Mimas (moon)

Mimas (, or as Greek Μίμᾱς, rarely Μίμανς) 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.

Mimas is the smallest known astronomical body of the solar system which has a near-spherical shape due to its self-gravitation.


Mimas was discovered by the astronomer William Herschel on 17 September, 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.


Mimas is named after one of the Titans in Greek mythology. 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. 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

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.

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.


Mimas has been imaged several times from moderate distances by the Cassini orbiter, the closest being at 63 000 km on 2005 August 01. Cassini's extended mission will include several non-targeted close approaches to Mimas. Improvements on the current best will occur during passes on 2008 October 24 and 2009 October 14. The closest will be on 2010 February 13 at 9 500 km.


Mimas in fiction and film

  • When seen from certain angles, Mimas closely resembles the Death Star in Star Wars Episode IV: A New Hope, which is also said to be several hundred kilometers in diameter. This is purely coincidental, as the first film was made 3 years before the first close-up photographs of Mimas were taken.
  • Mimas is featured in the book Red Dwarf: Infinity welcomes careful drivers as the moon Dave Lister lives on prior to his acceptance into the mining ship Red Dwarf.
  • Mimas is the site of a Federation way station in the Star Trek universe.

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