Mimas (moon)

Mimas

Mimas's anti-Saturn hemisphere with its large crater Herschel before Saturn's limb (Cassini, 2010-02-13). South is at the top.
Discovery
Discovered by	William Herschel
Discovery date	17 September 1789[1]
Designations
Alternate name(s)	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 twentieth 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 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."^[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), and this is accepted by the Cassini mission at JPL.^[14] Anglicisms such as Mimasian and Mimian are also very occasionally seen, but more commonly writers simply use the phrase 'of Mimas'.

Physical characteristics

Cassini view of Mimas's trailing hemisphere, showing craters up to 6 km deep and 1-km-deep chasmata (grooves).

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, the moon is not perfectly spherical; its longest axis is about 10% longer than the shortest. The ellipsoid shape of Mimas is especially noticeable in recent images from the Cassini probe.

Mimas' most distinctive feature is a colossal impact crater 130 kilometres (81 mi) 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 kilometres (3.1 mi) high, parts of its floor measure 10 kilometres (6.2 mi) deep, and its central peak rises 6 kilometres (3.7 mi) above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 4,000 kilometres (2,500 mi) 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 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 greater than 40 kilometres (25 mi) in diameter, but in the south polar region, craters greater than 20 kilometres (12 mi) 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

Two types of geological feature are officially recognized 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 co-rotation eccentricity resonance has been discovered with the G ring, whose inner edge is about 15,000 kilometres (9,300 mi) inside the orbit of Mimas.

Exploration

Mimas has been imaged several times by the Cassini orbiter. The closest flyby occurred on February 13, 2010, when Cassini passed by Mimas at 9,500 km (5,900 mi).

Unusual look

When seen from certain angles, Mimas closely resembles the Death Star, a space station known from the film Star Wars Episode IV: A New Hope, which is also said to be several hundred kilometres in diameter. This is purely coincidental, as the first film was made three years before the first close-up photographs of Mimas were taken.^[15]

In 2010, NASA revealed a temperature map of Mimas, using images obtained by Cassini. The warmest regions, which are along one edge of the moon, create a shape similar to the video game character Pac-Man, with the Herschel Crater assuming the role of an "edible dot" known from Pac-Man gameplay.^[16][17][18]

Gallery

Mimas as imaged by Cassini on August 1, 2005.	Mimas is the tiny white dot in the lower left. (Click to enlarge view).	Mimas, imaged by Cassini, looking notably egg-shaped.	Mimas, silhouetted against Saturn's northern latitudes.
Mimas, behind the F ring.	High-resolution view of Mimas's limb, showing striking albedo features on crater walls.	Subtle colour differences on Mimas are apparent in this false-colour view.

References

External links

• Cassini mission page - Mimas
• Mimas Profile at NASA's Solar System Exploration site
• The Planetary Society: Mimas
• Mimas page at The Nine Eight Planets
• Views of the Solar System - Mimas
• Cassini images of Mimas
• Images of Mimas at JPL's Planetary Photojournal
• Paul Schenk's Mimas blog entry and movie of Mimas's rotation on YouTube
• Mimas basemap (June 2008) from Cassini images
• Mimas atlas (October 2008) from Cassini images
• Mimas map with feature names from USGS Saturn system page

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