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Mimas , also designated Saturn I, is a
moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...
of
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
which was discovered in 1789 by
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

William Herschel
. It is named after Mimas, a son of
Gaia In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A be ...
in
Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psyc ...
. With a diameter of , it is the smallest
astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowled ...
that is known to still be rounded in shape because of
self-gravitation Self-gravity is the Gravity, gravitational force exerted on a body, or a group of bodies, by the body(ies) that allows it/them to be held together.Chamberlin, T. C. The Planetesimal Hypothesis. Journal of the Royal Astronomical Society of Canada, ...
. However, Mimas is not actually in
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ph ...

hydrostatic equilibrium
for its current rotation.


Discovery

Mimas was discovered by the
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

astronomer
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

William Herschel
on 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." The
40-foot telescope William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer of music. He frequently collaborated with his younger sister and ...
was a metal mirror reflecting telescope built by Herschel, with a aperture. The 40 feet refers to the length of the focus, not the aperture diameter as more common with modern telescopes.


Name

Mimas is named after one of the
Giant In folklore Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psychology), attitude that something is the case, or that so ...
s in
Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psyc ...
, Mimas. The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's son
John John is a common English name and surname: * John (given name) John is a common English name and surname: * John (given name) * John (surname), including a list of people who have the name John John may also refer to: New Testament Works ...
in his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope''. Saturn (the Roman equivalent of
Cronus In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practice ...
in Greek mythology) was the leader of the Titans, the generation before the Gods, and ruler of the world for some time. The Giants were the subsequent generation, and each group fought a great struggle against the Gods. The customary English pronunciation of the name is , though some people attempt a more 'authentic' pronunciation, . The Greek and Latin root of the name is ''Mimant-'', and so the English adjectival form is ''Mimantean'' or ''Mimantian'', either spelling pronounced ~ .


Physical characteristics

The surface area of Mimas is slightly less than the land area of
Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = , national_anthem = , image_map = , map_caption = , image_map2 ...

Spain
. 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, Mimas is noticeably
prolate A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric In mathematics, a quadric or quadric surface (quadric hypersurface in higher dimension thumb , 236px , The first four spatial dimensions, repres ...
; its longest axis is about 10% longer than the shortest. The ellipsoidal shape of Mimas is especially noticeable in some recent images from the '' Cassini'' probe. Mimas's most distinctive feature is a giant
impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller ...

impact crater
across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately high, parts of its floor measure deep, and its central peak rises above the crater floor. If there were a crater of an equivalent scale on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
(in relative size) it would be over in diameter, wider than
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
. The impact that made this crater must have nearly shattered Mimas: the surface antipodal to (opposite through the globe) Herschel is highly disrupted, indicating that the shock waves created by the Herschel impact propagated through the whole moon. 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 in diameter, but in the south polar region, there are generally no craters larger than in diameter. Three types of geological features are officially recognized on Mimas:
crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surface ...

crater
s,
chasma In planetary nomenclature, 1st edition (1881), predating IAU conventions Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can b ...
ta (chasms) and catenae (crater chains).


Orbital resonances

A number of features in
Saturn's rings The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely ...

Saturn's rings
are related to
resonances Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a Periodic function, periodically applied force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the syst ...
with Mimas. Mimas is responsible for clearing the material from the
Cassini Division The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely ...

Cassini Division
, the gap between Saturn's two widest rings, the
A Ring The rings of Saturn are the most extensive ring system A ring system is a disc or ring orbiting an astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, association, or ...
and
B Ring The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
. 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 RingG Ring may refer to: * Rings of Saturn, a planetary ring system around Saturn. * G-ring or Grothendieck ring, a type of commutative ring in algebra {{Disambig ...
was found to be in a 7:6 co-rotation eccentricity resonance with Mimas; the ring's inner edge is about inside Mimas's orbit. Mimas is also in a 2:1
mean-motion resonance . Conjunctions are highlighted by brief color changes. There are two Io-Europa conjunctions (green) and three Io-Ganymede conjunctions (grey) for each Europa-Ganymede conjunction (magenta). This diagram is not to scale. In celestial mechanics, ...
with the larger moon Tethys, and in a 2:3 resonance with the outer
F Ring The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
shepherd A shepherd or sheepherder is a person who tends, herds, feeds, or guards herds of sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), o ...
moonlet,
Pandora In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A bel ...
. A moon co-orbital with Mimas was reported by Stephen P. Synnott and Richard J. Terrile in 1982, but was never confirmed.


Anomalous libration

In 2014, researchers noted that the
libration In lunar astronomy, libration is the wagging or wavering of the Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the l ...

libration
al 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 In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ph ...
(an elongated
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
) or an
internal ocean The ocean (also the sea or the world ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
. However, in 2017 it was concluded that the presence of an ocean in Mimas' interior would have led to surface tidal stresses comparable to or greater than those on tectonically active
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
. 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. 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 Southwest Research Institute (SwRI), headquartered in San Antonio ("Cradle of Freedom") , image_map = Bexar SanAntonio.svg , mapsize = 220px , map_caption = Location within Bexar County , pushpi ...
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 to 31 km in thickness was found to match the visual and librational characteristics of Mimas as observed by ''Cassini''. Continued measurements of Mimas' surface heat flux will be needed in order to confirm this hypothesis.


Exploration

''
Pioneer 11 ''Pioneer 11'' (also known as ''Pioneer G'') is a robotic Robotics is an interdisciplinarity, interdisciplinary field that integrates computer science and engineering. Robotics involves design, construction, operation, and use of robots. T ...
'' flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on September 1, 1979. ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel ...
'' flew by in 1980, and ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'' in 1981. Mimas was 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 .


In popular culture

When seen from certain angles, Mimas resembles the
Death Star The Death Star is a fictional mobile space station and galactic Weapon of mass destruction, superweapon featured in the ''Star Wars'' Space opera, space-opera franchise. The first version, which appears in the original 1977 film Star Wars (film) ...
, a fictional
space station A space station, also known as an orbital station or an orbital space station, is a spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite alt=, A full-size model of the Ear ...

space station
and superweapon known from the 1977 film ''
Star Wars ''Star Wars'' is an American epic film, epic space opera multimedia franchise created by George Lucas, which began with the Star Wars (film), eponymous 1977 film and quickly became a worldwide popular culture, pop-culture Cultural impact of S ...
''. Herschel resembles the concave disc of the Death Star's "superlaser". This is coincidental, as the film was made nearly three years before Mimas was resolved well enough to see the crater. 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 is a 1980 maze A maze is a path or collection of paths, typically from an entrance to a goal. The word is used to refer both to branching tour puzzles through which the solver must find a route, and to simpler non-branching ("unicursa ...

Pac-Man
, with Herschel Crater assuming the role of an "edible dot" or "power pellet" known from Pac-Man gameplay.


Gallery

File:CassiniPhotographMimasSaturn.jpg, Mimas is the tiny white dot in the lower left. (Click to enlarge view.) File:Mimas (NASA) PIA06176.jpg, Mimas, silhouetted against Saturn's northern latitudes. File:Mimas and F ring PIA09806.jpg, Mimas, behind the
F Ring The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
. File:Mimas shape.jpg, Mimas viewed by ''Cassini'' looking distinctly egg-shaped. File:PIA20515 - Mimas' Mountain (cropped).jpg, Mimas and mountain
(October 22, 2016). File:Mimas before limb sharp (colored).jpg, Mimas before Saturn's limb (color added)
(February 13, 2010). File:Mimas PIA12569.jpg, Mimas mosaic with mostly high resolution. File:Color Near Herschel Crater.jpg, Mimas displays subtle color differences
(
false-color False color (or pseudo color) refers to a group of color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English ...
). File:Mimas limb sharp.jpg, Mimas' albedo features on crater walls (Herschel at lower right) File:Mimas (NASA) map.jpg, Mimas texture map File:NASA Spacecraft Sees 'Pac-Man' on Saturn Moon (4474329146).jpg, Temperature map overlay of Mimas, commonly said to resemble
Pac-Man is a 1980 maze A maze is a path or collection of paths, typically from an entrance to a goal. The word is used to refer both to branching tour puzzles through which the solver must find a route, and to simpler non-branching ("unicursa ...

Pac-Man
.


See also

*
List of natural satellites The Solar System's planets, and its list of possible dwarf planets, most likely dwarf planets, are known to be orbited by at least 219 natural satellites, or moons. At least List of gravitationally rounded objects of the Solar System#Satellites ...
* Mimas in fiction


References


External links


''Cassini'' mission page – Mimas

Mimas Profile
a
NASA's Solar System Exploration site



Google Mimas 3D
interactive map of the moon




''Cassini'' images of Mimas

Images of Mimas at JPL's Planetary Photojournal

3D shape model of Mimas
(requires WebGL)

and movie o
Mimas's rotation
on
YouTube YouTube is an American online video sharing and social media platform Social media are interactive technologies that allow the Content creation, creation or information sharing, sharing/exchange of information, ideas, career interests, an ...

YouTube
* Mima
global
an
polar
basemaps (June 2012) from ''Cassini'' images
Mimas atlas (July 2010) from ''Cassini'' images

Mimas nomenclature
an
Mimas map with feature names
from th
USGS planetary nomenclature page

Figure "J" is Mimas transiting Saturn in 1979, imaged by ''Pioneer 11''
{{Authority control 17890917 Discoveries by William Herschel