Deimos (moon)

Deimos (moon)

Overview
Deimos is the smaller and outer of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

's two moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 (the other being Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

). It is named after Deimos
Deimos (mythology)
In Greek mythology, Deimos was the personification of terror.He was the son of Ares and Aphrodite. He is the twin brother of Phobos and the goddess Enyo who accompanied Ares into battle, as well as his father's attendants, Trembling, Fear, Dread, and Panic...

, a figure representing dread
Fear
Fear is a distressing negative sensation induced by a perceived threat. It is a basic survival mechanism occurring in response to a specific stimulus, such as pain or the threat of danger...

 in Greek Mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. Its systematic designation is .



Deimos was discovered by Asaph Hall, Sr.
Asaph Hall
Asaph Hall III was an American astronomer who is most famous for having discovered the moons of Mars in 1877...

 at the US Naval Observatory in Washington, D.C on August 12, 1877, at about 07:48 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (given in contemporary sources as "August 11 14:40" Washington mean time
Washington mean time
Washington mean time was the time at the meridian through the center of the old dome atop the main building at the old US Naval Observatory at Washington, D.C. This Washington meridian was defined on 28 September 1850 by the United States Congress...

, using an astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time).
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Encyclopedia
Deimos is the smaller and outer of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

's two moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 (the other being Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

). It is named after Deimos
Deimos (mythology)
In Greek mythology, Deimos was the personification of terror.He was the son of Ares and Aphrodite. He is the twin brother of Phobos and the goddess Enyo who accompanied Ares into battle, as well as his father's attendants, Trembling, Fear, Dread, and Panic...

, a figure representing dread
Fear
Fear is a distressing negative sensation induced by a perceived threat. It is a basic survival mechanism occurring in response to a specific stimulus, such as pain or the threat of danger...

 in Greek Mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. Its systematic designation is .

Discovery




Deimos was discovered by Asaph Hall, Sr.
Asaph Hall
Asaph Hall III was an American astronomer who is most famous for having discovered the moons of Mars in 1877...

 at the US Naval Observatory in Washington, D.C on August 12, 1877, at about 07:48 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (given in contemporary sources as "August 11 14:40" Washington mean time
Washington mean time
Washington mean time was the time at the meridian through the center of the old dome atop the main building at the old US Naval Observatory at Washington, D.C. This Washington meridian was defined on 28 September 1850 by the United States Congress...

, using an astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time). Hall also discovered Phobos on August 18, 1877, at about 09:14 GMT, after deliberately searching for Martian moons.

The names, at first spelled Phobus and Deimus, were suggested by Henry Madan (1838–1901), Science Master of Eton
Eton College
Eton College, often referred to simply as Eton, is a British independent school for boys aged 13 to 18. It was founded in 1440 by King Henry VI as "The King's College of Our Lady of Eton besides Wyndsor"....

, from Book XV of the Iliad
Iliad
The Iliad is an epic poem in dactylic hexameters, traditionally attributed to Homer. Set during the Trojan War, the ten-year siege of the city of Troy by a coalition of Greek states, it tells of the battles and events during the weeks of a quarrel between King Agamemnon and the warrior Achilles...

, where Ares
Ares
Ares is the Greek god of war. He is one of the Twelve Olympians, and the son of Zeus and Hera. In Greek literature, he often represents the physical or violent aspect of war, in contrast to the armored Athena, whose functions as a goddess of intelligence include military strategy and...

 (the Roman god Mars) summons Dread (Deimos
Deimos (mythology)
In Greek mythology, Deimos was the personification of terror.He was the son of Ares and Aphrodite. He is the twin brother of Phobos and the goddess Enyo who accompanied Ares into battle, as well as his father's attendants, Trembling, Fear, Dread, and Panic...

) and Fear (Phobos
Phobos (mythology)
Phobos is the personification of horror in Greek mythology. He is the offspring of Ares and Aphrodite. He was known for accompanying Ares into battle along with his brother, Deimos, the goddess Enyo, and his father’s attendants. Timor is his Roman equivalent...

).

Characteristics



Physical


Deimos, like Mars' other moon Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

, has spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

s and densities
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 similar to those of a C-
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...

 or D-type asteroid
D-type asteroid
D-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors...

. Like most bodies of its size, Deimos is highly non-spherical with dimensions of 15 × 12.2 × 10.4 km. Deimos is composed of rock rich in carbonaceous material, much like C-type asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

 meteorites. It is cratered
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

. The regolith is highly porous
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 and has a radar estimated density of only 1.1 g/cm³. The two largest craters, Swift
Swift (Deimian crater)
Swift crater is a crater on Mars's moon Deimos. It is about in diameter. Swift crater is named after Jonathan Swift, who predicted the existence of the moons of Mars. Swift crater is one of two named features on Deimos, the other being Voltaire crater. On 10 July 2006, Mars Global Surveyor took an...

 and Voltaire
Voltaire (crater)
Voltaire is an impact crater on Mars' moon Deimos and is approximately across. Voltaire crater is named after François-Marie Arouet, a French Enlightenment writer who was better known by the pen name Voltaire. Voltaire crater is one of two named features on Deimos, the other being Swift crater...

, each measure about 3 kilometres across.

Orbital


Deimos' orbit is nearly circular and is close to Mars' equatorial plane. Mars' outer moon is possibly an asteroid that was perturbed by Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 into an orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed. Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination of atmospheric drag and tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s, although it is not clear that sufficient time is available for this to occur for Deimos.

As seen from the surface of Deimos, Mars would appear 1,000 times larger and 400 times brighter than the full Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 as seen from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, taking up one-eleventh of the width of a celestial hemisphere.

As seen from Mars, Deimos would have an angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 of no more than 2.5 minutes (sixty minutes make one degree) and would therefore appear almost star-like to the naked eye. At its brightest ("full moon") it would be about as bright as Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 is from Earth; at the first- or third-quarter phase it would be about as bright as Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...

. With a small telescope, a Martian observer could see Deimos' phases, which take 1.2648 days (Deimos' synodic period) to run their course.

Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets in the west. However, the Sun-synodic orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol
Timekeeping on Mars
Various schemes have been used or proposed to keep track of time and date on the planet Mars independently of Earth time and calendars.Mars has an axial tilt and a rotation period similar to those of Earth. Thus it experiences seasons of spring, summer, autumn and winter much like Earth, and its...

") of about 24.7 hours by such a small amount that 2.7 days elapse between its rising and setting for an equatorial observer.

Because Deimos’ orbit is relatively close to Mars and has only a very small inclination to Mars’ equator, it cannot be seen from Martian latitudes greater than 82.7°.

Solar transits




Deimos regularly passes in front of the Sun
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

 as seen from Mars. It is too small to cause a total eclipse
Total Eclipse
A total eclipse is an eclipse where either the Sun is entirely covered by the Moon, or the Earth's shadow entirely covers the Moon.Total Eclipse may also refer to:-Music:* Total Eclipse , a Goa trance music group...

, appearing only as a small black dot traveling across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during a transit of Venus
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...

 from Earth. On March 4, 2004 a transit of Deimos was photographed by Mars Rover Opportunity, and on March 13, 2004 a transit was photographed by Mars Rover Spirit.

Origin


The origin of the Martian moons is still controversial. The main hypotheses are that they formed either by capture
Capture
Capture may refer to:* Capture , to remove the opponent's piece from the board by taking it with one's own piece* Capture , situations in which a government agency created to act in the public interest instead acts in favor of other interests...

 or by accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

. Because of the similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

, with orbits that have been circularized either by atmospheric drag or tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s, as capture requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis
Geoffrey A. Landis
Geoffrey A. Landis is an American scientist, working for the National Aeronautics and Space Administration on planetary exploration, interstellar propulsion, solar power and photovoltaics...

 has pointed out that the capture could have occurred if the original body was a binary asteroid
Binary asteroid
A binary asteroid is a system of two asteroids orbiting their common center of mass, in analogy with binary stars. 243 Ida was the first binary asteroid to be identified when the Galileo spacecraft did a flyby in 1993...

 that separated due to tidal forces. The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

.

Named geological features



Only two geological features on Deimos have been given names. The craters Swift and Voltaire are named after writers who speculated on the existence of Martian moons before they were discovered.
Crater Named after Coordinates Diameter (m)
Swift
Swift (Deimian crater)
Swift crater is a crater on Mars's moon Deimos. It is about in diameter. Swift crater is named after Jonathan Swift, who predicted the existence of the moons of Mars. Swift crater is one of two named features on Deimos, the other being Voltaire crater. On 10 July 2006, Mars Global Surveyor took an...

 
Jonathan Swift
Jonathan Swift
Jonathan Swift was an Irish satirist, essayist, political pamphleteer , poet and cleric who became Dean of St...

 
12.5°N 358.2°W 1000
Voltaire
Voltaire (crater)
Voltaire is an impact crater on Mars' moon Deimos and is approximately across. Voltaire crater is named after François-Marie Arouet, a French Enlightenment writer who was better known by the pen name Voltaire. Voltaire crater is one of two named features on Deimos, the other being Swift crater...

 
Voltaire
Voltaire
François-Marie Arouet , better known by the pen name Voltaire , was a French Enlightenment writer, historian and philosopher famous for his wit and for his advocacy of civil liberties, including freedom of religion, free trade and separation of church and state...

 
22°N 3.5°W 1900

Namesakes


USS Deimos (AK-78)
USS Deimos (AK-78)
USS Deimos was a in the service of United States Navy in World War II. It was the first ship of the Navy to have borne the name Deimos, after one of the moons of Mars....

 was a United States Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 Crater class cargo ship
Crater class cargo ship
Crater-class cargo ship is a category of freighter that was constructed for use by the United States Navy during World War II under Maritime Commission EC2-S-C1 type....

named after the moon.

External links