Mimas#Anomalous libration and subsurface ocean

File:William_Herschel01.jpg Mimas was discovered by the astronomer William Herschel on 17 September 1789. He recorded his discovery as follows:

{{Quote|text=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.Herschel, William Philosophical Transactions of the Royal Society of London, Vol. 80, reported by {{cite journal| last=Arago| first=M.| title=Herschel| journal=Annual Report of the Board of Regents of the Smithsonian Institution| year=1871| pages=198–223| url=http://laplaza.org/~tom/People/Herschel.htm| access-date=2006-11-26| url-status=dead| archive-url=https://web.archive.org/web/20160113070818/http://laplaza.org/~tom/People/Herschel.htm| archive-date=2016-01-13}}|author=William Herschel}}

The 40-foot telescope was a metal mirror reflecting telescope built by Herschel, with a {{convert|48|in|mm|adj=on}} aperture. The 40 feet refers to the length of the focus, not the aperture diameter as is more common with modern telescopes.

File:John_F._Hershel.png, the astronomer who suggested that the moons of Saturn be named after the Titans and Giants]]

Mimas is named after one of the Giants 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.As reported by William Lassell, [http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43] (14 January 1848){{cite journal| last=Lassell| first= William | title = Satellites of Saturn: Observations of Mimas, the closest and most interior Satellite of Saturn| url=http://articles.adsabs.harvard.edu/full/seri/MNRAS/0008//0000042.000.html| journal=Monthly Notices of the Royal Astronomical Society| year=1848| volume=8| pages=42–43| access-date=26 November 2006|bibcode = 1848MNRAS...8...42L| doi=10.1093/mnras/8.3.42| doi-access=free}} Saturn (the Roman equivalent of Cronus in Greek mythology) was the leader of the Titans, the generation before the Gods, and rulers of the world for some time, while the Giants were the subsequent generation, and each group fought a great struggle against Zeus and the Olympians.

The customary English pronunciation of the name is {{IPAc-en|'|m|aɪ|m|ə|s}},{{Cite dictionary |url=http://www.lexico.com/definition/Mimas |archive-url=https://web.archive.org/web/20200327103308/https://www.lexico.com/definition/mimas |url-status=dead |archive-date=27 March 2020 |title=Mimas |dictionary=Lexico UK English Dictionary |publisher=Oxford University Press}}
{{MW|Mimas}}
{{Dict.com|Mimas}} or sometimes {{IPAc-en|'|m|iː|m|ə|s}}.{{Dict.com|Mimas}}

The Greek and Latin root of the name is Mimant- (cf. Italian Mimante, Russian Мимант for the mythological figure),{{Cite web |title=Charlton T. Lewis, Charles Short, A Latin Dictionary, Mĭmas |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=Mimas |website=www.perseus.tufts.edu}} and so the English adjectival form is Mimantean{{Cite web |url=http://saturn.jpl.nasa.gov/science/moons/mimas/ |title=JPL (ca. 2009) Cassini Equinox Mission: Mimas |access-date=10 February 2010 |archive-url=https://web.archive.org/web/20150905123839/http://saturn.jpl.nasa.gov/science/moons/mimas/ |archive-date=5 September 2015 |url-status=dead }} or Mimantian,Paul Schenk (2011), Geology of Mimas?, in 42nd Lunar and Planetary Science Conference either spelling pronounced {{IPAc-en|m|aɪ|ˈ|m|æ|n|t|i|ə|n}} ~ {{IPAc-en|m|ᵻ|ˈ|m|æ|n|t|i|ə|n}}.Jane Ellen Harrison (1908) "Orphic Mysteries", in Prolegomena to the study of Greek religion, page 514:

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek mu (the initial of Mimas) combined with the crook of the Saturn symbol as the symbol of Mimas (16px). This symbol is not widely used.{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}

{{Further|List of geological features on Mimas}}

File:MiniMimas.jpg and the Moon]]

Mimas is the smallest and innermost of Saturn's major moons. The surface area of Mimas is slightly less than the land area of Spain or California. 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 noticeably oblate; its longest axis is about 10% longer than the shortest. The ellipsoidal shape of Mimas is especially noticeable in some recent {{when|date=August 2024}} images from the Cassini probe. Mimas's most distinctive feature is a giant impact crater {{convert|139|km|abbr=on}} across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately {{convert|5|km|0|abbr=on}} high, parts of its floor measure {{convert|10|km|abbr=on|0}} deep, and its central peak rises {{convert|6|km|0|abbr=on}} above the crater floor. If there were a crater of an equivalent scale on Earth (in relative size) it would be over {{convert|4000|km|abbr=on}} in diameter, wider than Australia. The impact that made this crater must have nearly shattered Mimas: the surface antipodal to Herschel (opposite through the globe) is highly disrupted, indicating that the shock waves created by the Herschel impact propagated through the whole moon.{{cite book|title=Jupiter and Saturn|last=Elkins-Tanton|first=Linda E.|page=144|publisher=Infobase Publishing|year=2006|isbn=9781438107257}} See for example figure 4 of{{Cite journal |last1=Moore |first1=Jeffrey M. |last2=Schenk |first2=Paul M. |last3=Bruesch |first3=Lindsey S. |last4=Asphaug |first4=Erik |last5=McKinnon |first5=William B. |date=October 2004 |title=Large impact features on middle-sized icy satellites |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103504001666 |journal=Icarus |language=en |volume=171 |issue=2 |pages=421–443 |doi=10.1016/j.icarus.2004.05.009|bibcode=2004Icar..171..421M }} {{Subscription required}}

File:High Relief Mimas.jpg

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 {{convert|40|km|abbr=on}} in diameter, but in the south polar region, there are generally no craters larger than {{convert|20|km|abbr=on}} in diameter.

Three types of geological features are officially recognised on Mimas: craters, chasmata (chasms), and catenae (crater chains).

By studying Mimas's movement, researchers have found that it has a water ocean beneath {{Convert|20-30|km|mi|abbr=on}} 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.{{Cite journal |last1=Lainey |first1=V |last2=Rambaux |first2=N |last3=Tobie |first3=G |last4=Cooper |first4=N |last5=Zhang |first5=Q |last6=Noyelles |first6=B |last7=Baillié |first7=K |date=7 February 2024 |title=A recently formed ocean inside Saturn's moon Mimas |url=https://www.nature.com/articles/s41586-023-06975-9.epdf?sharing_token=vYAG4f049M5kK2gX7OfZg9RgN0jAjWel9jnR3ZoTv0MN0ukW__TUAf43EkTwisrqQJAdNax6CY-KTbuDYP9edZjI9EdTKQHqBQ7K4elhr_eA7eCcg60b2Fa-ecxLjrdaHL789s2t1YlS3e4-7nwjkbYcddy-Gnh3L73x_IXMKpuk6IXunnHmMERXqVHjOrcX0QRIEEolq8Efgnu3Kz1-6vBxMaerFbOz8sldPmErHwqNcmCFFYGSE3pLfIeaGK87Lisfz_gHA_dN9cqMDnl_asFvF8X4cvtD9qTYp0xwrzzdoKkTp2J5UhquPn7vGF4R |journal=Nature |language=en |volume=626 |issue=7998 |pages=280–282 |doi=10.1038/s41586-023-06975-9 |pmid=38326592 |bibcode=2024Natur.626..280L |s2cid=267546453 |issn=1476-4687}}

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 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, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance{{cite journal|last1= Hedman|first1= M.|last2= Burns|first2= J.|last3= Tiscareno|first3= M.|last4= Porco|first4= C.|last5= Jones|first5= G.|last6= Roussos|first6= E.|last7= Krupp|first7= N.|last8= Paranicas|first8= C.|last9= Kempf|first9= S.|title= The Source of Saturn's G Ring|journal= Science|volume= 317|issue= 5838|date= 3 August 2007|pages= 653–656|doi= 10.1126/science.1143964|pmid= 17673659|bibcode= 2007Sci...317..653H|s2cid= 137345}}{{Clarify|date=November 2012}} with Mimas; the ring's inner edge is about {{convert|15000|km|abbr=on}} inside Mimas's orbit.{{Citation needed|date=November 2011}}

Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora. A moon co-orbital with Mimas was reported by Stephen P. Synnott and Richard J. Terrile in 1982, but was never confirmed.{{Cite web|url=http://www.cbat.eps.harvard.edu/iauc/03600/03660.html|title = IAUC 3660: 1982 BB; Sats OF SATURN; P/SCHWASSMANN-WACHMANN 1}}Guinness Book of Astronomy, Patrick Moore,

Guinness Publishing, second edition, 1983 pp 110, 112

In 2014, researchers noted that the librational motion 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 in hydrostatic equilibrium (an elongated core) or an internal ocean.{{cite journal|last1= Tajeddine|first1= R.|last2= Rambaux|first2= N.|last3= Lainey|first3= V.|last4= Charnoz|first4= S.|last5= Richard|first5= A.|last6= Rivoldini|first6= A.|last7= Noyelles|first7= B.|title= Constraints on Mimas' interior from Cassini ISS libration measurements |journal= Science|volume= 346|issue= 6207|date= 17 October 2014|pages= 322–324|doi= 10.1126/science.1255299|pmid= 25324382|bibcode = 2014Sci...346..322T |s2cid= 206558386}} 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 active Europa. 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.{{cite journal|last1= Rhoden|first1=A. R.|last2= Henning|first2= W.|last3= Hurford|first3=T. A.|last4= Patthoff|first4=D. A.|last5= Tajeddine|first5= R.|title= The implications of tides on the Mimas ocean hypothesis|journal= Journal of Geophysical Research: Planets|volume=122|issue=2|pages=400–410|date= 24 February 2017|doi= 10.1002/2016JE005097|bibcode = 2017JGRE..122..400R |s2cid=132214182 }} The presence of an asymmetric mass anomaly associated with the crater Herschel was considered to be a more likely explanation for the libration.

In 2022, scientists at the Southwest Research Institute identified 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 by Cassini.{{cite news |last=Chang |first=Kenneth |title=An Ocean May Lurk Inside Saturn's 'Death Star' Moon - New research is converting some skeptics to the idea that tiny, icy Mimas may be full of liquid. |url=https://www.nytimes.com/2022/01/21/science/mimas-ocean-death-star.html |date=21 January 2022 |work=The New York Times |accessdate=22 January 2022 }} Continued measurements of Mimas's surface heat flux will be needed to confirm this hypothesis.{{cite press release |url=https://www.swri.org/press-release/swri-scientist-uncovers-evidence-internal-ocean-small-saturn-moon |title=SwRI scientist uncovers evidence for an internal ocean in small Saturn moon |work=Southwest Research Institute |date=19 January 2022 |access-date=20 January 2022}}

On 7 February 2024, researchers at the Paris Observatory announced the discovery that Mimas's orbit apsidally precesses slower 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, to explain the lack of geological activity on Mimas's cratered surface.{{Cite journal |last1=Lainey |first1=V. |last2=Rambaux |first2=N. |last3=Tobie |first3=G. |last4=Cooper |first4=N. |last5=Zhang |first5=Q. |last6=Noyelles |first6=B. |last7=Baillié |first7=K. |date=7 February 2024 |title=A recently formed ocean inside Saturn's moon Mimas |url=https://www.nature.com/articles/s41586-023-06975-9 |journal=Nature |language=en |volume=626 |issue=7998 |pages=280–282 |doi=10.1038/s41586-023-06975-9 |pmid=38326592 |bibcode=2024Natur.626..280L |s2cid=267546453 |issn=1476-4687}}

Pioneer 11 flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on 1 September 1979.{{cite web | url = http://www.dmuller.net/spaceflight/mission.php?mission=pioneer11&appear=black&mtype=scet&showimg=yes&dispwide=no | title = Pioneer 11 Full Mission Timeline | publisher = Dmuller.net | access-date = 26 February 2012 | archive-url = https://web.archive.org/web/20120303151608/http://www.dmuller.net/spaceflight/mission.php?mission=pioneer11&appear=black&mtype=scet&showimg=yes&dispwide=no | archive-date = 3 March 2012 | url-status = dead }} Voyager 1 flew by in 1980, and Voyager 2 in 1981.

Mimas was imaged several times by the Cassini orbiter, which entered into orbit around Saturn in 2004. A close flyby occurred on 13 February 2010, when Cassini passed by Mimas at {{convert|9500|km|mi|abbr=on}}.{{multiple image

| header = Mimas maps – June 2017

| header_align = center

| caption_align = center

| align = center

| direction = horizontal

| image1 = PIA17215-Mimas-NorthPoleMap-SaturnMoon-June2017.jpg

| alt1 =

| caption1 = North pole

| image2 = Map of Mimas 2017-01 PIA17214.jpg

| alt2 =

| caption2 = Global map

| image3 = PIA17215-Mimas-SouthPoleMap-SaturnMoon-June2017.jpg

| width3 = 200

| alt3 =

| caption3 = South pole

| total_width = 700

}}

File:NASA Spacecraft Sees 'Pac-Man' on Saturn Moon (4474329146).jpg

When seen from certain angles, Mimas resembles the Death Star, a fictional space station and superweapon known from the 1977 film Star Wars. Herschel resembles 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.{{cite web | url = https://www.newscientist.com/article.ns?id=dn6999 | title = Saturn's moon is Death Star's twin|date=11 February 2005|access-date=21 August 2008|publisher=New Scientist|first=Kelly|last=Young|quote=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. }}

In 2010, NASA revealed a temperature map of Mimas, using images obtained by Cassini. The warmest regions, which are along one edge of Mimas, create a shape similar to the video game character Pac-Man, with Herschel Crater assuming the role of an "edible dot" or "power pellet" known from Pac-Man gameplay.{{cite web | url = http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100329.html | title = 1980s Video Icon Glows on Saturn Moon|date=29 March 2010|access-date=2 April 2010|publisher=NASA|first=Jia-Rui C.|last=Cook }}{{cite web | url = http://www.nasa.gov/mission_pages/cassini/multimedia/pia12867.html | title = Bizarre Temperatures on Mimas|date=29 March 2010|access-date=2 April 2010|publisher=NASA }}{{cite web | url = https://www.telegraph.co.uk/science/space/7537170/Saturn-moon-looks-like-Pac-Man-in-image-taken-by-Nasa-spacecraft.html | archive-url = https://web.archive.org/web/20100402005025/http://www.telegraph.co.uk/science/space/7537170/Saturn-moon-looks-like-Pac-Man-in-image-taken-by-Nasa-spacecraft.html | url-status = dead | archive-date = 2 April 2010 | title = Saturn moon looks like Pac-Man in image taken by Nasa spacecraft|date=30 March 2010|access-date=2 April 2010|publisher=The Daily Telegraph }}

• List of natural satellites
• Mimas in fiction
• Extraterrestrial liquid water
• Ocean world

{{Reflist

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{{cite web

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| access-date = 10 October 2007

}}

{{cite web

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{{cite book| doi = 10.1007/978-1-4020-9217-6_24| last1 = Roatsch| first1 = T.| last2 = Jaumann| first2 = R.| last3 = Stephan| first3 = K.| last4 = Thomas| first4 = P. C.| year = 2009| chapter = Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data| title = Saturn from Cassini-Huygens| pages = 763–781| isbn = 978-1-4020-9216-9| ref = {{sfnRef|Roatsch Jaumann et al.|2009}}}}

{{cite journal| doi = 10.1126/science.1134681| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = 9 February 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| s2cid = 21932253| ref = {{sfnRef|Verbiscer French et al.|2007}}| url = https://www.science.org/doi/abs/10.1126/science.1134681| access-date = 20 December 2011| url-access = subscription}} (supporting online material, table S1)

{{cite journal| last=Herschel| first=W.| title=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| journal=Philosophical Transactions of the Royal Society of London| year=1790| volume=80| pages=11| doi=10.1098/rstl.1790.0001 | jstor=106823|bibcode=1790RSPT...80....1H | doi-access=free }}

}}

{{Commons}}

{{Spoken Wikipedia|Mimas (moon).ogg|date=10 January 2010}}

• [https://web.archive.org/web/20150905123839/http://saturn.jpl.nasa.gov/science/moons/mimas/ Cassini mission page – Mimas]
• [https://web.archive.org/web/20070801204734/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Mimas Mimas Profile] at [http://solarsystem.nasa.gov/ NASA's Solar System Exploration site]
• [https://web.archive.org/web/20060927042133/http://www.planetary.org/explore/topics/our_solar_system/saturn/mimas.html The Planetary Society: Mimas]
• [https://www.google.com/maps/space/mimas/@2.2166766,-120.6422896,11481535m/data=!3m1!1e3 Google Mimas 3D], interactive map of the moon
• [http://www.nineplanets.org/mimas.html Mimas page at The Nine Planets]
• [http://www.solarviews.com/eng/mimas.htm Views of the Solar System – Mimas]
• [http://ciclops.org/search.php?search=Mimas Cassini images of Mimas]
• [http://photojournal.jpl.nasa.gov/target/Mimas Images of Mimas at JPL's Planetary Photojournal]
• [https://3d-asteroids.space/moons/S1-Mimas 3D shape model of Mimas] (requires WebGL)
• [http://stereomoons.blogspot.com/2010/02/mimas-rising.html Paul Schenk's Mimas blog entry] and movie of [https://www.youtube.com/watch?v=CwWBZFIBkr0 Mimas's rotation] on YouTube
• Mimas [http://www.ciclops.org/view/7311/Map_of_Mimas_-_June_2012 global] and [http://www.ciclops.org/view/7312/Mimas_Polar_Maps_-_June_2012 polar] basemaps (June 2012) from Cassini images
• [http://www.ciclops.org/view.php?id=6467 Mimas atlas (July 2010) from Cassini images]
• [http://planetarynames.wr.usgs.gov/Page/MIMAS/target Mimas nomenclature] and [http://planetarynames.wr.usgs.gov/images/mimas_comp.pdf Mimas map with feature names] from the [http://planetarynames.wr.usgs.gov/ USGS planetary nomenclature page]
• [https://lh5.googleusercontent.com/-nvePMCOxbew/TXqwuSFdvzI/AAAAAAAAAUw/CNhsEQXwSDU/postercorrect.jpg Figure "J" is Mimas transiting Saturn in 1979, imaged by Pioneer 11] [http://planetimages.blogspot.com/2011/03/wanted-in-digital-formatand-my-2011.html from here]

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Category:Moons with a prograde orbit