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Tethys (moon)

Tethys (moon)

Overview
Tethys or Saturn III is a mid-sized moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 about 1060 km (658.7 mi) across. It was discovered by G. D. Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

 in 1684 and is named after titan Tethys
Tethys (mythology)
In Greek mythology, Tethys , daughter of Uranus and Gaia was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus...

 of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. Tethys is pronounced (icon or ˈ;In US dictionary transcription, . and in Greek it is Τηθύς.

Tethys has a low density of 0.98 g/cm3 indicating that it is made of water ice with just a small fraction of rock.
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Encyclopedia
Tethys or Saturn III is a mid-sized moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 about 1060 km (658.7 mi) across. It was discovered by G. D. Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

 in 1684 and is named after titan Tethys
Tethys (mythology)
In Greek mythology, Tethys , daughter of Uranus and Gaia was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus...

 of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. Tethys is pronounced (icon or ˈ;In US dictionary transcription, . and in Greek it is Τηθύς.

Tethys has a low density of 0.98 g/cm3 indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A small amount of an unidentified dark material is present as well. The surface of Tethys is very bright, being second brightest among the moons of Saturn after Enceladus, and neutral in color.

Tethys is heavily cratered and cut by a number large scale faults/graben. The largest impact crater—Odysseus
Odysseus (crater)
Odysseus is the largest crater on Saturn's moon Tethys. It is 445 km across, more than 2/5 of the moon's diameter, and is one of the larger craters in the Solar System. It is situated in the western part of leading hemisphere of the moon—the latitude and longitude of its center are 32.8°N and...

 is about 400 km in diameter, while the largest graben—Ithaca Chasma
Ithaca Chasma
Ithaca Chasma is a valley on Saturn's moon Tethys, named after the island of Ithaca, in Greece. It is on average 100 km wide, 3 to 5 km deep and 2,000 km long, running approximately three-quarters of the way around Tethys' circumference, making it one of the longer valleys in the...

 is about 100 km wide and more 2000 km long. These two largest surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Like all 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 by several space probes including Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...

 (1979), Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

 (1980), Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 (1981), and Cassini
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

 since 2004.

Discovery and naming


Tethys was discovered by Giovanni Domenico Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

 in 1684 together with Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

, another moon of Saturn. He had also discovered two moons, Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

 and Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

 earlier, in 1671–72. Cassini observed all these moons using a large aerial telescope
Aerial telescope
An aerial telescope is a type of very-long-focal-length refracting telescope built in the second half of the 17th century that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held...

 he set up on the grounds of the Paris Observatory
Paris Observatory
The Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world...

.

Cassini named the four new moons as Sidera Lodoicea
Sidera Lodoicea
Sidera Lodoicea is the name given by the astronomer Giovanni Domenico Cassini to the four moons of Saturn discovered by him in the years 1671, 1672, and 1684 and published in his Découverte de deux nouvelles planètes autour de Saturne in 1673 and in the Journal des sçavans in 1686...

("the stars of Louis") to honour king Louis XIV of France
Louis XIV of France
Louis XIV , known as Louis the Great or the Sun King , was a Bourbon monarch who ruled as King of France and Navarre. His reign, from 1643 to his death in 1715, began at the age of four and lasted seventy-two years, three months, and eighteen days...

. By the end of the seventeenth century, astronomers fell into the habit of referring to them and Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 as Saturn I through Saturn V (Tethys, Dione, Rhea, Titan, Iapetus). Once Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

 and Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 were discovered in 1789, the numbering scheme was extended to Saturn VII by bumping the older five moons up two slots. The discovery of Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...

 in 1848 changed the numbers one last time, bumping Iapetus up to Saturn VIII. Henceforth, the numbering scheme would remain fixed.

The modern names of all seven satellites of Saturn come from John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 (son of William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

, discoverer of Mimas and Enceladus). In his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, he suggested the names of the Titans
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

, sisters and brothers of Kronos
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

 (the Greek analogue of Saturn), be used.

Tethys is named after the titan Tethys
Tethys (mythology)
In Greek mythology, Tethys , daughter of Uranus and Gaia was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus...

 of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. It is also designated Saturn III or S III Tethys. The correct adjectival form of the moon's name is Tethyan, although other forms are also used.

Orbit


Tethys orbits Saturn at a distance of about 295000 km (about 4.4 Saturn's radii) from the center of the planet. The orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 is negligible, while the orbital inclination is about 1 degree. The moon is locked in an inclination resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

, which, however, does not cause any noticeable orbital eccentricity and tidal heating.

The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemipshere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

The co-orbital moons Telesto
Telesto (moon)
Telesto is a moon of Saturn. It was discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations, and was provisionally designated '. In the following months, several other apparitions were observed: , , and ....

 and Calypso
Calypso (moon)
Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated ' . Several other apparitions of it were recorded in the following months: , , , and...

 are located within Tethys' Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

s and , 60 degrees ahead and behind Tethys in its orbit respectively.

Physical characteristics


At 1066 km in diameter, Tethys is the 16th largest moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, and is more massive than all known moons smaller than itself combined.The masses of smaller spherical moons are (in kg): Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

—1.1, Miranda
Miranda (moon)
-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...

—0.6, Proteus
Proteus (moon)
Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology...

—0.5, Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

—0.4. The total mass of remaining moons is about 0.9. So, the total mass of all moons smaller than Tethys is about 3.5. (See List of moons by diameter)
The density of Tethys is 0.98 g/cm³, indicating that it is composed almost entirely of water-ice. The mass of rocky material can not exceed 6% of the mass of this moon.

It is not known whether Tethys is differentiated into a rocky core and ice mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

. However, if it is differentiated, the radius of the core is about 145 km. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with this moon having a homogeneous interior. The existence of a subsurface ocean—a layer of liquid water in the interior of Tethys - is considered unlikely.

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. The radar albedo of the Tethyan surface is also very high. The leading hemisphere of Tethys is by 10–15% brighter than the trailing one.

The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material. The visible spectrum of the moon is flat and featureless, while in the near-infrared strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible. No compound other than crystalline water ice has been unambiguously identified on Tethys. (Possible constituents include organics, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

.) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—Iapetus and Hyperion. The most probable candidate is nanophase iron or hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

. Measurements of the thermal emission as well as radar observations by the Cassini spacecraft show that the icy regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 on the surface of Tethys is structurally complex and has a large porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 exceeding 95%.

Surface features


Color patterns


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. The leading hemisphere also reddens slightly as the apex
Apex
Apex may refer to:- Biology :* Apex , the tip of the spire of the shell of a gastropod* Apex is the Apical meristem or its remnant on a flower...

 of the motion is approached, although without any noticeable darkening. 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 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.

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. The band is almost certainly caused by the influence of energetic electrons from the Saturnian magnetosphere with energies greater than about 1 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...

. These particle drift in the direction opposite to the rotation of the planet and preferentially impact areas on the leading hemisphere close to the equator.

Geology


The geology of Tethys is relatively simple. It surface is mostly made of hilly cratered terrain dominated by craters more than 40 km in diameter. A smaller part 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
Trough (geology)
In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....

s.

The western part of the leading hemisphere of Tethys is dominated by a large impact crater called Odysseus
Odysseus (crater)
Odysseus is the largest crater on Saturn's moon Tethys. It is 445 km across, more than 2/5 of the moon's diameter, and is one of the larger craters in the Solar System. It is situated in the western part of leading hemisphere of the moon—the latitude and longitude of its center are 32.8°N and...

, whose 450 km diameter is nearly 2/5 of that of Tethys itself. The crater is now quite flat or more precisely, its floor conforms to Tethys' spherical shape. This is most likely due to the viscous relaxation of the Tethyan icy crust over geologic time. Nevertheless the rim
Rim (craters)
The rim of a crater is the part that extends above the height of the local surface, usually in a circular or elliptical pattern. In a more specific sense, the rim may refer to the circular or elliptical edge that represents the uppermost tip of this raised portion...

 crest 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.

The second major feature seen on Tethys is a huge valley called Ithaca Chasma
Ithaca Chasma
Ithaca Chasma is a valley on Saturn's moon Tethys, named after the island of Ithaca, in Greece. It is on average 100 km wide, 3 to 5 km deep and 2,000 km long, running approximately three-quarters of the way around Tethys' circumference, making it one of the longer valleys in the...

, about 100 km wide and 3 km deep. It is more than 2000 km in length, approximately 3/4 of the way around Tethys' circumference. 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.

It is thought that Ithaca Chasma formed as Tethys' 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
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 between Dione and Tethys early in the solar system's history that led to orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 and tidal heating of Tethys' interior. The ocean would have frozen after the moons escaped from the resonance. 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 outmost ring graben of Odysseus. 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.
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
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

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.

Impact craters and chronology


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.

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 Solar System formation
Formation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...

 4.56 billion years ago. 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. The age of Ithaca Chasma is intermediate between the two ages mentioned above.

Origin and evolution


Tethys is thought to have formed from an accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...

 or subnebula; a disc of gas and dust that either existed around Saturn for some time after its formation. 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
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 were likely present as well, though their abundances are not well constrained.

The extremely water ice rich composition of Tethys remains unexplained. The conditions in the Saturnian sub-nebula likely favored conversion of the molecular nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 into ammonia and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, respectively. This can partially explain why Saturnian moons including Tethys contain more water ice than outer Solar System bodies like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 or Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

 as the oxygen freed from carbon monoxide would react with the hydrogen forming water. One of the most interesting explanations proposed is that the rings and inner moons accreted from the tidally striped ice-rich crust of a Titan like moon before it was swallowed by Saturn.

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' outer layer, reaching a maximum temperature of around 155 K at a depth of about 29 km. After the end of formation due to thermal conduction, the subsurface layer cooled, while the interior heated up. The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust reaching estimates of 5.7 MPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

, which likely led to cracking.

Since Tethys lacks substantial rock content, the heating by decay of radioactive elements is unlikely to have played a significant role in further evolution of this moon. 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. Still, present knowledge of the evolution of Tethys is very limited.

Exploration


Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...

 flew by Saturn in 1979, and its closest approach to Tethys was 329,197 km on September 1, 1979.

One year later, on 12 November 1980, Voyager 1 flew at the minimal distance of 415,670 km from Tethys. Its twin spacecraft, Voyager 2, passed as close as 93,000 km from the moon on 1 September 1981. While both spacecraft took images of Tethys, the resolution of Voyager's 1 images did not exceed 15 km, and only those obtained by Voyager 2 had a resolution as high as 2 km. The first geological feature discovered in 1980 by Voyager 1 was Ithaca Chasma. Later in 1981 Voyager 2 revealed that it almost circled the moon running for 270°. Voyager 2 also discovered the Odysseus crater. Tethys was the best imaged Saturnian satellite by the Voyagers.

The Cassini
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

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 September 24, 2005 at the distance of 1503 km. In addition to this flyby the spacecraft performed seven non-targeted flybys during its primary and equinox missions in 2004–2010 at distances of tens of thousands of kilometers.

Another flyby of Tethys took place on August 14, 2010 (during the solstice mission) at a distance of 38300 km, when the fourth-largest crater on Tethys, Penelope
Penelope (crater)
Penelope is the fourth largest impact crater on Tethys, which is one of Saturn's moons. It is 208 kilometers wide, and is located near the equator in the center of trailing hemisphere of the moon at 10.8°S, 249.2°W. It is approximately opposite to the largest crater on Tethys—Odysseus.Penelope...

, which is 207 km wide, was imaged. More non-targeted flybys are planned for the solstice mission in 2011–2017.

Cassini's observations allowed high resolution maps of Tethys to be produced with the resolution of 0.29 km. The spacecraft obtained spatially resolved near-infrared spectra of Tethys showing that its surface is made of water ice mixed with a dark material, while the far-infrared observations constrained the bolometric bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...

. The radar observations at the wavelength of 2.2 cm showed that the ice regolith has a complex structure and is very porous. 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.

Future missions to Tethys and the Saturn system are uncertain, but one possibility is the
Titan Saturn System Mission
Titan Saturn System Mission
Titan Saturn System Mission was a joint NASA/ESA proposal for an exploration of Saturn and its moons Titan and Enceladus, where many complex phenomena have been revealed by the recent Cassini–Huygens mission...

.

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