape_velocity = {{V2|0.6174|531.1|3}} km/s{{efn|name=escape velocity}}
| rotation = [[Synchronous rotation|synchronous]]{{sfn|Jaumann Clark et al.|2009|p=659}}
| axial_tilt = zero
| albedo = {{plainlist |
* {{val|1.229|0.005}} ([[geometric albedo|geometric]]){{sfn|Verbiscer French et al.|2007}}
* {{val|0.80|0.15}} ([[bond albedo|bond]]){{sfn|Jaumann Clark et al.|2009|p=662|loc=Table 20.4}}
* {{val|0.67|0.11}} (bolometric bond){{sfn|Howett Spencer et al.|2010|p=581|loc=Table 7}}
  }}
| magnitude = 10.2{{sfn|Observatorio ARVAL}}
| single_temperature = {{val|86|1|u=K}}{{sfn|Stone|Miner|1982}}
| note = no
}}

'''Tethys''' ({{IPAc-en|ˈ|t|iː|θ|ɪ|s|,_|ˈ|t|ɛ|θ|ɪ|s}}), or '''Saturn III''', is the fifth-largest [[natural satellite|moon]] of [[Saturn]], measuring about {{convert|1060|km|mi|abbr=on}} across. It was discovered by [[Giovanni Domenico Cassini]] in 1684, and is named after the [[Titans|titan]] [[Tethys (mythology)|Tethys]] of [[Greek mythology]].

Tethys has a low density of 0.98&nbsp;g/cm<sup>3</sup>, the lowest of all the major moons in the [[Solar System|solar system]], indicating that it is made of water ice with just a small fraction of rock. This was confirmed by the [[spectroscopy]] of its surface, which identified water ice as the dominant surface material. A further, smaller amount of an unidentified dark material is present as well. The surface of Tethys is very bright, the second-brightest of the [[moons of Saturn]] after [[Enceladus]], and neutral in color.

Tethys is heavily [[Crater|cratered]] and cut by a number of large faults and trench-like [[graben]]. The largest impact crater, [[Odysseus (crater)|Odysseus]], is about 400&nbsp;km in diameter, whereas the largest graben, [[Ithaca Chasma]], is about 100&nbsp;km wide and more than 2,000&nbsp;km long; the two surface features may be related. A small part of the surface is covered by smooth plains that may be [[cryovolcanic]] in origin. Like the other regular moons of Saturn, Tethys formed from the Saturnian sub-[[nebula]]—a disk of gas and dust that surrounded Saturn soon after its formation.

Tethys has been approached and observed by several space probes, including ''[[Pioneer 11]]'' (1979), ''[[Voyager 1]]'' (1980) and ''[[Voyager 2]]'' (1981), with ''[[Cassini–Huygens|Cassini-Huygens]]'' observing the moon the most, and in greatest detail, during its extensive mission to the [[Moons of Saturn|Saturnian system]] (2004-2017).

== Discovery and naming ==
[[File:Giovanni Cassini.jpg|left|thumb|upright|Giovanni Domenico Cassini, discoverer of Tethys]]
Tethys was discovered by [[Giovanni Domenico Cassini]] in 1684 together with [[Dione (moon)|Dione]], another moon of Saturn. He had also discovered two moons, [[Rhea (moon)|Rhea]] and [[Iapetus (moon)|Iapetus]] earlier, in 1671–72.{{sfn|Van Helden|1994}} Cassini observed all of these moons using a large [[aerial telescope]] he set up on the grounds of the [[Paris Observatory]].{{sfn|Price|2000|p=279}}

Cassini named the four new moons as ''[[Sidera Lodoicea]]'' ("the stars of Louis") to honour king [[Louis XIV of France]].{{sfn|Cassini|1686–1692}} By the end of the seventeenth century, astronomers fell into the habit of referring to them and [[Titan (moon)|Titan]] as ''Saturn&nbsp;I'' through ''Saturn&nbsp;V'' (Tethys, Dione, Rhea, Titan, Iapetus).{{sfn|Van Helden|1994}} Once [[Mimas (moon)|Mimas]] and [[Enceladus (moon)|Enceladus]] were discovered in 1789 by [[William Herschel]], the numbering scheme was extended to ''Saturn&nbsp;VII'' by bumping the older five moons up two slots. The discovery of [[Hyperion (moon)|Hyperion]] in 1848 changed the numbers one last time, bumping Iapetus up to ''Saturn&nbsp;VIII''. Henceforth, the numbering scheme would remain fixed.

[[File:John_F._Hershel.png|left|thumb|upright|[[John Herschel]], the astronomer who suggested that the moons of Saturn be named after the Titans and Giants]]
The modern names of all seven satellites of Saturn come from [[John Herschel]] (son of [[William Herschel]], discoverer of Mimas and Enceladus).{{sfn|Van Helden|1994}} In his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope'',{{sfn|Lassell|1848}} he suggested the names of the [[Titan (mythology)|Titans]], sisters and brothers of [[Cronus|Kronos]] (the Greek analogue of Saturn), be used. Tethys is named after the titaness [[Tethys (mythology)|Tethys]].{{sfn|Van Helden|1994}} It is also designated '''Saturn&nbsp;III''' or '''S III Tethys'''.

The name ''Tethys'' has two customary pronunciations, with either a 'long' or a 'short' ''e'': {{IPAc-en|'|t|iː|θ|ᵻ|s}}<ref>{{MW|Tethys}}<br>{{dict.com|Tethys}}</ref> or {{IPAc-en|'|t|ɛ|θ|ᵻ|s}}.<ref>{{OED|Tethys}}<br>{{Cite dictionary |url=http://www.lexico.com/definition/Tethys |archive-url=https://web.archive.org/web/20200327105314/https://www.lexico.com/definition/tethys |url-status=dead |archive-date=27 March 2020 |title=Tethys |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> The conventional adjectival form of the name is ''Tethyan'',<ref>{{OED|Tethys}}<br>{{MW|Tethys}}</ref> again with either a long or a short ''e''.

== Orbit ==
Tethys orbits Saturn at a distance of about 295,000&nbsp;km (about 4.4 Saturn's radii) from the center of the planet. Its [[orbital eccentricity]] is negligible, and its [[orbital inclination]] is about 1°. Tethys is locked in an inclination [[orbital resonance|resonance]] with [[Mimas (moon)|Mimas]]; however, due to the low gravity of the respective bodies, this interaction does not cause any noticeable orbital eccentricity or tidal heating.{{sfn|Matson Castillo-Rogez et al.|2009|pp=604–05}}

The Tethyan orbit lies deep inside the [[magnetosphere of Saturn]], so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.{{sfn|Khurana Russell et al.|2008|pp=466–67}}
=== Trojans ===
Tethys has two [[co-orbital configuration|co-orbital moons]], [[Telesto (moon)|Telesto]] and [[Calypso (moon)|Calypso]], orbiting near Tethys's [[Lagrange point]]s {{L4}} (60° ahead) and {{L5}} (60° behind) respectively.

{| class="wikitable"
|+Tethys trojans 
!Name
!Diameter (km)
!Semi-major axis (km)
!Mass (kg)
!Discovery date
|-
|Tethys
|1 062
| 294 619
|(6. 175 ± 0.000 146) × 10<sup>20</sup>
|11 March 1684
|-
|[[Telesto (moon)|Telesto]]
|24.6 ± 0.6 
|295 000
|≈4 × 10<sup>15</sup>
|8 April 1980
|-
|[[Calypso (moon)|Calypso]]
| 19 ± 0.8
|295 000
|≈2 × 10<sup>15</sup>
|13 March 1980
|}

== Physical characteristics ==
<!---
[[File:Map of Tethys PIA 14931 Jun 2012.jpg|thumb|left|300px|Tethys basemap (trailing hemisphere on the right)]]
--->
[[File:Tethys_size_corrected.png|left|thumb|Size comparison between Tethys(lower left), the [[Moon]] (upper left) and Earth]]
Tethys is the [[List of moons|16th-largest moon]] in the [[Solar System]], with a radius of 531&nbsp;km.{{sfn|Roatsch Jaumann et al.|2009|p=765|loc=Tables 24.1–2}} Its mass is about {{val|6.17|e=20|u=kg}}(0.000103 Earth mass),<ref name="Jacobson2022"/> which is less than 1% of the [[Moon]]. Despite its relatively small mass, it is more massive than all known moons smaller than itself combined.<ref>{{Cite web |last=Williams |first=Matt |date=23 October 2015 |title=Saturn's Moon Tethys |url=https://www.universetoday.com/15430/saturns-moon-tethys/ |access-date=14 November 2023 |website=Universe Today |language=en-US}}</ref>

The density of Tethys is 0.98&nbsp;g/cm<sup>3</sup>, indicating that it is composed almost entirely of water-ice.{{sfn|Thomas Burns et al.|2007}}
It is also the fifth-largest of Saturn's moons.
It is not known whether Tethys is differentiated into a rocky core and ice [[mantle (geology)|mantle]]. However, if it is differentiated, the radius of the core does not exceed 145&nbsp;km, and its mass is below 6% of the total mass. Due to the action of tidal and rotational forces, Tethys has the shape of [[triaxial ellipsoid]]. The dimensions of this ellipsoid are consistent with it having a homogeneous interior.{{sfn|Thomas Burns et al.|2007}} The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely.{{sfn|Hussmann Sohl et al.|2006}}

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System, with a visual albedo of 1.229. This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by [[Enceladus]]'s south polar geysers.{{sfn|Verbiscer French et al.|2007}} The radar albedo of the Tethyan surface is also very high.{{sfn|Ostro West et al.|2006}} The leading hemisphere of Tethys is brighter by 10–15% than the trailing one.{{sfn|Filacchione Capaccioni et al.|2007}}

The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of darker materials. The visible spectrum of Tethys is flat and featureless, whereas in the [[near-infrared]] strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0&nbsp;μm wavelengths are visible.{{sfn|Filacchione Capaccioni et al.|2007}} No compound other than crystalline water ice has been unambiguously identified on Tethys.{{sfn|Jaumann Clark et al.|2009|pp=651–654}} (Possible constituents include [[Organic compound|organics]], [[ammonia]] and [[carbon dioxide]].) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—[[Iapetus (moon)|Iapetus]] and [[Hyperion (moon)|Hyperion]]. The most probable candidate is [[nanophase]] iron or [[hematite]].{{sfn|Jaumann Clark et al.|2009|pp=654–656}} Measurements of the [[thermal emission]] as well as radar observations by the Cassini spacecraft show that the icy [[regolith]] on the surface of Tethys is structurally complex{{sfn|Ostro West et al.|2006}} and has a large [[porosity]] exceeding 95%.{{sfn|Carvano Migliorini et al.|2007}}

{{Annotated image 
|image        = Color map of Tethys PIA18439 Nov. 2014.jpg 
|image-width  = 600  
|width=600
| height      = 305 <!-- to crop the lower part of the image -->
| float       = center 
| annotations = <!-- this parameter must be there, empty or not! -->
| caption = {{center|Enhanced-color map (27.2 MB) showing reddening of trailing hemisphere (left) and bluish band on leading hemisphere}}}}
{{Annotated image 
| image       = Color Tethys polar maps PIA18439 Nov. 2014.jpg
| image-width = 600
| height      = 285 <!-- to crop the lower part of the image -->
| float       = center 
| annotations = <!-- this parameter must be there, empty or not! -->
| caption     = {{center|Enhanced-color maps<br/>northern and southern hemispheres}}
}}
{{Annotated image 
| image       = PIA18439-SaturnMoon-Tethys-20141104-fig2.jpg
| image-width = 600
| height      = 280 <!-- to crop the lower part of the image -->
| float       = center 
| annotations = <!-- this parameter must be there, empty or not! -->
| caption     = {{center|Enhanced-color maps<br/>trailing and leading hemispheres}}
}}

{{Gallery |align=center |width=180
|File:PIA19638-Tethys-SaturnMoon-TrailingHemisphere-Standard-20150411.jpg|{{center|Tethys—Trailing hemisphere—Standard processing<br/>(11 April 2015).}}
|File:PIA19638-Tethys-SaturnMoon-TrailingHemisphere-Enhanced-20150411.jpg|{{center|Tethys—Trailing hemisphere—Enhanced processing<br/>(11 April 2015).}}
|File:PIA19636-SaturnMoon-Tethys-Cassini-20150411.jpg|{{center|Tethys—Trailing hemisphere—Enhanced-color<br/>(11 April 2014)}}}}


== 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' 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}}

== Origin and evolution ==
[[File:PIA18350-SaturnRingsTethys-20150307.jpg|thumb|right|Tethys (lower right) near [[Saturn]] and its [[rings of Saturn|rings]]]]
{{multiple image |header=Tethys and<br/>[[Rings of Saturn|Saturn's rings]] |caption_align=center |align=left |width=150 |direction=vertical
|image1=PIA18355-SaturnMoon-Tethys-20151123.jpg
|caption1=Tethys and the rings of Saturn
|image2=PIA18283-SaturnRings-TethysHyperionPrometheus-20140714.jpg
|caption2=Tethys, [[Hyperion (moon)|Hyperion]] and [[Prometheus (moon)|Prometheus]]
|image3=PIA18353-SaturnRingsMoons-JanusTethys-20151027.jpg
|caption3=Tethys and [[Janus (moon)|Janus]]
}}
Tethys is thought to have formed from an [[accretion disc]] or subnebula; a disc of gas and dust that existed around Saturn for some time after its formation.{{sfn|Johnson|Estrada|2009|pp=59–60}} The low temperature at the position of Saturn in the Solar nebular means that water ice was the primary solid from which all moons formed. Other more volatile compounds like [[ammonia]] and [[carbon dioxide]] were likely present as well, though their abundances are not well constrained.{{sfn|Matson Castillo-Rogez et al.|2009|pp=582–83}}

The extremely water-ice-rich composition of Tethys remains unexplained. The conditions in the Saturnian sub-nebula likely favored conversion of the molecular [[nitrogen]] and [[carbon monoxide]] into ammonia and [[methane]], respectively.{{sfn|Johnson|Estrada|2009|pp=65–68}} This can partially explain why Saturnian moons including Tethys contain more water ice than outer Solar System bodies like [[Pluto]] or [[Triton (moon)|Triton]] as the oxygen freed from carbon monoxide would react with the hydrogen forming water.{{sfn|Johnson|Estrada|2009|pp=65–68}} One of the most interesting explanations proposed is that the rings and inner moons accreted from the tidally stripped ice-rich crust of a Titan-like moon before it was swallowed by Saturn.{{sfn|Canup|2010}}

The accretion process probably lasted for several thousand years before the moon was fully formed. Models suggest that impacts accompanying accretion caused heating of Tethys's outer layer, reaching a maximum temperature of around 155&nbsp;K at a depth of about 29&nbsp;km.{{sfn|Squyres Reynolds et al.|1988|p=8788|loc=Table 2}} After the end of formation due to [[thermal conduction]], the subsurface layer cooled and the interior heated up.{{sfn|Squyres Reynolds et al.|1988|pp=8791–92}} The cooling near-surface layer contracted and the interior expanded. This caused strong [[Stress (mechanics)|extensional stresses]] in Tethys's crust reaching estimates of 5.7 [[Pascal (unit)|MPa]], which likely led to cracking.{{sfn|Hillier|Squyres|1991}}

Because Tethys lacks substantial rock content, the heating by decay of radioactive elements is unlikely to have played a significant role in its further evolution.{{sfn|Matson Castillo-Rogez et al.|2009|p=590}} This also means that Tethys may have never experienced any significant melting unless its interior was heated by tides. They may have occurred, for instance, during the passage of Tethys through an orbital resonance with Dione or some other moon.{{sfn|Matson Castillo-Rogez et al.|2009|pp=604–05}} Still, present knowledge of the evolution of Tethys is very limited.

== Exploration ==
[[File:Tethys.gif|thumb|Animation of Tethys's rotation]]

''[[Pioneer 11]]'' flew by Saturn in 1979, and its closest approach to Tethys was 329,197&nbsp;km on 1 September 1979.{{sfn|Muller, Pioneer 11 Full Mission Timeline}}

One year later, on 12 November 1980, ''Voyager 1'' flew 415,670&nbsp;km from Tethys.{{sfn|Stone|Miner|1981}} Its twin spacecraft, ''Voyager 2'', passed as close as 93,010&nbsp;km from the moon on 26 August 1981.{{sfn|Muller, Missions to Tethys}}{{sfn|Voyager Mission Description}}{{sfn|Stone|Miner|1982}} Although both spacecraft took images of Tethys, the resolution of ''Voyager 1'''s images did not exceed 15&nbsp;km, and only those obtained by ''Voyager 2'' had a resolution as high as 2&nbsp;km.{{sfn|Stone|Miner|1982}} The first geological feature discovered in 1980 by ''Voyager 1'' was Ithaca Chasma.{{sfn|Stone|Miner|1981}} Later in 1981 ''Voyager 2'' revealed that it almost circled the moon running for 270°. ''Voyager 2'' also discovered the Odysseus crater.{{sfn|Stone|Miner|1982}} Tethys was the Saturnian satellite most fully imaged by the ''Voyagers''.{{sfn|Moore Schenk et al.|2004|pp=424–30}}
[[File:PIA18318-SaturnMoon-Tethys-20150411.jpg|thumb|left|150px|Tethys near Saturn (11 April 2015).]]
The ''[[Cassini-Huygens|Cassini]]'' spacecraft entered orbit around Saturn in 2004. During its primary mission from June 2004 through June 2008 it performed one very close targeted flyby of Tethys on 24 September 2005 at the distance of 1,503&nbsp;km. In addition to this flyby the spacecraft performed many non-targeted flybys during its primary and equinox missions since 2004, at distances of tens of thousands of kilometers.{{sfn|Muller, Missions to Tethys}}{{sfn|Jaumann Clark et al.|2009|pp=639–40|loc=Table 20.2 at p. 641}}{{sfn|Seal|Buffington|2009|pp=725–26}}

Another flyby of Tethys took place on 14 August 2010 (during the solstice mission) at a distance of 38,300&nbsp;km, when the fourth-largest crater on Tethys, [[Penelope (crater)|Penelope]], which is 207&nbsp;km wide, was imaged.{{sfn|Cook|2010}} More non-targeted flybys were planned for the solstice mission in 2011–2017.{{sfn|Cassini Solstice Mission}}

''Cassini''{{'}}s observations allowed high-resolution maps of Tethys to be produced with the resolution of 0.29&nbsp;km.{{sfn|Roatsch Jaumann et al.|2009|p=768}} The spacecraft obtained spatially resolved near-infrared spectra of Tethys showing that its surface is made of water ice mixed with a dark material,{{sfn|Filacchione Capaccioni et al.|2007}} whereas the far-infrared observations constrained the bolometric [[bond albedo]].{{sfn|Howett Spencer et al.|2010|p=581|loc=Table 7}} The radar observations at the wavelength of 2.2&nbsp;cm showed that the ice regolith has a complex structure and is very porous.{{sfn|Ostro West et al.|2006}} The observations of plasma in the vicinity of Tethys demonstrated that it is a geologically dead body producing no new plasma in the Saturnian magnetosphere.{{sfn|Khurana Russell et al.|2008|pp=472–73}}

Future missions to Tethys and the Saturn system are uncertain, but one possibility is the [[Titan Saturn System Mission]].

== Quadrangles ==
[[File:Tethys quadrangles PIA10357.png|thumb|The quadrangles of Tethys]]
Tethys is divided into 15 [[Quadrangle (geography)|quadrangles]]:
{{div col}}
# North Polar Area
# Anticleia
# Odysseus
# Alcinous
# Telemachus
# Circe
# Polycaste
# Theoclymenus
# Penelope
# Salmoneus
# Ithaca Chasma
# Hermione
# Melanthius
# Antinous
# South Polar Area
{{Div col end}}

== Tethys in fiction ==
{{Further|Saturn's moons in fiction}}

== See also ==
* [[List of former planets]]
* [[List of geological features on Tethys]]

== Notes ==

{{Notelist
| notes =

{{efn
| name = surface gravity
| Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'' : <math>Gm/r^2</math>.
}}

{{efn
| name = escape velocity
| Escape velocity derived from the mass ''m'', the gravitational constant ''G'' and the radius ''r'' : {{math|{{radical|2''Gm''/''r''}}}}.
}}

<!--  {{efn
| name = dict-def
| {{respell|TEE|thəs}} or {{respell|TETH|əs}}-->
}}

<!--unused{{efn
| name = moon masses
| The masses of smaller spherical moons are (in kg): [[Enceladus (moon)|Enceladus]]—1.1{{e|20}}, [[Miranda (moon)|Miranda]]—0.6{{e|20}}, [[Proteus (moon)|Proteus]]—0.5{{e|20}}, [[Mimas (moon)|Mimas]]—0.4{{e|20}}, [[Hi'iaka (moon)|Hi'iaka]]—0.179{{e|20}}. The total mass of remaining moons is about 0.9{{e|20}}. So, the total mass of all moons smaller than Tethys is about 3.679{{e|20}}. Note that [[Dysnomia (moon)|Dysnomia]] and [[Vanth (moon)|Vanth]] do not have good mass estimates at this time and may result in the smaller satellite total exceeding Tethys's mass (See [[List of moons by diameter]])
}}-->

== Citations ==

{{Reflist
| colwidth = 30em
| refs =

<ref name=saturnian_sat_fact>{{cite web
 |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html
 |title=Saturnian Satellite Fact Sheet
 |author=Williams D. R.
 |publisher=NASA
 |date=22 February 2011
|access-date=16 September 2014
|archive-url=https://web.archive.org/web/20140712071124/http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html
 |archive-date=12 July 2014
}}</ref>

}}

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{{Refend}}

== External links ==

{{Spoken Wikipedia|Tethys (moon).ogg|date=14 January 2010}}
{{Commons category|Tethys}}
* [https://web.archive.org/web/20070609142406/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Tethys Tethys Profile] at [http://solarsystem.nasa.gov/ NASA's Solar System Exploration Site]
* [http://www.solarviews.com/cap/sat/vtethys1.htm Movie of Tethys's rotation by Calvin J. Hamilton (based on ''Voyager'' images)]
* [https://web.archive.org/web/20060927044313/http://www.planetary.org/explore/topics/saturn/tethys.html The Planetary Society: Tethys]
* [http://ciclops.org/search.php?x=20&y=7&search=Tethys ''Cassini'' images of Tethys] {{Webarchive|url=https://web.archive.org/web/20110813151249/http://ciclops.org/search.php?x=20&y=7&search=Tethys |date=13 August 2011 }}
* [http://photojournal.jpl.nasa.gov/target/Tethys Images of Tethys at JPL's Planetary Photojournal]
* [http://space.frieger.com/asteroids/asteroids.php?id=S3 3D shape model of Tethys] (requires WebGL)
* Movie of [https://web.archive.org/web/20100601190100/http://sos.noaa.gov/videos/Tethys.mov Tethys' rotation] from the National Oceanic and Atmospheric Administration
* Tethys [http://www.ciclops.org/view/6533/Map_of_Tethys_-_August_2010 global] {{Webarchive|url=https://web.archive.org/web/20130424042753/http://www.ciclops.org/view/6533/Map_of_Tethys_-_August_2010 |date=24 April 2013 }} and [http://www.ciclops.org/view/6534/Tethys_Polar_Maps_-_August_2010 polar] {{Webarchive|url=https://web.archive.org/web/20130424051858/http://www.ciclops.org/view/6534/Tethys_Polar_Maps_-_August_2010 |date=24 April 2013 }} basemaps (August 2010) from ''Cassini'' images
* [http://ciclops.org/view.php?id=5074 Tethys atlas (August 2008) from ''Cassini'' images] {{Webarchive|url=https://web.archive.org/web/20120226150137/http://ciclops.org/view.php?id=5074 |date=26 February 2012 }}
* [http://planetarynames.wr.usgs.gov/Page/TETHYS/target Tethys nomenclature] and [http://planetarynames.wr.usgs.gov/images/tethys_comp.pdf Tethys map with feature names] from the [http://planetarynames.wr.usgs.gov/ USGS planetary nomenclature page]
* [https://www.google.com/maps/space/tethys/@3.3425001,163.2130808,11340642m/data=!3m1!1e3 Google Tethys 3D], interactive map of the moon

{{Tethys|state=uncollapsed}}
{{Moons of Saturn}}
{{Solar System moons (compact)}}
{{Saturn}}
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[[Category:Tethys (moon)| ]]
[[Category:Astronomical objects discovered in 1684|16840321]]
[[Category:Discoveries by Giovanni Domenico Cassini]]
[[Category:Moons with a prograde orbit]]