Mimas

Mimas

Discovery
Discovered by	William Herschel
Discovery date	September 17, 1789[1]
Designations
Alternate name	Saturn I
Adjective	Mimantean
Orbital characteristics
Mean orbit radius	185 520 km [2]
Eccentricity	0.020 2[3]
Orbital period	0.942 421 8 d[3]
Inclination	1.51° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Dimensions	414.8×394.4×381.4 km (0.0311 Earths)[4]
Mean radius	198.30 ± 0.30 km[5]
Surface area	~490 000 km²
Volume	~32 900 000 km3
Mass	(3.749 3 ± 0.003 1) × 1019 kg [5][6] (6.3 × 10−6 Earths)
Mean density	1.147 9 ± 0.005 3 g/cm3 [5]
Equatorial surface gravity	0.063 6 m/s2 (0.648%g)
Escape velocity	0.159 km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.962 ± 0.004 (geometric)[7]
Temperature	~64 K
Apparent magnitude	12.9 [8]

Mimas (pronounced /ˈmaɪməs/,^[9] or as Greek Μίμᾱς, rarely Μίμανς) is a moon of Saturn which was discovered in 1789 by William Herschel.^[10] It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I.

By diameter, Mimas is the 20th largest moon in the solar system. However, Mimas is the smallest known astronomical body that is thought to be rounded in shape due to self-gravitation.

Discovery

Mimas was discovered by the astronomer William Herschel on September 17, 1789. He recorded his discovery as follows: "The great light of my forty-foot telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation."^[11]

Name

Mimas is named after one of the Titans 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.^[12][13] He named them after Titans specifically because Saturn (the Roman equivalent of Kronos in Greek mythology), was the leader of the Titans.

According to Liddell and Scott's Greek-English Lexicon, the adjectival form of Mimas would be Mimantean (the genitive case is Latin Mimantis, Greek Μῑμάντος). In practice, anglicisms such as Mimasian and Mimian are very occasionally seen, but more commonly writers simply use the phrase 'of Mimas'.

Physical characteristics

High-resolution Cassini view of Mimas, showing craters up to 6 km deep, and 1-km-deep grooves.

Mimas' low density (1.17) indicates that it is composed mostly of water ice with only a small amount of rock. Due to the tidal forces acting on it, the moon is not perfectly spherical; its longest axis is about 10% longer than the shortest. The somewhat ovoid shape of Mimas is especially noticeable in recent images from the Cassini probe.

Mimas' most distinctive feature is a colossal impact crater 130 km across, named Herschel after the moon's discoverer. Herschel's diameter is almost a third of the moon's own diameter; its walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 4000 km in diameter, wider than Canada. The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon's body.

The 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 greater than 40 km in diameter, but in the south polar region, craters greater than 20 km are generally lacking. This suggests that some process removed the larger craters from these areas, or that something prevented larger stellar bodies from hitting the south polar region.

Map of Mimas

Scientists officially recognise two types of geological features on Mimas: craters and chasmata (chasms). (See also: List of geological features on Mimas)

Relationship with the rings of Saturn

Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, A ring and B ring. Particles at the inner edge of the Cassini division are in a 2:1 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. Other resonances with Mimas are also responsible for other features in Saturn's rings: the boundary between the C and B ring is at the 3:1 resonance and the outer F ring shepherd, Pandora, is at the 3:2 resonance. More recently, a 7:6 corotation eccentricity resonance has been discovered with the G ring, whose inner edge is about 15 000 km inside the orbit of Mimas.

Exploration

Mimas has been imaged several times from moderate distances by the Cassini orbiter, the closest being at 63 000 km on August 1, 2005. Cassini's extended mission will include several non-targeted close approaches to Mimas. Improvements on the current best will occur during passes on October 24, 2008 and October 14, 2009. The closest will be on February 13, 2010 at 9 500 km.

Gallery

Mimas, imaged by Cassini, looking notably egg-shaped

Mimas, silhouetted against Saturn's northern latitudes

Mimas, behind the F ring

See also

• Polar bulge

References

External links

• Cassini mission page - Mimas
• Mimas Profile by NASA's Solar System Exploration
• Journal of raw images from Cassini
• The Planetary Society: Mimas
• The Nine Planets - Mimas
• Views of the Solar System - Mimas

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