Tethys (moon)
![]() Tethys imaged by theCassiniorbiter, April 2015 | |
Discovery | |
---|---|
Discovered by | G. D. Cassini |
Discovery date | 11 March 1684 |
Designations | |
Designation | Saturn III |
Pronunciation | /ˈtɛθəs/[1]or/ˈtiːθəs/[2] |
Named after | ΤηθύςTēthys |
Adjectives | Tethyan[3]/ˈtɛθiən,ˈtiː-/[1][2] |
Orbital characteristics | |
294619km | |
Eccentricity | 0.0001[4] |
1.887802d[5] | |
Averageorbital speed | 11.35 km/s |
Inclination | 1.12° (to Saturn's equator) |
Satellite of | Saturn |
Physical characteristics | |
Dimensions | 1076.8 × 1057.4 × 1052.6 km[6] |
531.1±0.6 km[6][7] | |
Mass | 6.1749×1020kg[7](1.03×10−4Earths) |
Meandensity | 0.9840±0.0033 g/cm3[7] |
0.146m/s2 [a] | |
0.394 km/s[b] | |
synchronous[8] | |
zero | |
Albedo | |
Temperature | 86±1 K[12] |
10.2[13] |
Tethys(/ˈtiːθɪs,ˈtɛθɪs/), orSaturn III,is the fifth-largestmoonofSaturn,measuring about 1,060 km (660 mi) across. It was discovered byGiovanni Domenico Cassiniin 1684, and is named after thetitanTethysofGreek mythology.
Tethys has a low density of 0.98 g/cm3,the lowest of all the major moons in thesolar system,indicating that it is made of water ice with just a small fraction of rock. This was confirmed by thespectroscopyof 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, being the second-brightest of themoons of SaturnafterEnceladus,and neutral in color.
Tethys is heavilycrateredand cut by a number of large faults and trench-likegraben.The largest impact crater,Odysseus,is about 400 km in diameter, whereas the largest graben,Ithaca Chasma,is about 100 km wide and more than 2,000 km long; the two surface features may be related. A small part of the surface is covered by smooth plains that may becryovolcanicin 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, includingPioneer 11(1979),Voyager 1(1980) andVoyager 2(1981), withCassini-Huygensobserving the moon the most, and in greatest detail, during its extensive mission to theSaturnian system(2004-2017).
Discovery and naming[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Giovanni_Cassini.jpg/170px-Giovanni_Cassini.jpg)
Tethys was discovered byGiovanni Domenico Cassiniin 1684 together withDione,another moon of Saturn. He had also discovered two moons,RheaandIapetusearlier, in 1671–72.[14]Cassini observed all of these moons using a largeaerial telescopehe set up on the grounds of theParis Observatory.[15]
Cassini named the four new moons asSidera Lodoicea( "the stars of Louis" ) to honour kingLouis XIV of France.[16]By the end of the seventeenth century, astronomers fell into the habit of referring to them andTitanasSaturn IthroughSaturn V(Tethys, Dione, Rhea, Titan, Iapetus).[14]OnceMimasandEnceladuswere discovered in 1789 byWilliam Herschel,the numbering scheme was extended toSaturn VIIby bumping the older five moons up two slots. The discovery ofHyperionin 1848 changed the numbers one last time, bumping Iapetus up toSaturn VIII.Henceforth, the numbering scheme would remain fixed.
![](https://upload.wikimedia.org/wikipedia/commons/thumb/b/be/John_F._Hershel.png/170px-John_F._Hershel.png)
The modern names of all seven satellites of Saturn come fromJohn Herschel(son ofWilliam Herschel,discoverer of Mimas and Enceladus).[14]In his 1847 publicationResults of Astronomical Observations made at the Cape of Good Hope,[17]he suggested the names of theTitans,sisters and brothers ofKronos(the Greek analogue of Saturn), be used. Tethys is named after the titanessTethys.[14]It is also designatedSaturn IIIorS III Tethys.
The nameTethyshas two customary pronunciations, with either a 'long' or a 'short'e:/ˈtiːθɪs/[18]or/ˈtɛθɪs/.[19]The conventional adjectival form of the name isTethyan,[20]again with either a long or a shorte.
Orbit[edit]
Tethys orbits Saturn at a distance of about 295,000 km (about 4.4 Saturn's radii) from the center of the planet. Itsorbital eccentricityis negligible, and itsorbital inclinationis about 1°. Tethys is locked in an inclinationresonancewithMimas;however, due to the low gravity of the respective bodies, this interaction does not cause any noticeable orbital eccentricity or tidal heating.[21]
The Tethyan orbit lies deep inside themagnetosphere 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.[22]
Trojans[edit]
Tethys has twoco-orbital moons,TelestoandCalypso,orbiting near Tethys'sLagrange pointsL4(60° ahead) andL5(60° behind) respectively.
Name | Diameter (km) | Semi-major axis (km) | Mass (kg) | Discovery date |
---|---|---|---|---|
Tethys | 1 062 | 294 619 | (6. 175 ± 0.000 146) × 1020 | 11 March 1684 |
Telesto | 24.6 ± 0.6 | 295 000 | ≈4 × 1015 | 8 April 1980 |
Calypso | 19 ± 0.8 | 295 000 | ≈2 × 1015 | 13 March 1980 |
Physical characteristics[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/3/32/Tethys_size_corrected.png/220px-Tethys_size_corrected.png)
Tethys is the16th-largest moonin theSolar System,with a radius of 531 km.[6]Its mass is about6.17×1020kg(0.000103 Earth mass),[7]which is less than 1% of theMoon.Despite its relatively small mass, it is more massive than all known moons smaller than itself combined.[23]
The density of Tethys is 0.98 g/cm3,indicating that it is composed almost entirely of water-ice.[24] It is also the fifth-largest of Saturn's moons. It is not known whether Tethys is differentiated into a rocky core and icemantle.However, if it is differentiated, the radius of the core does not exceed 145 km, and its mass is below 6% of the total mass. Due to the action of tidal and rotational forces, Tethys has the shape oftriaxial ellipsoid.The dimensions of this ellipsoid are consistent with it having a homogeneous interior.[24]The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely.[25]
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 byEnceladus's south polar geysers.[9]The radar albedo of the Tethyan surface is also very high.[26]The leading hemisphere of Tethys is brighter by 10–15% than the trailing one.[27]
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 thenear-infraredstrong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible.[27]No compound other than crystalline water ice has been unambiguously identified on Tethys.[28](Possible constituents includeorganics,ammoniaandcarbon dioxide.) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—IapetusandHyperion.The most probable candidate isnanophaseiron orhematite.[29]Measurements of thethermal emissionas well as radar observations by the Cassini spacecraft show that the icyregolithon the surface of Tethys is structurally complex[26]and has a largeporosityexceeding 95%.[30]
Surface features[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg/250px-PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg)
Color patterns[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/PIA19637-SaturnMoon-Tethys-RedArcs-Cassini-20150411.jpg/200px-PIA19637-SaturnMoon-Tethys-RedArcs-Cassini-20150411.jpg)
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.[31]The leading hemisphere also reddens slightly as theapexof the motion is approached, although without any noticeable darkening.[31]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 theE-ringonto 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 themagnetosphere of Saturn,which co-rotates with the planet.[32]
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.[33]The band is almost certainly caused by the influence of energetic electrons from the Saturnian magnetosphere with energies greater than about 1MeV.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.[34]Temperature maps of Tethys obtained byCassini,have shown this bluish region is cooler at midday than surrounding areas, giving the satellite a "Pac-man" -like appearance at mid-infrared wavelengths.[35]
Geology[edit]
The surface of Tethys mostly consists of hilly cratered terrain dominated by craters more than 40 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 aschasmataandtroughs.[36]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/0/01/PIA07738_Tethys_mosaic_contrast-enhanced.jpg/220px-PIA07738_Tethys_mosaic_contrast-enhanced.jpg)
The western part of the leading hemisphere of Tethys is dominated byOdysseus,a large impact basin whose 450 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, therimcrest of Odysseus is elevated by approximately 5 km above the mean satellite radius. The central complex of Odysseus features a central pit 2–4 km deep surrounded by massifs elevated by 6–9 km above the crater floor, which itself is about 3 km below the average radius.[36]
The second major feature seen on Tethys is a huge valley calledIthaca Chasma,about 100 km wide and 3 km deep. It is more than 2,000 km in length, approximately 3/4 of the way around Tethys's circumference.[36]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.[37]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Tethys_near_true.jpg/220px-Tethys_near_true.jpg)
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 2:3orbital resonancebetween Dione and Tethys early in the Solar System's history that led toorbital eccentricityandtidal heatingof Tethys's interior. The ocean would have frozen after the moons escaped from the resonance.[38]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.[36]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.[37]
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 theseismic wavesproduced 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.[36][39]
Impact craters and chronology[edit]
The majority of Tethyan impact craters are of a simple central peak type. Those more than 150 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.[40]
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 theSolar System formation4.56 billion years ago.[41]The youngest unit lies within Odysseus crater with an estimated age from 3.76 to 1.06 billion years, depending on the absolute chronology used.[41]Ithaca Chasma is older than Odysseus.[42]
Origin and evolution[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/PIA18350-SaturnRingsTethys-20150307.jpg/220px-PIA18350-SaturnRingsTethys-20150307.jpg)
Tethys is thought to have formed from anaccretion discor subnebula; a disc of gas and dust that existed around Saturn for some time after its formation.[43]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 likeammoniaandcarbon dioxidewere likely present as well, though their abundances are not well constrained.[44]
The extremely water-ice-rich composition of Tethys remains unexplained. The conditions in the Saturnian sub-nebula likely favored conversion of the molecularnitrogenandcarbon monoxideinto ammonia andmethane,respectively.[45]This can partially explain why Saturnian moons including Tethys contain more water ice than outer Solar System bodies likePlutoorTritonas the oxygen freed from carbon monoxide would react with the hydrogen forming water.[45]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.[46]
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 K at a depth of about 29 km.[47]After the end of formation due tothermal conduction,the subsurface layer cooled and the interior heated up.[48]The cooling near-surface layer contracted and the interior expanded. This caused strongextensional stressesin Tethys's crust reaching estimates of 5.7MPa,which likely led to cracking.[49]
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.[50]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.[21]Still, present knowledge of the evolution of Tethys is very limited.
Exploration[edit]
![](https://upload.wikimedia.org/wikipedia/commons/4/47/Tethys.gif)
Pioneer 11flew by Saturn in 1979, and its closest approach to Tethys was 329,197 km on 1 September 1979.[51]
One year later, on 12 November 1980,Voyager 1flew 415,670 km from Tethys.[52]Its twin spacecraft,Voyager 2,passed as close as 93,010 km from the moon on 26 August 1981.[53][54][12]Although both spacecraft took images of Tethys, the resolution ofVoyager 1's images did not exceed 15 km, and only those obtained byVoyager 2had a resolution as high as 2 km.[12]The first geological feature discovered in 1980 byVoyager 1was Ithaca Chasma.[52]Later in 1981Voyager 2revealed that it almost circled the moon running for 270°.Voyager 2also discovered the Odysseus crater.[12]Tethys was the Saturnian satellite most fully imaged by theVoyagers.[36]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/PIA18318-SaturnMoon-Tethys-20150411.jpg/150px-PIA18318-SaturnMoon-Tethys-20150411.jpg)
TheCassinispacecraft 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 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.[53][55][56]
Another flyby of Tethys took place on 14 August 2010 (during the solstice mission) at a distance of 38,300 km, when the fourth-largest crater on Tethys,Penelope,which is 207 km wide, was imaged.[57]More non-targeted flybys were planned for the solstice mission in 2011–2017.[58]
Cassini's observations allowed high-resolution maps of Tethys to be produced with the resolution of 0.29 km.[59]The spacecraft obtained spatially resolved near-infrared spectra of Tethys showing that its surface is made of water ice mixed with a dark material,[27]whereas the far-infrared observations constrained the bolometricbond albedo.[11]The radar observations at the wavelength of 2.2 cm showed that the ice regolith has a complex structure and is very porous.[26]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.[60]
Future missions to Tethys and the Saturn system are uncertain, but one possibility is theTitan Saturn System Mission.
Quadrangles[edit]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/72/Tethys_quadrangles_PIA10357.png/220px-Tethys_quadrangles_PIA10357.png)
Tethys is divided into 15quadrangles:
- North Polar Area
- Anticleia
- Odysseus
- Alcinous
- Telemachus
- Circe
- Polycaste
- Theoclymenus
- Penelope
- Salmoneus
- Ithaca Chasma
- Hermione
- Melanthius
- Antinous
- South Polar Area
Tethys in fiction[edit]
See also[edit]
Notes[edit]
- ^Surface gravity derived from the massm,thegravitational constantGand the radiusr:.
- ^Escape velocity derived from the massm,the gravitational constantGand the radiusr:√2Gm/r.
Citations[edit]
- ^ab"Tethys".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
- ^ab"Tethys".Merriam-Webster Dictionary.
- ^JPL (2009)Cassini Equinox Mission: Tethys
- ^Jacobson 2010 SAT339.
- ^Williams D. R. (22 February 2011)."Saturnian Satellite Fact Sheet".NASA. Archived fromthe originalon 12 July 2014.Retrieved16 September2014.
- ^abcRoatsch Jaumann et al. 2009,p. 765, Tables 24.1–2.
- ^abcdJacobson, Robert. A. (1 November 2022)."The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole*".The Astronomical Journal.164(5): 199.Bibcode:2022AJ....164..199J.doi:10.3847/1538-3881/ac90c9.S2CID252992162.
- ^Jaumann Clark et al. 2009,p. 659.
- ^abVerbiscer French et al. 2007.
- ^Jaumann Clark et al. 2009,p. 662, Table 20.4.
- ^abHowett Spencer et al. 2010,p. 581, Table 7.
- ^abcdStone & Miner 1982.
- ^Observatorio ARVAL.
- ^abcdVan Helden 1994.
- ^Price 2000,p. 279.
- ^Cassini 1686–1692.
- ^Lassell 1848.
- ^"Tethys".Merriam-Webster Dictionary.
"Tethys".Dictionary Unabridged(Online). n.d. - ^"Tethys".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
"Tethys".LexicoUK English Dictionary.Oxford University Press.Archived fromthe originalon 27 March 2020. - ^"Tethys".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
"Tethys".Merriam-Webster Dictionary. - ^abMatson Castillo-Rogez et al. 2009,pp. 604–05.
- ^Khurana Russell et al. 2008,pp. 466–67.
- ^Williams, Matt (23 October 2015)."Saturn's Moon Tethys".Universe Today.Retrieved14 November2023.
- ^abThomas Burns et al. 2007.
- ^Hussmann Sohl et al. 2006.
- ^abcOstro West et al. 2006.
- ^abcFilacchione Capaccioni et al. 2007.
- ^Jaumann Clark et al. 2009,pp. 651–654.
- ^Jaumann Clark et al. 2009,pp. 654–656.
- ^Carvano Migliorini et al. 2007.
- ^abSchenk Hamilton et al. 2011,pp. 740–44.
- ^Schenk Hamilton et al. 2011,pp. 750–53.
- ^Schenk Hamilton et al. 2011,pp. 745–46.
- ^Schenk Hamilton et al. 2011,pp. 751–53.
- ^"Cassini Finds a Video Gamers' Paradise at Saturn".NASA. 26 November 2012.Retrieved26 November2012.
- ^abcdefMoore Schenk et al. 2004,pp. 424–30.
- ^abJaumann Clark et al. 2009,pp. 645–46, 669.
- ^Chen & Nimmo 2008.
- ^Jaumann Clark et al. 2009,pp. 650–51.
- ^Jaumann Clark et al. 2009,p. 642.
- ^abDones Chapman et al. 2009,pp. 626–30.
- ^Giese et al. 2007.
- ^Johnson & Estrada 2009,pp. 59–60.
- ^Matson Castillo-Rogez et al. 2009,pp. 582–83.
- ^abJohnson & Estrada 2009,pp. 65–68.
- ^Canup 2010.
- ^Squyres Reynolds et al. 1988,p. 8788, Table 2.
- ^Squyres Reynolds et al. 1988,pp. 8791–92.
- ^Hillier & Squyres 1991.
- ^Matson Castillo-Rogez et al. 2009,p. 590.
- ^Muller, Pioneer 11 Full Mission Timeline.
- ^abStone & Miner 1981.
- ^abMuller, Missions to Tethys.
- ^Voyager Mission Description.
- ^Jaumann Clark et al. 2009,pp. 639–40, Table 20.2 at p. 641.
- ^Seal & Buffington 2009,pp. 725–26.
- ^Cook 2010.
- ^Cassini Solstice Mission.
- ^Roatsch Jaumann et al. 2009,p. 768.
- ^Khurana Russell et al. 2008,pp. 472–73.
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External links[edit]
![](https://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Tethys ProfileatNASA's Solar System Exploration Site
- Movie of Tethys's rotation by Calvin J. Hamilton (based onVoyagerimages)
- The Planetary Society: Tethys
- Cassiniimages of TethysArchived13 August 2011 at theWayback Machine
- Images of Tethys at JPL's Planetary Photojournal
- 3D shape model of Tethys(requires WebGL)
- Movie ofTethys' rotationfrom the National Oceanic and Atmospheric Administration
- TethysglobalArchived24 April 2013 at theWayback MachineandpolarArchived24 April 2013 at theWayback Machinebasemaps (August 2010) fromCassiniimages
- Tethys atlas (August 2008) fromCassiniimagesArchived26 February 2012 at theWayback Machine
- Tethys nomenclatureandTethys map with feature namesfrom theUSGS planetary nomenclature page
- Google Tethys 3D,interactive map of the moon