Tethys (moon)
| Tethys | |
|---|---|
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| Saturn's moon Tethys, captured by the Cassini spacecraft | |
| Central body | Saturn |
| Properties of the orbit | |
| Major semi-axis | 294,619 km |
| Periapsis | 294,589 km |
| Apoapsis | 294,648 km |
| eccentricity | 0.0001 |
| Orbit inclination | 1.120 ° |
| Orbital time | 1,887802 d |
| Mean orbital velocity | 11.35 km / s |
| Physical Properties | |
| Albedo | 0.80 ± 0.15 |
| Apparent brightness | 10.2 mag |
| Medium diameter | 1062.2 ± 1.2 (1076.8 × 1057.4 × 1052.6) km |
| Dimensions | 6.17449 ± 0.00132 x 10 20 kg |
| surface | 3,544,561 km 2 |
| Medium density | 0.984 ± 0.003 g / cm 3 |
| Sidereal rotation | 1.887802 days |
| Axis inclination | 0.034 ° |
| Acceleration of gravity on the surface | 0.147 m / s 2 |
| Escape speed | 394 m / s |
| Surface temperature | −187 ± 1 ° C (86 ± 1) K |
| discovery | |
| Explorer | |
| Date of discovery | March 21, 1684 |
| Remarks | Simply bound rotation |
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| The positions of the inner moons of Saturn in Saturn's ring system, from inside to outside Pan, Atlas, Prometheus, Pandora, Janus & Epimetheus, Mimas, Enceladus, Tethys, Dione and Rhea | |
Tethys (also Saturn III ) is the fifteenth and fifth largest of the 82 known moons of the planet Saturn and is one of the icy moons .
Discovery and naming
Tethys was discovered on March 21, 1684 by the French astronomer of Italian origin Giovanni Domenico Cassini .
Tethys is together with Dione the fourth and fifth respectively discovered Saturn moon and the eighth and ninth respectively discovered moon in the entire solar system . Due to its orbit, which was the third closest to Saturn at that time, it was designated as the third innermost of the seven previously known large moons of Saturn by the International Astronomical Union (IAU) with the Roman number III .
The moon was named after the titan Tethys from 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 in the universe and had about 3,000 daughters (including Europe ), called the Oceanids . One of her granddaughters was Thetis , later the mortal Peleus the Achilles gave birth. During the titan fight , Tethys raised Hera . She was not happy about the placement of Callisto and Arcas as the constellations Big and Little Bear in the sky, so she asked her nurse Tethys for help. Tethys cursed the constellations for wandering around the sky forever and never sinking below the horizon.
The geological equatorial ocean Tethys is named after this goddess .
The name "Tethys" and a further seven moons of Saturn was proposed by Wilhelm Herschel's son, the astronomer John Herschel , in an 1847 publication "Results of Astronomical Observations made at the Cape of Good Hope " . They should be named after siblings of the titan Kronos , who corresponds to the Roman Saturn .
Track properties
Orbit
Tethys orbits Saturn in a prograde , almost perfectly circular orbit at an average distance of 294,619 km (approx. 4,888 Saturn radii) from its center (or the center of gravity ), i.e. approx. 234,351 km above its cloud ceiling. The orbital eccentricity is 0.0001, the orbit is 1.12 ° inclined to the equator of Saturn , so it is almost in the equatorial plane of the planet. Due to the very low eccentricity, the orbit varies by about 59 km in the distance to Saturn.
The orbit of the next inner moon Enceladus is on average about 56,671 km from the orbit of Tethys, the distances of the orbits of the next outer moons Dione and their Trojan moons Helene and Polydeuces are on average about 82,777 km.
The Tethys Trojan moons Telesto and Calypso orbit the planet at Tethys' Lagrange points L 4 and L 5 , each 60 ° in front of and behind the moon, on the same orbit.
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 time of Jupiter's moon Io . Tethys takes 12 hours and about 25.3 minutes longer to complete one orbit than its inner neighbor Enceladus.
Tethys' orbit is deep in Saturn's magnetosphere , so the plasma that rotates with the planet will hit the next hemisphere. It is also hit by energetic particles ( electrons and ions ).
Railway resonances
Tethys is in gravitational interaction with its neighborhood. In addition to the Trojan moons Telesto and Calypso (1: 1 orbit resonance ), Tethys runs almost in a 4: 3 orbit resonance with the next inner moon Enceladus and almost in a 2: 3 resonance with its outer orbit neighbor Dione around Saturn.
Tethys and her two Trojan moons orbit Saturn within the E-Ring , so the surfaces of the moons are in constant bombardment by micrometeorites .
rotation
The rotation time of Tethys is equal to its orbital time . Both take place within one day, 21 hours, 18 minutes and 26.1 seconds. Tethys, like the Earth's moon and all major satellites of the gas giants , has a synchronous rotation . So it always points to Saturn with the same hemisphere . The equatorial plane is inclined 0.034 ° from the orbit.
Physical Properties
size
Tethys is roughly spherical, with an average diameter of 1062.2 km. The exact dimensions are 1076.8 km × 1057.4 km × 1052.6 km. The deviation of about 0.9% is due to the tidal forces of Saturn, which gives the moon the shape of a three-axis ellipsoid . The long axis is aligned with Saturn, the middle axis is between the leading and following hemispheres and the shortest axis is between the poles . Tethys is the fifth largest moon of Saturn and ranks in the entire solar system on the 16th place among all planetary moons as well as the 31st place of all known bodies at all (as of March 2012).
In terms of size, Tethys can best be compared with the largest main belt - asteroid Ceres or its slightly larger neighbor Dione .
The total area of Tethys is approximately 3,544,561 km².
internal structure
Tethys is an icy celestial body, similar to the large Saturn moons Dione and Rhea . Its low density of 0.984 g / cm 3 indicates that it is largely composed of water ice . The mass of rock cannot exceed 6% of the total mass of the moon. It is not known whether Tethys has a differentiated body. If so, the rock core would have a diameter of about 290 km, about a third of the total diameter of the moon. The dimensions of the ellipsoid indicate a homogeneous interior. The existence of an underground ocean 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 being bombarded by the fine water ice particles of the E-ring. As a result, the leading hemisphere is around 10 to 15% brighter than the following, as it virtually “sweeps up” the ring material on its orbit. Temperatures on Tethys are around −187 ° C (86 K ).
Tethys' surface is badly cratered and has numerous cracks. Two different types of geological regions could be identified, a region with numerous impact craters and a dark, less cratered band that runs across the moon. The latter region is an indication that the surface was geologically active at a later date, with older areas of the surface being renewed. The exact cause of the dark discoloration of the ribbon is not known. One possible explanation could be images of the space probe Galileo , which examined the moons of Jupiter Ganymede and Callisto . Both moons have bright polar caps formed by large ice deposits on the slopes of craters. From a distance, the polar caps appear brighter than the pattern formed by thousands of unresolved smaller and icy craters. Tethys' surface may have been formed similarly and consists of polar regions with an indistinct pattern of light ice with a darker zone between them.
Odysseus crater
In the western hemisphere, the huge Odysseus crater is the most noticeable feature. It covers almost 3.5 percent of the entire surface, and its diameter of 445 km is a good 40 percent of the moon's diameter. The crater appears relatively shallow and resembles the craters on Jupiter's moon Callisto, with the lack of the typical ring walls and central mountain that can be found on Mercury or the Earth's moon . This is believed to be due to the fact that the weak icy crust of Tethys collapsed over geological time.
Ithaca Chasma
The second noticeable feature on Tethys is a huge valley, Ithaca Chasma , which is approximately 100 km wide and 3 to 5 km deep. With a length of 2000 km it runs about three quarters around the moon. It could have been formed when liquid water inside the moon froze and tore the surface as a result of expansion. According to another theory, Ithaca Chasma may have been formed when the impact of the huge body that made up Odysseus' crater caused shock waves to ripple through the moon and crack the fragile crust on the opposite side.
exploration
Tethys has an apparent magnitude of 10.3 m , which is a 1: 6300 of the central planet. A larger telescope is required to observe Tethys .
Tethys has been visited by four spacecraft to date , namely the flyby probes Pioneer 11 on September 1, 1979, Voyager 1 on November 12, 1980 and Voyager 2 on August 25, 1981, and finally Cassini-Huygens , which operated from July 1, 2004 to May 15 September 2017 orbited Saturn. On September 24, 2005, the probe passed Tethys at a distance of 1503 km and has not come any closer since then. During a flyby in April 2015, the probe sent images of strange red streaks from the surface of Tethys, which are believed to be geologically very young.
Web links
- Current Cassini images from Tethys , a picture gallery in the field of planetology and remote sensing at the Free University of Berlin under the direction of Cassini team member Gerhard Neukum
- Voyager 1 and 2 - Atlas of Six Saturnian Satellites , NASA maps of six Saturn moons based on images from the Voyager probes
- Saturn, Tethys, Rings and Shadows APOD April 5, 2015
Individual evidence
- ↑ Paul Rincon: Saturn overtakes Jupiter as planet with most moons. BBC , October 7, 2019, accessed March 20, 2020 .
- ^ Cassini: An Extract of the Journal Des Scavans. of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris. Phil. Trans. 1686 16: 79-85; doi : 10.1098 / rstl.1686.0013 ( full text )
- ↑ NASA Cassini Solstice Mission: Red Arcs on Tethys
| further inside | Saturn moons | further outside |
| Enceladus |
Semi- major axis (km) Tethys 294.700 |
Telesto |