Tethys (moon)

Giovanni Cassini

Tethys (including Saturn III) is the fifteenth and fifth-largest of the 62 known moons of the planet Saturn.

• 3.1 size
• 3.2 Internal structure
• 3.3 Surface 3.3.1 Odysseus Crater
• 3.3.2 Ithaca Chasma

Discovery and designation

Tethys was discovered on March 21, 1684 by the French astronomer Giovanni Cassini Italian origin.

Tethys Dione together with the 4th and 5th discovered Saturn's moon and the 8th or 9th discovered moon in the solar system. He was the third- innermost of the seven previously known large moons of Saturn by the International Astronomical Union ( IAU) designated by the Roman numbering III Through his time at the third closest to Saturn orbit.

It was named after the moon Titan Tethys in Greek mythology. She was a daughter of Uranus and Gaia, and was married to her brother Okeanos. Tethys was the mother of the main rivers of the universe and had about 3,000 daughters ( including Europe ), called the Oceanids. One of her granddaughters was Thetis, from the mortal Peleus to Achilles gave birth later. During the titanic struggle Tethys Hera moved on. This was not pleased with the placement of Callisto and Arcas as constellations of the Great and Little Bear in the sky, so she asked her nurse Tethys for help. Tethys cursed the constellations forever to wander around the sky and never drop below the horizon.

After this goddess and the geological equatorial Tethys Ocean is named.

The name " Tethys " and seven other moons of Saturn was suggested by William Herschel's son, the astronomer John Herschel, in a 1847 published paper " Results of Astronomical Observations made ​​at the Cape of Good Hope ." They should be named after siblings of the Titans Cronus, corresponding to the Roman Saturn.

Web properties

Orbit

Tethys orbits Saturn on a prograde, nearly perfectly circular orbit at an average distance of 294,619 kilometers (about 4,888 Saturn radii ) from the center (or the center of gravity ), or about 234,351 km on whose cloud tops. The orbital eccentricity is 0.0001, the orbit is inclined 1.12 ° relative to the equator of Saturn, that is nearly in the equatorial plane of the planet. Due to the very low eccentricity, the web varies in distance to Saturn at about 59 km.

The orbits of the next inner moon Enceladus is removed an average of about 56,671 km from the orbit of Tethys, the distances of the orbits of the moons Dione and their next outer Trojan moons Helene and Polydeuces are on average about 82,777 km.

The Tethys Trojan moons Telesto and Calypso run in Tethys ' Lagrange Points L4 and L5, respectively 60 ° and behind the moon, on the same orbit around the planet.

Tethys orbits Saturn in 1 day, 21 hours, 18 minutes and 26.1 seconds. This is about 2 hours and 50.9 minutes longer than the orbital period of Jupiter's moon Io. Tethys required for one revolution about 12 hours, and 25.3 minutes longer than the inner adjacent Enceladus.

The orbit of Tethys lies deep in the magnetosphere of Saturn, so that the plasma co-rotates with the planet that meets the following hemisphere. It is thus also hit by energetic particles (electrons and ions).

Train resonances

Tethys is in gravitational interaction with its neighborhood. In addition to the Trojan moons Telesto and Calypso ( 1:1 orbital resonance ) Tethys runs almost in a 4:3 orbital resonance with the next inner moon Enceladus and approximately in a 2:3 resonance with its outer rail neighbor Dione around Saturn.

Tethys and its two Trojan moons orbit Saturn within the E ring, so that the surfaces of the moons are in a constant bombardment by micrometeorites.

Rotation

The rotation time of Tethys is equal to its orbital period. Both take place within one day 21 hours, 18 minutes and 26.1 seconds. Tethys has with how the Earth's moon and all the major satellites of the gas giant, in a synchronous rotation. So he is always with the same hemisphere to Saturn. The equatorial plane is inclined 0.034 degrees from the orbit.

Physical Properties

Size

Tethys is approximately spherical, with an average diameter of 1062.2 km. The exact dimensions are 1076.8 × 1057.4 × 1052.6 km. The deviation of about 0.9 % is due to the tidal forces of Saturn, which gives the shape of a triaxial ellipsoid moon. The longitudinal axis is aligned with Saturn, the central axis is between leading and following hemisphere and the shortest axis between the poles. Tethys is the fifth largest moon of Saturn and is ranked in the solar system in 16th place in all the planet 's moons and the 31st place all previously known body at all (as of March 2012).

From the size Tethys is best compared with the largest main belt asteroids Ceres or the slightly larger neighbor Dione.

The total area of ​​3,544,561 km ² Tethys is about, which corresponds approximately to the area of ​​India, with Bangladesh and Nepal.

Internal construction

Tethys is an icy celestial bodies, similar to Saturn's large moons Dione and Rhea. Their low density of 0.984 g/cm3 indicates that it is largely composed of water ice. The mass of rock may not exceed 6% of the total mass of the moon. It is not known whether Tethys has a differentiated body. If so, the rocky core had a diameter of about 290 km, that is approximately one third of the total diameter of the Moon. The dimensions of the ellipsoid indicate a homogeneous interior. The existence of an underground Eismondozeans is considered unlikely.

Surface

The surface of Tethys is very bright, it reflects 80 % of the incident sunlight. This high Albedo is the result of the bombardment of the fine water ice of the E ring. The leading hemisphere is brighter by about 10 to 15% as the following, as they continue their orbit quasi " sweeps up " the ring material. On Tethys, temperatures of about -187 ° C ( this corresponds to 86 Kelvin).

Tethys ' surface is heavily cratered and has numerous cracks. Two different types of geological regions could be identified, a region with numerous impact craters and a dark, less verkratertes belt passing over the moon. The latter region is an indication that the surface was geologically active at a later time, with older areas of the surface were replaced. The exact cause of dark discoloration of the tape is not known. A possible explanation could provide images of the Galileo spacecraft, which examined the Jupiter moons Ganymede and Callisto. Both moons have bright polar caps, which were formed of large ice deposits on the slopes of craters. From a distance, the polar ice caps appear brighter than the pattern that is formed by thousands, unresolved smaller and icy craters. Tethys ' surface could have been similarly formed and consists of polar regions with an indistinct pattern of bright ice, between which a darker zone.

Odysseus Crater

In the Western Hemisphere, the huge crater Odysseus is the most prominent feature. It covers almost 3.5 % of the total surface, and its diameter is 400 km of well 40 % of the diameter of the moon. The crater appears to be relatively flat and similar to craters on Jupiter's moon Callisto, with typical circular walls and the central peak is missing, as they are found on Mercury or the Earth's moon. This is probably due to the fact that the weak icy crust of Tethys over geological time has broken away.

Ithaca Chasma

The second striking feature on Tethys is a huge valley, Ithaca Chasma, which is about 100 km wide and three to five kilometers deep. With a length of 2000 km, it runs to about three-quarters around the moon. It could have been made, ausfror as liquid water in the interior of the moon and the surface ripped open due to expansion. Another theory Ithaca Chasma could have arisen, as the impact of the huge body, formed the crater Odysseus, shock waves ran through the moon and the fragile crust on the opposite side broke up.

Research

Tethys has an apparent magnitude of 10.3 m, which is 1:6300 of the central planet. To observe Tethys, you need a larger telescope.

Tethys was attended by current four spacecraft, notably by the flyby probe Pioneer 11 on 1 September 1979, Voyager 1 on 12 November 1980 and Voyager 2 on August 25, 1981 and finally Cassini -Huygens, the Saturn since July 1, 2004 orbits. The next flyby of Cassini took place on 24 September 2005, when the spacecraft Tethys happened at a distance of 1503 km.