Mimas (moon)

Mimas (moon)
Mimas Cassini.jpg
Mimas with its large crater Herschel
(Cassini, 2010-02-13)
Discovered by William Herschel
Discovery date 17 September 1789[1]
Alternate name(s) Saturn I
Periapsis 181,902 km
Apoapsis 189,176 km
Eccentricity 0.0196
Orbital period 0.942 d
Inclination 1.574 ° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions 415.6×393.4×381.2 km (0.0311 Earths)[3]
Mean radius 198.2 ± 0.4 km[3]
Surface area ~490 000 km2
Volume ~32 900 000 km3
Mass (3.749 3 ± 0.003 1)×1019 kg[4][5]
(6.3×10−6 Earths)
Mean density 1.1479 ± 0.007 g/cm3[3]
Equatorial surface gravity 0.064 m/s2 (0.648%g)
Escape velocity 0.159 km/s
Rotation period synchronous
Axial tilt zero
Albedo 0.962 ± 0.004 (geometric)[6]
Temperature ~64 K
Apparent magnitude 12.9 [7]

Mimas (play /ˈmməs/;[8] Greek: Μίμᾱς) is a moon of Saturn which was discovered in 1789 by William Herschel.[9] It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I.

By diameter (396 kilometers) the twentieth-largest moon in the Solar System, Mimas is the smallest known astronomical body that is thought to be rounded in shape due to 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."[10]


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.[11][12] He named them after Titans specifically because Saturn (the Roman equivalent of Kronos in Greek mythology), was the leader of the Titans and ruler of the world for some time.

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 Australia. 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.[citation needed]

The surface of Mimas 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.

Map of Mimas

Two types of geological feature are officially recognized on Mimas: craters and chasmata (chasms).

Orbital resonances

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 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 ring is in a 3:1 resonance with Mimas. Recently, the G ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas; the ring's inner edge is about 15,000 kilometres (9,300 mi) inside Mimas' orbit.[citation needed]

Mimas is also in a 4:2 mean motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F ring shepherd moonlet, Pandora.


Pioneer 11 flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on September 1, 1979.[13] Voyager 1 flew by in 1980, and Voyager 2 in 1981.

Mimas has been 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).

Unusual resemblances

When seen from certain angles, Mimas closely resembles the Death Star, a fictional space station known from the film Star Wars Episode IV: A New Hope, which is said to be roughly 140 kilometres in diameter. This resemblance stems from the fact that Herschel can appear in Mimas' northern hemisphere, much like the concave disc of the Death Star's "superlaser". This is purely coincidental, as the first film was made three years before the first close-up photographs of Mimas were taken.[14]

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 Herschel Crater assuming the role of an "edible dot" known from Pac-Man gameplay.[15][16][17]

In popular culture

  • The Red Dwarf novel Infinity Welcomes Careful Drivers is partially set on Mimas. Mimas is owned by the Spanish government, and is home to many bars and brothels, with a heavy organized-crime presence.
  • "The First Duty" (1992), episode of Star Trek: The Next Generation. Mimas is the site of an evacuation station to which four Starfleet cadets, including Wesley Crusher, transport after their vessels collide.



  1. ^ "Imago Mundi: La Découverte des satellites de Saturne" (in French). http://www.cosmovisions.com/SaturneChrono02.htm. 
  2. ^ Harvey, Samantha (April 11, 2007). "NASA: Solar System Exploration: Planets: Saturn: Moons: Mimas: Facts & Figures". NASA. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mimas&Display=Facts. Retrieved 2007-10-10. 
  3. ^ a b c Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.  edit
  4. ^ Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E. et al. (December 2006). "The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data". The Astronomical Journal 132 (6): 2520–2526. Bibcode 2006AJ....132.2520J. doi:10.1086/508812. 
  5. ^ Jacobson, R. A.; et al. (2005). "The GM values of Mimas and Tethys and the libration of Methone". Astronomical Journal 132 (2): 711. Bibcode 2006AJ....132..711J. doi:10.1086/505209. 
  6. ^ Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science 315 (5813): 815. Bibcode 2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992.  p. 815 (supporting online material, table S1)
  7. ^ "Classic Satellites of the Solar System". Observatorio ARVAL. http://www.oarval.org/ClasSaten.htm. Retrieved 2007-09-28. 
  8. ^ In US dictionary transcription, US dict: mī′·məs.
  9. ^ Herschel, W. (1790). "Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; With Remarks on the Construction of Its Ring, Its Atmosphere, Its Rotation on an Axis, and Its Spheroidical Figure". Philosophical Transactions of the Royal Society of London 80 (0): 1–20. doi:10.1098/rstl.1790.0001. 
  10. ^ Herschel, William Philosophical Transactions of the Royal Society of London, Vol. 80, reported by Arago, M. (1871). "Herschel". Annual Report of the Board of Regents of the Smithsonian Institution: 198–223. http://laplaza.org/~tom/People/Herschel.htm. Retrieved 2006-11-26. 
  11. ^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
  12. ^ Lassell, William (1848). "Satellites of Saturn: Observations of Mimas, the closest and most interior Satellite of Saturn". Monthly Notices of the Royal Astronomical Society 8: 42. Bibcode 1848MNRAS...8...42L. http://articles.adsabs.harvard.edu/full/seri/MNRAS/0008//0000042.000.html. Retrieved 2006-11-26. 
  13. ^ Pioneer 11 Full Mission Timeline
  14. ^ Young, Kelly (2005-02-11). "Saturn's moon is Death Star's twin". New Scientist. http://www.newscientist.com/article.ns?id=dn6999. Retrieved 2008-08-21. "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." 
  15. ^ Cook, Jia-Rui C. (2010-03-29). "1980s Video Icon Glows on Saturn Moon". NASA. http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100329.html. Retrieved 2010-04-02. 
  16. ^ "Bizarre Temperatures on Mimas". NASA. 2010-03-29. http://www.nasa.gov/mission_pages/cassini/multimedia/pia12867.html. Retrieved 2010-04-02. 
  17. ^ "Saturn moon looks like Pac-Man in image taken by Nasa spacecraft". The Daily Telegraph. 2010-03-30. http://www.telegraph.co.uk/science/space/7537170/Saturn-moon-looks-like-Pac-Man-in-image-taken-by-Nasa-spacecraft.html. Retrieved 2010-04-02. 

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