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Mimas, also designated Saturn I, is the seventh-largest
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. With a
mean diameter In applied sciences, the equivalent radius (or mean radius) is the radius of a circle or sphere with the same perimeter, area, or volume of a non-circular or non-spherical object. The equivalent diameter (or mean diameter) (D) is twice the equiva ...
of , Mimas is the smallest
astronomical body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical object, physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ...
known 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 is Herschel, one of the largest craters relative to the size of the parent body in the Solar System. Herschel measures across, about one-third of Mimas's mean diameter, and formed from an extremely energetic
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
. The crater's name is derived from the discoverer of Mimas,
William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...
, in 1789. The moon's presence has created one of the largest 'gaps' in Saturn's ring, named the
Cassini Division Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range fro ...
, due to
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
destabilising the particles' orbit there.


Discovery

Mimas was discovered by the
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...
on 17 September 1789. He recorded his discovery as follows: The
40-foot telescope William Herschel's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope constructed between 1785 and 1789 at Observatory House in Slough, England. It used a diameter primary mirror with a focal length (h ...
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 is more common with modern telescopes.


Name

Mimas is named after one of the
Giant In folklore, giants (from Ancient Greek: ''wiktionary:gigas, gigas'', cognate wiktionary:giga-, giga-) are beings of humanoid appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''gia ...
s in
Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
,
Mimas Mimas, also designated Saturn I, is the seventh-largest natural satellite of Saturn. With a mean diameter of , Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15  ...
. 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 (surname) John may also refer to: New Testament Works * Gospel of John, a title often shortened to John * First Epistle of John, often shortened to 1 John * Second E ...
in his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope''. Saturn (the Roman equivalent of
Cronus In ancient Greek religion and Greek mythology, mythology, Cronus, Cronos, or Kronos ( or ; ) was the leader and youngest of the Titans, the children of Gaia (Earth) and Uranus (mythology), Uranus (Sky). He overthrew his father and ruled dur ...
in Greek mythology) was the leader of the Titans, the generation before the
Gods A deity or god is a supernatural being considered to be sacred and worthy of worship due to having authority over some aspect of the universe and/or life. The ''Oxford Dictionary of English'' defines ''deity'' as a God (male deity), god or god ...
, and rulers of the world for some time, while the Giants were the subsequent generation, and each group fought a great struggle against
Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...
and the Olympians. The customary English pronunciation of the name is , or sometimes . The Greek and Latin root of the name is ''Mimant-'' (cf. Italian ''Mimante'', Russian ''Мимант'' for the mythological figure), and so the English adjectival form is ''Mimantean'' or ''Mimantian'', either spelling pronounced ~ . 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 A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
symbols, proposed a Greek mu (the initial of Mimas) combined with the crook of the Saturn symbol as the symbol of Mimas (). This symbol is not widely used.


Physical characteristics

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 Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
or
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
. 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 In Christianity (specifically the Roman Catholic, Orthodox, Lutheran, Anglican and Methodist traditions), an oblate is a person associated with a Benedictine monastery or convent who is specifically dedicated to God and service. Oblates are i ...
; its longest axis is about 10% longer than the shortest. The
ellipsoidal An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as the ze ...
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 a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
(in relative size) it would be over in diameter, wider than
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
. The impact that made this crater must have nearly shattered Mimas: the surface
antipodal Antipode or Antipodes may refer to: Mathematics * Antipodal point, the diametrically opposite point on a circle or ''n''-sphere, also known as an antipode * Antipode, the convolution inverse of the identity on a Hopf algebra Geography * Antipodes ...
to Herschel (opposite through the globe) is highly disrupted, indicating that the shock waves created by the Herschel impact propagated through the whole moon. See for example figure 4 of 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 recognised on Mimas:
crater A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...
s,
chasma In planetary nomenclature, a chasma (''plural'': chasmata ) is a deep, elongated, steep-sided Depression (geology), depression. As of 2020, the International Astronomical Union, IAU has named 122 such features in the Solar System, on Venus (63), ...
ta (chasms), and catenae (crater chains). By studying Mimas's movement, researchers have found that it has a water ocean beneath 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.


Orbital resonances

A number of features in
Saturn's rings Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range fro ...
are related to
resonances Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
with Mimas. Mimas is responsible for clearing the material from the
Cassini Division Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range fro ...
, the gap between Saturn's two widest rings, the
A Ring Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range fro ...
and
B Ring Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range from ...
. 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 with Mimas; the ring's inner edge is about inside Mimas's orbit. Mimas is also in a 2:1
mean-motion resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
with the larger moon Tethys, and in a 2:3 resonance with the outer
F Ring Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of rocky material. Particles range from ...
shepherd A shepherd is a person who tends, herds, feeds, or guards flocks of sheep. Shepherding is one of the world's oldest occupations; it exists in many parts of the globe, and it is an important part of Pastoralism, pastoralist animal husbandry. ...
moonlet,
Pandora In Greek mythology, Pandora was the first human woman created by Hephaestus on the instructions of Zeus. As Hesiod related it, each god cooperated by giving her unique gifts. Her other name—inscribed against her figure on a white-ground '' ky ...
. A moon co-orbital with Mimas was reported by
Stephen P. Synnott Stephen P. Synnott (born 1946) is an American astronomer and Voyager scientist at JPL, and expert in spacecraft optical navigation techniques. He has discovered several natural satellites of outer Solar System planets such as Metis, Puck, Lari ...
and Richard J. Terrile in 1982, but was never confirmed.


Anomalous libration and subsurface ocean

In 2014, researchers noted that the
libration In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...
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, also called hydrostatic balance and 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. I ...
(an elongated
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...
) or an internal ocean. 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 Europa may refer to: Places * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliffs, Alexan ...
. 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, Texas, is an independent and nonprofit applied research and development (R&D) organization. Founded in 1947 by oil businessman Tom Slick, it provides contract research and deve ...
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''. Continued measurements of Mimas's surface heat flux will be needed to confirm this hypothesis. On 7 February 2024, researchers at the
Paris Observatory The Paris Observatory (, ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Ban ...
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.


Exploration

''
Pioneer 11 ''Pioneer 11'' (also known as ''Pioneer G'') is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to Exploration ...
'' flew by Saturn in 1979, and its closest approach to Mimas was 104,263 km on 1 September 1979. ''
Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar medium, interstellar space beyond the Sun's heliosphere. It was launched 16 days afte ...
'' flew by in 1980, and ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
'' 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 .


In popular culture

When seen from certain angles, Mimas resembles the
Death Star The Death Star is a fictional space station and superweapon featured in the ''Star Wars'' space-opera franchise. Constructed by the autocratic Galactic Empire, the Death Star is capable of obliterating entire planets, and serves to enforce ...
, a fictional
space station A space station (or orbital station) is a spacecraft which remains orbital spaceflight, in orbit and human spaceflight, hosts humans for extended periods of time. It therefore is an artificial satellite featuring space habitat (facility), habitat ...
and superweapon known from the 1977 film ''
Star Wars ''Star Wars'' is an American epic film, epic space opera media franchise created by George Lucas, which began with the Star Wars (film), eponymous 1977 film and Cultural impact of Star Wars, quickly became a worldwide popular culture, pop cu ...
''. 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. 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 ''Pac-Man,'' originally called in Japan, is a 1980 maze video game developed and published by Namco for arcades. In North America, the game was released by Midway Manufacturing as part of its licensing agreement with Namco America. The pla ...
, with Herschel Crater assuming the role of an "edible dot" or "power pellet" known from Pac-Man gameplay.


See also

*
List of natural satellites Of the Solar System's eight planets and its list of possible dwarf planets, nine most likely dwarf planets, six planets and seven dwarf planets are known to be orbited by at least 430 natural satellites, or moons. At least List of gravitational ...
* Mimas in fiction *
Extraterrestrial liquid water Extraterrestrial may refer to: Science * Extraterrestrial life, life that occurs outside of Earth and that probably did not originate from Earth Media * ''Extraterrestrial'' (TV program), a program on the National Geographic Channel * '' Extrate ...
*
Ocean world An ocean world, ocean planet or water world is a type of planet or natural satellite that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the s ...


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 social media and online video sharing platform owned by Google. YouTube was founded on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim who were three former employees of PayPal. Headquartered in ...
* 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 Moons with a prograde orbit