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This article is about the moon of Saturn. For other uses, seeMimas (disambiguation).
Mimas
Mimas imaged by theCassiniorbiter, February 2010. Mimas's surface is dominated by craters; the large crater at the right isHerschel
Discovery
Discovered byWilliam Herschel
Discovery date17 September 1789[1]
Designations
Designation
Saturn I
Pronunciation/ˈmaɪməs/[2]or as Greco-LatinMimas(approximated/ˈmiːməs/)
Named after
ΜίμαςMimās
AdjectivesMimantean,[3]Mimantian[4](both/mɪˈmæntiən/)
Orbital characteristics [5]
Periapsis181902km
Apoapsis189176km
185539km
Eccentricity0.0196
0.942421959d
14.28 km/s (calculated)
Inclination1.574°(to Saturn's equator)
Satellite ofSaturn
Physical characteristics
Dimensions415.6 × 393.4 × 381.2 km
(0.0311 Earths)[6]
198.2±0.4 km[6][7]
490000–500000km2
Volume32600000±200000km3
Mass(3.75094±0.00023)×1019kg [7]
(6.3×10−6Earths)
1.1501±0.0070 g/cm3[7]
0.064m/s2(0.00648g)
0.159 km/s
synchronous
zero
Albedo0.962±0.004(geometric)[8]
Temperature≈ 64 K
12.9 [9]

Mimas,also designatedSaturn I,is the seventh-largestnatural satelliteofSaturn.With amean diameterof 396.4 kilometres or 246.3 miles, Mimas is the smallestastronomical bodyknown to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15 g/cm3,indicates that it is composed mostly of water ice with only a small amount of rock, and study of Mimas's motion suggests that it may have a liquid ocean beneath its surface ice. The surface of Mimas is heavily cratered and shows little signs of recent geological activity. A notable feature of Mimas's surface isHerschel,one of the largest craters relative to the size of the parent body in the Solar System. Herschel measures 139 kilometres (86 miles) across, about one-third of Mimas's mean diameter,[10]and formed from an extremely energeticimpact event.The crater's name is derived from the discoverer of Mimas,William Herschel,in 1789. The moon's presence has created one of the largest 'gaps' inSaturn's ring,named theCassini Division,due toorbital resonancedestabilizing the particles' orbit there.

Discovery[edit]

William Herschel, discoverer of Mimas

Mimas was discovered by theastronomerWilliam Herschelon 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.[11][12]

— William Herschel

The40-foot telescopewas 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 is more common with modern telescopes.

Name[edit]

John Herschel,the astronomer who suggested that the moons of Saturn be named after the Titans and Giants

Mimas is named after one of theGiantsinGreek mythology,Mimas.The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's sonJohnin his 1847 publicationResults of Astronomical Observations made at the Cape of Good Hope.[13][14]Saturn (the Roman equivalent ofCronusin Greek mythology) was the leader of the Titans, the generation before theGods,and rulers of the world for some time, while the Giants were the subsequent generation, and each group fought a great struggle againstZeusand the Olympians.

The customary English pronunciation of the name is/ˈmaɪməs/,[15]or sometimes/ˈmiːməs/.[16]

The Greek and Latin root of the name isMimant-(cf. ItalianMimante,RussianМимантfor the mythological figure),[17]and so the English adjectival form isMimantean[18]orMimantian,[19]either spelling pronounced/maɪˈmæntiən/~/mɪˈmæntiən/.[20]

Physical characteristics[edit]

Further information:List of geological features on Mimas
Size comparison between Mimas (lower left), theMoon(upper left) and Earth
A close up picture of Mimas's largest crater and its surrounding terrain,Herschel

The surface area of Mimas is slightly less than the land area ofSpainorCalifornia.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. As a result of the tidal forces acting on it, Mimas is noticeablyoblate;its longest axis is about 10% longer than the shortest. Theellipsoidalshape of Mimas is especially noticeable in some recent images from theCassiniprobe. Mimas's most distinctive feature is a giantimpact crater139 km (86 mi) across, namedHerschelafter 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 onEarth(in relative size) it would be over 4,000 km (2,500 mi) in diameter, wider thanAustralia.The impact that made this crater must have nearly shattered Mimas: the surfaceantipodalto Herschel (opposite through the globe) is highly disrupted, indicating that the shock waves created by the Herschel impact propagated through the whole moon.[21]See for example figure 4 of[22]

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), andcatenae(crater chains).

By studying Mimas's movement, researchers have found that it has a water ocean beneath 20–30 km (12–19 mi) of surface ice. The ocean formed within the last 25 million years, perhaps even the last 2-3 million years, and is thought to be warmed by Saturn's tidal forces.[23]

Orbital resonances[edit]

A number of features inSaturn's ringsare related toresonanceswith Mimas. Mimas is responsible for clearing the material from theCassini Division,the gap between Saturn's two widest rings, theA RingandB Ring.Particles in theHuygens Gapat 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, theG Ringwas found to be in a 7:6 co-rotation eccentricity resonance[24][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 2:1mean-motion resonancewith the larger moonTethys,and in a 2:3 resonance with the outerF Ringshepherdmoonlet,Pandora.A moon co-orbital with Mimas was reported byStephen P. SynnottandRichard J. Terrilein 1982, but was never confirmed.[25][26]

Anomalous libration and subsurface ocean[edit]

In 2014, researchers noted that thelibrationalmotion 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 inhydrostatic equilibrium(an elongatedcore) or aninternal ocean.[27]However, in 2017 it was concluded that the presence of an ocean in Mimas's interior would have led to surface tidal stresses comparable to or greater than those on tectonically activeEuropa.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.[28]The presence of an asymmetric mass anomaly associated with the crater Herschel was considered to be a more likely explanation for the libration.[28]

In 2022, scientists at theSouthwest Research Instituteidentified a tidal heating model for Mimas that produced an internal ocean without any surface cracking or visible tidal stresses. The presence of an internal ocean concealed by a stable icy shell between 24 and 31 km in thickness was found to match the visual and librational characteristics of Mimas as observed byCassini.[29]Continued measurements of Mimas's surface heat flux will be needed in order to confirm this hypothesis.[30]

On February 7, 2024, researchers at theParis Observatoryannounced the discovery that Mimas's orbitapsidally precessesslower than predicted if it were a solid body, which further supports the existence of a subsurface ocean in Mimas. The researchers estimated the ocean to be located 20 to 30 km below the surface, consistent with previous estimates. The researchers suggest that Mimas's ocean must be very young, less than 25 million years old, in order to explain the lack of geological activity on Mimas's cratered surface.[31]

Exploration[edit]

Pioneer 11flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on September 1, 1979.[32]Voyager 1flew by in 1980, andVoyager 2in 1981.

Mimas was imaged several times by theCassiniorbiter,which entered into orbit around Saturn in 2004. A close flyby occurred on February 13, 2010, whenCassinipassed by Mimas at 9,500 km (5,900 mi).

Mimas maps – June 2017
North pole
Global map
South pole

In popular culture[edit]

A temperature map overlay of Mimas that looks like Pac-Man

When seen from certain angles, Mimas resembles theDeath Star,a fictionalspace stationand superweapon known from the 1977 filmStar Wars.Herschelresembles the concave disc of the Death Star's "superlaser". This is a coincidence, as the film was made nearly three years before Mimas was resolved well enough to see the crater.[33]

In 2010, NASA revealed a temperature map of Mimas, using images obtained byCassini.The warmest regions, which are along one edge of Mimas, create a shape similar to the video game characterPac-Man,with Herschel Crater assuming the role of an "edible dot" or"power pellet"known from Pac-Man gameplay.[34][35][36]

Gallery[edit]

  • An image of Mimas taken by Voyager 1 from a distance of 550,000 km (340,000 miles) on November 12, 1980[37] during its flyby of Saturn.
    An image of Mimas taken by Voyager 1 from a distance of 550,000 km (340,000 miles) on November 12, 1980[37]during its flyby of Saturn.
  • The south pole of Mimas taken by Voyager 1, 127,000 km (79,000 miles) away with a resolution of 1 km/pixel.[38]
    The south pole of Mimas taken by Voyager 1, 127,000 km (79,000 miles) away with a resolution of 1 km/pixel.[38]
  • Cassini's image of Mimas passing in front of Saturn's rings, imaged on February 20, 2005.[39]
    Cassini's image of Mimas passing in front of Saturn's rings, imaged on February 20, 2005.[39]
  • Mimas (smallest) together with Rhea and Titan in crescent shape, forming a triple crescent.[40]
    Mimas (smallest) together withRheaandTitanin crescent shape, forming a triple crescent.[40]
  • The side of Mimas that always points away from Saturn, imaged by Cassini on November 19, 2016 at a distance of 85,000 kilometers (53,000 miles).[41]
    The side of Mimas that always points away from Saturn, imaged by Cassini on November 19, 2016 at a distance of 85,000 kilometers (53,000 miles).[41]
  • One of Cassini's final close approach to Mimas, imaged from a distance of 41,230 kilometers (25,620 miles) on January 30, 2017. Sunlight reflecting from Saturn is causing the moon to shine on the left.[42]
    One of Cassini's final close approach to Mimas, imaged from a distance of 41,230 kilometers (25,620 miles) on January 30, 2017. Sunlight reflecting from Saturn is causing the moon to shine on the left.[42]
  • A high-relief image of Mimas by Cassini on January 30, 2017. [43] The shapes and the texture of its many overlapping craters can clearly be seen.
    A high-relief image of Mimas by Cassini on January 30, 2017.[43]The shapes and the texture of its many overlapping craters can clearly be seen.

See also[edit]

References[edit]

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  4. ^Harrison (1908)Prolegomena to the study of Greek religion,ed. 2, p. 514
  5. ^ Harvey, Samantha (April 11, 2007)."NASA: Solar System Exploration: Planets: Saturn: Moons: Mimas: Facts & Figures".NASA.Retrieved2007-10-10.
  6. ^ab 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.ISBN978-1-4020-9216-9.
  7. ^abcJacobson, Robert. A. (1 November 2022)."The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole*".The Astronomical Journal.164(5): 199.Bibcode:2022AJ....164..199J.doi:10.3847/1538-3881/ac90c9.S2CID252992162.
  8. ^ Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007)."Enceladus: Cosmic Graffiti Artist Caught in the Act".Science.315(5813): 815.Bibcode:2007Sci...315..815V.doi:10.1126/science.1134681.PMID17289992.S2CID21932253.Retrieved20 December2011.(supporting online material, table S1)
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  10. ^"Herschel".Gazetteer of Planetary Nomenclature.USGS Astrogeology Research Program.
  11. ^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.80:11.Bibcode:1790RSPT...80....1H.doi:10.1098/rstl.1790.0001.JSTOR106823.
  12. ^Herschel, WilliamPhilosophical Transactions of the Royal Society of London,Vol. 80, reported byArago, M. (1871)."Herschel".Annual Report of the Board of Regents of the Smithsonian Institution:198–223. Archived fromthe originalon 2016-01-13.Retrieved2006-11-26.
  13. ^As reported byWilliam Lassell,Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43(January 14, 1848)
  14. ^Lassell, William (1848)."Satellites of Saturn: Observations of Mimas, the closest and most interior Satellite of Saturn".Monthly Notices of the Royal Astronomical Society.8:42–43.Bibcode:1848MNRAS...8...42L.doi:10.1093/mnras/8.3.42.Retrieved2006-11-26.
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    "Mimas".Merriam-Webster Dictionary.
    "Mimas".Dictionary Unabridged(Online). n.d.
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  19. ^Paul Schenk (2011), Geology of Mimas?, in42nd Lunar and Planetary Science Conference
  20. ^Jane Ellen Harrison (1908) "Orphic Mysteries", inProlegomena to the study of Greek religion,page 514:
  21. ^Elkins-Tanton, Linda E. (2006).Jupiter and Saturn.Infobase Publishing.p. 144.ISBN9781438107257.
  22. ^Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S.; Asphaug, Erik; McKinnon, William B. (October 2004)."Large impact features on middle-sized icy satellites".Icarus.171(2): 421–443.doi:10.1016/j.icarus.2004.05.009.
  23. ^Lainey, V; Rambaux, N; Tobie, G; Cooper, N; Zhang, Q; Noyelles, B; Baillié, K (2024-02-07)."A recently formed ocean inside Saturn's moon Mimas".Nature.626(7998): 280–282.Bibcode:2024Natur.626..280L.doi:10.1038/s41586-023-06975-9.ISSN1476-4687.PMID38326592.S2CID267546453.
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