Phobos (moon)
Overview
 
Phobos is the larger and closer of the two natural satellites 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...

. Both moons were discovered in 1877. With a mean radius of 11.1 km (6.9 mi), Phobos is 7.24 times as massive as Deimos. It is named after the Greek god
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...

 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...

 (which means "fear"), a son of 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...

 (Mars).

A small, irregularly shaped object, Phobos orbits about 9377 km (5,826.6 mi) from the center of Mars, closer to its primary
Primary (astronomy)
A primary is the main physical body of a gravitationally-bound, multi-object system. This body contributes most of the mass of that system and will generally be located near its center of mass....

 than any other known planetary moon.
Encyclopedia
Phobos is the larger and closer of the two natural satellites 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...

. Both moons were discovered in 1877. With a mean radius of 11.1 km (6.9 mi), Phobos is 7.24 times as massive as Deimos. It is named after the Greek god
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...

 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...

 (which means "fear"), a son of 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...

 (Mars).

A small, irregularly shaped object, Phobos orbits about 9377 km (5,826.6 mi) from the center of Mars, closer to its primary
Primary (astronomy)
A primary is the main physical body of a gravitationally-bound, multi-object system. This body contributes most of the mass of that system and will generally be located near its center of mass....

 than any other known planetary moon. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater
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...

, Stickney crater. It orbits so close to the planet that it moves around Mars faster than Mars itself rotates. As a result, from the surface of Mars it appears to rise in the west, move rapidly across the sky (in 4 h 15 min or less) and set in the east. Due to its short orbital period and tidal interactions, Phobos's orbital radius is decreasing and it will eventually either impact the surface of Mars or break up into a planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

.

Discovery

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

 on August 18, 1877, at the United States Naval Observatory
United States Naval Observatory
The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...

 in Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....

, at about 09:14 Greenwich Mean Time
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...

 (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 17 at 16:06 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...

). Hall also discovered Deimos
Deimos (moon)
Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...

, Mars's other moon, on August 12, 1877 at about 07:48 UTC. The names, originally spelled Phobus and Deimus respectively, 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"....

, based on 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...

, in which the god 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...

 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...

).

Physical characteristics

Phobos is one of the least reflective bodies in the Solar System. Spectroscopically it appears to be similar to the 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...

s, and is apparently of composition similar to 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...

 material. Phobos's density is too low to be solid rock, and it is known to have significant porosity
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%...

. These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface 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:...

 layer lacks hydration, but ice below the regolith is not ruled out.

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. Recent images from Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...

indicate that Phobos is covered with a layer of fine-grained 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:...

 at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.

Phobos is highly non-spherical, with dimensions of .

Phobos is heavily crater
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...

ed, with one of the craters near the equator having a central peak despite the moon's small size. The most prominent surface feature is Stickney crater, named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name. As with Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

's crater Herschel, the impact that created Stickney must have nearly shattered Phobos. Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 metres (98.4 ft) deep, 100 metre wide, and up to 20 kilometres (12.4 mi) in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...

spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspect that they have been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as crater chain
Crater chain
A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena , as specified by the International Astronomical Union's rules on planetary nomenclature....

s, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events. Grooves are also seen on asteroid 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

, but scientists are not currently suggesting a similar mechanism for forming those grooves.

The unique Kaidun meteorite
Kaidun meteorite
Kaidun is a meteorite that fell on 3 December 1980 on a Soviet military base near what is now Al-Khuraybah in Yemen. A fireball was observed travelling from the northwest to the southeast, and a single stone weighing about 2 kilograms was recovered from a small impact pit...

 is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.

Named geological features

Geological features on Phobos are named after astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s who studied Phobos and people and places from Jonathan Swift
Jonathan Swift
Jonathan Swift was an Irish satirist, essayist, political pamphleteer , poet and cleric who became Dean of St...

's Gulliver's Travels
Gulliver's Travels
Travels into Several Remote Nations of the World, in Four Parts. By Lemuel Gulliver, First a Surgeon, and then a Captain of Several Ships, better known simply as Gulliver's Travels , is a novel by Anglo-Irish writer and clergyman Jonathan Swift that is both a satire on human nature and a parody of...

. There is one named regio
Regio
Regio is the Latin word for ‘region’ and the Italian word for ‘royal’It may refer to:*Regio - in planetary geology, a large area of a planet or moon that is strongly differentiated in colour or albedo**A regio of Venus:...

, Laputa Regio, and one named planitia, Lagado Planitia; both are named after places in Gulliver's Travels (the fictional Laputa
Laputa
Laputa is a fictional place from the book Gulliver's Travels by Jonathan Swift.Laputa is a fictional flying island or rock, about 4.5 miles in diameter, with an adamantine base, which its inhabitants can maneuver in any direction using magnetic levitation...

, a flying island, and Lagado
Lagado
Lagado is a fictional city from the book of Gulliver's Travels by Jonathan Swift.Lagado is the capital of the nation Balnibarbi, which is ruled by a tyrannical king from a flying island called Laputa. Lagado is on the ground below Laputa, and also has access to Laputa at any given time to proceed...

, imaginary capital of the fictional nation Balnibarbi). The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

. Several craters have been named.
Crater Named after Coordinates
Clustril  Character in Gulliver's Travels 60°N 91°W
D'Arrest  Heinrich Louis d'Arrest
Heinrich Louis d'Arrest
Heinrich Louis d'Arrest was a German astronomer, born in Berlin. His name is sometimes given as Heinrich Ludwig d'Arrest....

, astronomer
39°S 179°W
Drunlo  Character in Gulliver's Travels 36.5°N 92°W
Flimnap  Character in Gulliver's Travels 60°N 350°W
Grildrig  Character in Gulliver's Travels 81°N 195°W
Gulliver  Main character of Gulliver's Travels 62°N 163°W
Hall  Asaph Hall
Asaph Hall
Asaph Hall III was an American astronomer who is most famous for having discovered the moons of Mars in 1877...

, discoverer of Phobos
80°S 210°W
Limtoc  Character in Gulliver's Travels 11°S 54°W
Öpik  Ernst J. Öpik, astronomer 7°S 297°W
Reldresal  Character in Gulliver's Travels 41°N 39°W
Roche  Édouard Roche
Édouard Roche
Édouard Albert Roche was a French astronomer and mathematician, who is best known for his work in the field of celestial mechanics...

, astronomer
53°N 183°W
Sharpless  Bevan Sharpless
Bevan Sharpless
Bevan P. Sharpless was an American astronomer. He concluded in 1944 that the orbit of Phobos was decaying and reported his observations at the US Naval Observatory in the Astronomical Journal in 1945. A crater on Phobos is named after him....

, astronomer
27.5°S 154°W
Shklovsky  Iosif Shklovsky
Iosif Shklovsky
Iosif Samuilovich Shklovsky was a Soviet astronomer and astrophysicist...

, astronomer
24°N 248°W
Skyresh  Character in Gulliver's Travels 52.5°N 320°W
Stickney
Stickney (crater)
Stickney is the largest crater on Phobos, which is a satellite of Mars. It is located at and is in diameter, taking up a substantial proportion of the moon's surface.It is named after Chloe Angeline Stickney Hall, wife of Phobos's discoverer, Asaph Hall...

 
Angeline Stickney
Angeline Stickney
Chloe Angeline Stickney Hall , suffragist, abolitionist, and mathematician, was the wife of astronomer Asaph Hall...

, wife of Asaph Hall
1°N 49°W
Todd  David Peck Todd
David Peck Todd
David Peck Todd was a noted American astronomer. He produced a complete set of photographs of the 1882 transit of Venus.-Biography:...

, astronomer
9°S 153°W
Wendell  Oliver Wendell
Oliver Wendell
Oliver Wendell was an American astronomer. A crater on Phobos is named after him.-References:...

, astronomer
1°S 132°W

Orbital characteristics

Phobos's unusually close orbit around its parent planet produces some unusual effects. It orbits Mars below the synchronous orbit
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...

 radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Since it is close to the surface and in an equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

ial orbit, it cannot be seen above the horizon from latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s greater than 70.4°. Its orbit is so low that its 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...

, as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

 it is 0.20°, one-third as wide as 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...

. By comparison, the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's synodic period) to run their course, a mere 13 seconds longer than Phobos's sidereal period.

As seen from Phobos, Mars would appear 6,400 times larger and 2,500 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...

 appears from Earth, taking up a quarter of the width of a celestial hemisphere. The Mars-Phobos Lagrange 1 point is 2.5 kilometres (1.6 mi) above Stickney Crater, which is unusually close to the surface.

Solar transits

An observer situated on the Martian surface, in a position to observe Phobos, would see regular transits
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...

 of the moon across the Sun. Several of these transits have been photographed by the Mars Rover Opportunity. During the transits, Phobos's shadow is cast on the surface of Mars; an event which has been photographed by several spacecraft. Phobos is not large enough to cover the Sun's disk, and so cannot 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...

.

Future destruction

Because Phobos's orbital period is shorter than a Martian day, tidal deceleration is decreasing its orbital radius at the rate of about 20 metres (65.6 ft) per century. In an estimated 11 million years it will either impact the surface of Mars or, more probably, break up into a planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

. Given Phobos' irregular shape and assuming that it is a pile of rubble
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...

 (specifically a Mohr-Coulomb body
Mohr-Coulomb theory
Mohr–Coulomb theory is a mathematical model describing the response of brittle materials such as concrete, or rubble piles, to shear stress as well as normal stress. Most of the classical engineering materials somehow follow this rule in at least a portion of their shear failure envelope...

), it has been calculated that Phobos is currently stable with respect to tidal forces. But it is estimated that Phobos will pass the Roche Limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...

 for a rubble pile when its orbital radius drops by a little over 2000 kilometres (1,242.7 mi) to about 7100 kilometres (4,411.7 mi). Newer calculations suggests this will happen in just 7.6 million years from now. At this distance, Phobos will probably begin to break up and form a ring system which will continue to spiral slowly into Mars.

Origin

The origin of the Martian moons is still controversial. Phobos and Deimos both have much in common with carbonaceous C-type asteroid
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...

s, with 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...

, and density
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...

 very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. Both moons have very circular orbits which lie almost exactly in Mars's 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 probably 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. Capture also requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis 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 under tidal forces.

Phobos could be a second-generation Solar System object that coalesced
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...

 in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.

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 large 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...

. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos' volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...

 meteorites, again pointing away from an asteroidal origin. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...

 for the origin of Earth's moon.

Exploration

Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9
Mariner 9
Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...

in 1971, followed by Viking 1
Viking 1
Viking 1 was the first of two spacecraft sent to Mars as part of NASA's Viking program. It was the first spacecraft to successfully land on Mars and perform its mission, and until May 19, 2010 held the record for the second longest Mars surface mission of 6 years and 116 days .- Mission :Following...

in 1977, Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...

in 1998 and 2003, Mars Express in 2004, 2008, and 2010, and Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

in 2007 and 2008. On August 25, 2005, the Spirit Rover
Spirit rover
Spirit, MER-A , is a robotic rover on Mars, active from 2004 to 2010. It was one of two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity , landed on the other side of the planet...

, with an excess of energy due to wind blowing dust off of its solar panels, took several short-exposure photographs of the night sky from the surface of Mars. Phobos and Deimos are both clearly visible in the photograph. The only dedicated Phobos probes have been the Soviet Phobos 1 and Phobos 2, both launched in July 1988. The first was lost en route to Mars, while the second returned unusual data and images but failed shortly before beginning its detailed examination of the moon's surface.

Future missions

The Russian Space Agency launched a sample return mission to Phobos in 2011, called Phobos-Grunt
Phobos-Grunt
Fobos-Grunt or Phobos-Grunt was an attempted Russian sample return mission to Phobos, one of the moons of Mars. It was launched on 9 November 2011 at 02:16 local time from the Baikonur Cosmodrome, but subsequent rocket burns intended to set the craft on a course for Mars failed, leaving it...

. The return capsule includes a life science experiment of The Planetary Society, called Living Interplanetary Flight Experiment
Living Interplanetary Flight Experiment
The Living Interplanetary Flight Experiment is an interplanetary mission developed by the Planetary Society. It consists of sending selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft in 2011, which is a sample-return...

, or LIFE. A second contributor to this mission is the China National Space Administration
China National Space Administration
The China National Space Administration is the national space agency of the People's Republic of China responsible for the national space program. It is responsible for planning and development of space activities...

, which is sending a surveying satellite called "Yinghuo-1", which will be released in the orbit of Mars, and a soil-grinding and sieving system for the scientific payload of the Phobos lander.

In 2007, the European aerospace subsidiary EADS Astrium
EADS Astrium
Astrium Satellites is one of the three business units of Astrium, a subsidiary of EADS. It is a European space manufacturer involved in the manufacture of spacecraft used for science, Earth observation and telecommunication, as well as the equipment and subsystems used therein and related ground...

 was reported to have been developing a mission to Phobos as a technology demonstrator. Astrium is involved in developing a European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 plan for a sample return mission to Mars, as part of the ESA's Aurora programme
Aurora Programme
The Aurora programme is a human spaceflight programme of the European Space Agency established in 2001 with the primary objectives of creating, and then implementing, a European long-term plan for exploration of the Solar System using robotic spacecraft and human spaceflight...

, and sending a mission to the low gravity Phobos is seen as a good opportunity for testing and proving the technologies required for an eventual sample return mission to Mars. The mission is envisioned to start in 2016, and last for three years. The company plans to use a "mothership", which would be propelled by an ion engine, releasing a lander to the surface of Phobos. The lander would perform some tests and experiments, gather samples in a capsule, then return to the mothership and head back to Earth where the samples would be jettisoned for recovery on the surface.

In 2007, the Canadian Space Agency funded a study by Optech and the Mars Institute
Mars Institute
The Mars Institute is an international non-governmental organization created with the goals of advancing the scientific study and exploration of Mars, conducting high-quality, peer-reviewed research, and educating the public on the progress toward and benefits of Mars exploration. It was...

 for an unmanned mission to Phobos known as PRIME (Phobos Reconnaissance and International Mars Exploration). A proposed landing site for the PRIME spacecraft is at the "Phobos monolith
Phobos monolith
The Phobos monolith is a surface feature on the moon Phobos, which orbits Mars. A monolith is a geological feature consisting of a single massive piece of rock. Monoliths also occur naturally on Earth, but it has been suggested that the Phobos monolith may be a piece of impact ejecta...

", a bright object near Stickney which casts a prominent shadow Astronaut Buzz Aldrin
Buzz Aldrin
Buzz Aldrin is an American mechanical engineer, retired United States Air Force pilot and astronaut who was the Lunar Module pilot on Apollo 11, the first manned lunar landing in history...

 referred to this "monolith
Monolith
A monolith is a geological feature such as a mountain, consisting of a single massive stone or rock, or a single piece of rock placed as, or within, a monument...

" in a July 22, 2009 interview with C-Span
C-SPAN
C-SPAN , an acronym for Cable-Satellite Public Affairs Network, is an American cable television network that offers coverage of federal government proceedings and other public affairs programming via its three television channels , one radio station and a group of websites that provide streaming...

: "We should go boldly where man has not gone before. Fly by the comets, visit asteroids, visit the moon of Mars. There’s a monolith there. A very unusual structure on this potato shaped object that goes around Mars once in seven hours. When people find out about that they’re going to say ‘Who put that there? Who put that there?’ The universe put it there. If you choose, God put it there...” The PRIME mission would be composed of an orbiter and lander, and each would carry 4 instruments designed to study various aspects of Phobos's geology. , PRIME does not have a projected launch date.

In 2008, NASA Glenn Research Center began studying a Phobos and Deimos sample return mission that would use solar electric propulsion. The study gave rise to the "Hall" mission concept, a New Frontiers-class mission currently under further study.

Phobos has been proposed as an early target for a manned mission to Mars
Manned mission to Mars
A manned mission to Mars has been the subject of science fiction, engineering, and scientific proposals throughout the 20th century and into the 21st century...

. The tele-operation of robotic scouts on Mars by humans on Phobos could be conducted without significant time delay, and planetary protection
Planetary protection
Planetary protection is the term used to describe a guiding principle in design of an interplanetary mission that aims to prevent biological contamination of both the target celestial body and the Earth. This principle arises from the scientific need to preserve planetary conditions for future...

 concerns in early Mars exploration might be addressed by such an approach. Phobos has also been proposed as an early target for a manned mission to Mars
Manned mission to Mars
A manned mission to Mars has been the subject of science fiction, engineering, and scientific proposals throughout the 20th century and into the 21st century...

 because a landing on Phobos would be considerably less difficult and expensive than a landing on the surface of Mars itself. A lander bound for Mars would need to be capable of atmospheric entry and subsequent return to orbit, without any support facilities (a capacity which has never been attempted in a manned spacecraft), or would require the creation of support facilities in-situ (a "colony or bust" mission); a lander intended for Phobos could be based on equipment designed for lunar and asteroid landings. The human exploration of Phobos could serve as a catalyst for the human exploration of Mars and be exciting and scientifically valuable in its own right.

"Hollow Phobos" suggestions

In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.

Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular
Secular phenomena
In astronomy, secular phenomena are contrasted with phenomena observed to repeat periodically. In particular, astronomical ephemerides use secular to label the longest-lasting or non-oscillatory perturbations in the motion of planets, as opposed to periodic perturbations which exhibit repetition...

 acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

 of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin. Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light — one calculation yielded a hollow iron sphere 16 kilometres (9.9 mi) across but less than 6 cm thick. In a February 1960 letter to the journal Astronautics, Fred Singer
Fred Singer
Siegfried Fred Singer is an Austrian-born American physicist and emeritus professor of environmental science at the University of Virginia...

, then science advisor to U.S. President Dwight D. Eisenhower
Dwight D. Eisenhower
Dwight David "Ike" Eisenhower was the 34th President of the United States, from 1953 until 1961. He was a five-star general in the United States Army...

, said of Shklovsky's theory:

If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.


Subsequently, the systemic data errors that Singer predicted were found to exist, and the claim was called into doubt, and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist. Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr. The secular acceleration is now attributed to tidal effects, which had not been considered in the earlier studies. The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm3. Current observations are consistent with Phobos being a rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...

. In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one.

However, mapping by the Mars Express probe and subsequent volume calculations do suggest the presence of voids within the moon and indicate that it is not a solid chunk of rock but a porous body instead. The porosity
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%...

 of Phobos was calculated to be 30% ± 5%, or a quarter to a third of the moon being hollow. This void space is mostly on small scales (millimeters to ~1-m), between individual grains and boulders.

Namesakes

USS Phobos (AK-129)
USS Phobos (AK-129)
USS Phobos was an commissioned by the U.S. Navy for service in World War II. She was responsible for delivering troops, goods and equipment to locations in the war zone....

 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.

See also

  • Phobos and Deimos in fiction
    Phobos and Deimos in fiction
    Phobos and Deimos, the two moons of Mars, are locations frequently mentioned in works of science fiction.-Phobos:*In part 3 chapter 3 of Jonathan Swift's famous satire Gulliver's Travels, a fictional work written in 1726, the astronomers of Laputa are described as having discovered two satellites...

  • Phobos-Grunt
    Phobos-Grunt
    Fobos-Grunt or Phobos-Grunt was an attempted Russian sample return mission to Phobos, one of the moons of Mars. It was launched on 9 November 2011 at 02:16 local time from the Baikonur Cosmodrome, but subsequent rocket burns intended to set the craft on a course for Mars failed, leaving it...

     mission
  • Transit of Phobos from Mars
    Transit of Phobos from Mars
    A transit of Phobos across the Sun as seen from Mars takes place when Phobos passes directly between the Sun and a point on the surface of Mars, obscuring a large part of the Sun's disc for an observer on Mars. During a transit, Phobos can be seen from Mars as a large black disc rapidly moving...

  • Phobos monolith
    Phobos monolith
    The Phobos monolith is a surface feature on the moon Phobos, which orbits Mars. A monolith is a geological feature consisting of a single massive piece of rock. Monoliths also occur naturally on Earth, but it has been suggested that the Phobos monolith may be a piece of impact ejecta...


External links

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