|Discovered by||William Herschel|
|Discovery date||17 September 1789|
|Pronunciation||/ / or as Greco-Latin Mimas (approximated / /)|
|Adjectives||Mimantean, Mimantian (both / /)|
Average orbital speed
|14.28 km/s (calculated)|
|Inclination||1.574° (to Saturn's equator)|
|Dimensions||415.6 × 393.4 × 381.2 km|
|Mass||(3.7493±0.0031)×1019 kg |
|0.064 m/s2 (0.00648 g)|
|Temperature||≈ 64 K|
Mimas // , also designated Saturn I, 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.
With a diameter of 396 kilometres (246 mi), it is the smallest astronomical body that is known to still be rounded in shape because of self-gravitation. However, Mimas is not actually in hydrostatic equilibrium for its current rotation.
Mimas was discovered by the astronomer William Herschel on 17 September 1789. He recorded his discovery as follows: "I continued my observations constantly, whenever the weather would permit; and the great light of the forty-feet speculum was now of so much use, that I also, on the 17th of September, detected the seventh satellite, when it was at its greatest preceding elongation."
The 40-foot telescope was a metal mirror reflecting telescope built by Herschel, with a 48-inch (1,200 mm) aperture. The 40 feet refers to the length of the focus, not the aperture diameter as more common with modern telescopes.
Mimas is named after one of the Giants in Greek mythology, Mimas. The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.Saturn (the Roman equivalent of Cronus in Greek mythology) was the leader of the Titans, the generation before the Gods, and ruler of the world for some time. The Giants were the subsequent generation, and each group fought a great struggle against the Gods.
The customary English pronunciation of the name is // , though some people attempt a more 'authentic' pronunciation, // .
The Greek and Latin root of the name is Mimant-, // ~ // .and so the English adjectival form is Mimantean or Mimantian, either spelling pronounced
The surface area of Mimas is slightly less than the land area of Spain. The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock. Due to the tidal forces acting on it, Mimas is noticeably prolate; its longest axis is about 10% longer than the shortest. The ellipsoidal shape of Mimas is especially noticeable in some recent images from the Cassini probe.
Mimas's most distinctive feature is a giant impact crater 130 km (81 mi) across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately 5 km (3 mi) high, parts of its floor measure 10 km (6 mi) deep, and its central peak rises 6 km (4 mi) above the crater floor. If there were a crater of an equivalent scale on Earth (in relative size) it would be over 4,000 km (2,500 mi) in diameter, wider than Australia. The impact that made this crater must have nearly shattered Mimas: the surface antipodal to (opposite through the globe) Herschel is highly disrupted, indicating that the shock waves created by the Herschel impact propagated through the whole moon.
The Mimantean surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel. Although Mimas is heavily cratered, the cratering is not uniform. Most of the surface is covered with craters larger than 40 km (25 mi) in diameter, but in the south polar region, there are generally no craters larger than 20 km (12 mi) in diameter.
Three types of geological features are officially recognized on Mimas: craters, chasmata (chasms) and catenae (crater chains).
A number of features in Saturn's rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A Ring and B Ring. Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 orbital resonance with Mimas. They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. The boundary between the C and B rings is in a 3:1 resonance with Mimas. Recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance[ clarification needed ] with Mimas; the ring's inner edge is about 15,000 km (9,300 mi) inside Mimas's orbit.[ citation needed ]
Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora. A moon co-orbital with Mimas was reported by Stephen P. Synnott and Richard J. Terrile in 1982, but was never confirmed.
In 2014, researchers noted that the librational motion of Mimas has a component that cannot be explained by its orbit alone, and concluded that it was due to either an interior that is not in hydrostatic equilibrium (an elongated core) or an internal ocean.However, in 2017 it was concluded that the presence of an ocean in Mimas' interior would have led to surface tidal stresses comparable to or greater than those on tectonically active Europa. Thus, the lack of evidence for surface cracking or other tectonic activity on Mimas argues against the presence of such an ocean; as the formation of a core would have also produced an ocean and thus the nonexistent tidal stresses, that possibility is also unlikely. The presence of an asymmetric mass anomaly associated with the crater Herschel may be a more likely explanation for the libration.
Pioneer 11 flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on September 1, 1979. Voyager 1 flew by in 1980, and Voyager 2 in 1981.
Mimas was imaged several times by the Cassini orbiter, which entered into orbit around Saturn in 2004. A close flyby occurred on February 13, 2010, when Cassini passed by Mimas at 9,500 km (5,900 mi).
When seen from certain angles, Mimas resembles the Death Star, a fictional space station and superweapon known from the 1977 film Star Wars . Herschel resembles the concave disc of the Death Star's "superlaser". This is coincidental, as the film was made nearly three years before Mimas was resolved well enough to see the crater.
In 2010, NASA revealed a temperature map of Mimas, using images obtained by Cassini. The warmest regions, which are along one edge of Mimas, create a shape similar to the video game character Pac-Man, with Herschel Crater assuming the role of an "edible dot" or "power pellet" known from Pac-Man gameplay.
Miranda, also designated Uranus V, is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is perpendicular to the ecliptic and shares Uranus' extreme seasonal cycle.
Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.
Tethys, or Saturn III, is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology.
Iapetus is the third-largest natural satellite of Saturn and the eleventh-largest in the Solar System. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon and a distinctive color pattern.
Janus is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Janus.
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.
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. Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator.
Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 26. In late 1985 it was officially named after Pandora from Greek mythology. It is also designated Saturn XVII.
Hyperion, also known as Saturn VII (7), is a moon of Saturn discovered by William Cranch Bond, his son George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered.
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 S/1980 S 25. Several other apparitions of it were recorded in the following months: S/1980 S 29, S/1980 S 30, S/1980 S 32, and S/1981 S 2. In 1983 it was officially named after Calypso of Greek mythology. It is also designated Saturn XIV or Tethys C.
Dione is a moon of Saturn. It was discovered by Italian astronomer Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology. It is also designated Saturn IV.
Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometers (310 mi) in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains.
Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on March 18, 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.
Prometheus is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 27.
The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan, which is larger than the planet Mercury. Saturn has 83 moons with confirmed orbits that are not embedded in its rings – of which only 13 have diameters greater than 50 kilometers – as well as dense rings that contain millions of embedded moonlets and innumerable smaller ring particles. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, Enceladus, which emits jets of gas and dust from its south-polar region, and Iapetus, with its contrasting black and white hemispheres.
Methone is a small, egg-shaped moon of Saturn that orbits out past Saturn's ring system, between the orbits of Mimas and Enceladus. It was discovered in 2004, though it wasn't until 2012 that it was imaged in detail by the Cassini spacecraft.
Pallene is a very small natural satellite of Saturn. It is one of three small moons known as the Alkyonides that lie between the orbits of the larger Mimas and Enceladus. It is also designated Saturn XXXIII.
The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, newer data from Cassini suggested they formed relatively late.
Herschel is a large crater in the leading hemisphere of the Saturnian moon Mimas, centered on the equator at 112° longitude. It is named after the 18th-century astronomer William Herschel, who discovered Mimas in 1789.
Polydeuces, or Saturn XXXIV, is a small natural satellite of Saturn that is co-orbital with the moon Dione and librates around its trailing Lagrangian point (L5). Its diameter is estimated to be 2–3 km. Dione's other co-orbital moon is Helene, which is bigger and located at the leading L4 point.
Saturn's diminutive moon, Mimas, poses as the Death Star – the planet-destroying space station from the movie Star Wars – in an image recently captured by NASA's Cassini spacecraft.
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