Phobos (moon)

Phobos (moon)
Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23, 2008. Stickney crater, the largest, is on the right side.
Discovered by Asaph Hall
Discovery date August 18, 1877
Alternate name(s) Mars I
Adjective Phobian
Epoch J2000
Periapsis 9,235.6 km (5,738.7 mi)
Apoapsis 9,518.8 km (5,914.7 mi)
Semi-major axis 9,377.2 km (5,826.7 mi)[1]
Eccentricity 0.0151
Orbital period 0.31891023 d
(7 h 39.2 min)
Average orbital speed 2.138 km/s (1.328 mi/s)
Inclination 1.093° (to Mars's equator)
0.046° (to local Laplace plane)
26.04° (to the ecliptic)
Satellite of Mars
Physical characteristics
Dimensions 26.8 × 22.4 × 18.4 km (11.4 mi)[2]
Mean radius 11.1 km (6.9 mi)[3]
(0.0021 Earths)
Surface area 6,100 km2 (2,400 sq mi)
(11.9 µEarths)
Volume 5,680 km3 (1,360 cu mi)[4]
(5.0 nEarths)
Mass 1.072×1016 kg[5]
(1.8 nEarths)
Mean density 1.876 g/cm3[6]
Equatorial surface gravity 0.0084–0.0019 m/s2
(8.4–1.9 mm/s2)
(860–190 µg)
Escape velocity 11.3 m/s (40 km/h)[5]
Rotation period synchronous
Equatorial rotation velocity 11.0 km/h (6.8 mph) (at longest axis' tips)
Axial tilt
Albedo 0.071[3]
Temperature ~233 K
Apparent magnitude 11.3[7]

Phobos (play /ˈfbəs/ foh-bəs; Greek: Φόβος; systematic designation: Mars I) is the largest and closest of the two natural satellites of Mars. 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 Phobos (which means "fear"), a son of Ares (Mars).

A small, irregularly shaped object, Phobos orbits about 9,377 km (5,827 mi) from the center of Mars, closer to its primary than any other known planetary moon. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater, 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.



Phobos was discovered by astronomer Asaph Hall on August 18, 1877, at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time (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).[8][9][10] Hall also discovered Deimos, 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, based on Book XV of the Iliad, in which the god Ares summons Dread (Deimos) and Fear (Phobos).[11][12]

Physical characteristics

A mosaic of three separate images taken by Viking 1 on October 19, 1978. The large crater (mostly in darkness) on the upper left is Stickney.

Phobos is one of the least reflective bodies in the Solar System. Spectroscopically it appears to be similar to the D-type asteroids,[13] and is apparently of composition similar to carbonaceous chondrite material.[14] Phobos's density is too low to be solid rock, and it is known to have significant porosity.[15][16][17] These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface regolith layer lacks hydration,[18][19] but ice below the regolith is not ruled out.[20]

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.[21] Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith 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.[22]

Phobos is highly non-spherical, with dimensions of 27 × 22 × 18 km.[2]

Phobos is heavily cratered,[23] with one of the craters near the equator having a central peak despite the moon's small size.[24] 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's crater Herschel, the impact that created Stickney must have nearly shattered Phobos.[25] Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 meters (98 ft) deep, 100 to 200 meters (330 to 660 ft) wide, and up to 20 kilometers (12 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 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 chains, 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.[26] Grooves are also seen on asteroid 4 Vesta, but scientists are not currently suggesting a similar mechanism for forming those grooves.[27]

The unique Kaidun meteorite 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.[28][29]

Named geological features

Enhanced-color view of Stickney Crater by the Mars Reconnaissance Orbiter.
Some of the named craters of Phobos. C = Clustril; D = Drunlo; F = Flimnap; L = Limtoc; R = Reldresal; S = Stickney; Sk = Skyresh. Grildrig is on the horizon below Skyresh and Flimnap.

Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels.[30] There is one named regio, Laputa Regio, and one named planitia, Lagado Planitia; both are named after places in Gulliver's Travels (the fictional Laputa, a flying island, and Lagado, imaginary capital of the fictional nation Balnibarbi).[31] The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler. Several craters have been named.[32]

Crater Named after Coordinates
Clustril Character in Gulliver's Travels 60°N 91°W / 60°N 91°W / 60; -91 (Clustril)
D'Arrest Heinrich Louis d'Arrest, astronomer 39°S 179°W / 39°S 179°W / -39; -179 (D'Arrest)
Drunlo Character in Gulliver's Travels 36°30′N 92°00′W / 36.5°N 92°W / 36.5; -92 (Drunlo)
Flimnap Character in Gulliver's Travels 60°N 350°W / 60°N 350°W / 60; -350 (Flimnap)
Grildrig Character in Gulliver's Travels 81°N 195°W / 81°N 195°W / 81; -195 (Grildrig)
Gulliver Main character of Gulliver's Travels 62°N 163°W / 62°N 163°W / 62; -163 (Gulliver)
Hall Asaph Hall, discoverer of Phobos 80°S 210°W / 80°S 210°W / -80; -210 (Hall)
Limtoc Character in Gulliver's Travels 11°S 54°W / 11°S 54°W / -11; -54 (Limtoc)
Öpik Ernst J. Öpik, astronomer 7°S 297°W / 7°S 297°W / -7; -297 (Öpik)
Reldresal Character in Gulliver's Travels 41°N 39°W / 41°N 39°W / 41; -39 (Reldresal)
Roche Édouard Roche, astronomer 53°N 183°W / 53°N 183°W / 53; -183 (Roche)
Sharpless Bevan Sharpless, astronomer 27°30′S 154°00′W / 27.5°S 154°W / -27.5; -154 (Sharpless)
Shklovsky Iosif Shklovsky, astronomer 24°N 248°W / 24°N 248°W / 24; -248 (Skyresh)
Skyresh Character in Gulliver's Travels 52°30′N 320°00′W / 52.5°N 320°W / 52.5; -320 (Skyresh)
Stickney Angeline Stickney, wife of Asaph Hall 1°N 49°W / 1°N 49°W / 1; -49 (Stickney)
Todd David Peck Todd, astronomer 9°S 153°W / 9°S 153°W / -9; -153 (Todd)
Wendell Oliver Wendell, astronomer 1°S 132°W / 1°S 132°W / -1; -132 (Wendell)

Orbital characteristics

Orbits of Phobos and Deimos (to scale), seen from above Mars' north pole. Phobos is orbiting Mars 3.96 times faster than Deimos.

Phobos's unusually close orbit around its parent planet produces some unusual effects. It orbits Mars below the synchronous orbit 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 equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. Its orbit is so low that its angular diameter, 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 it is 0.20°, one-third as wide as the full Moon as seen from Earth. By comparison, the Sun 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 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.

Phobos transits Sun, as seen by Mars Rover Opportunity

Solar transits

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

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 (66 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.[33] Given Phobos' irregular shape and assuming that it is a pile of rubble (specifically a Mohr-Coulomb body), 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 for a rubble pile when its orbital radius drops by a little over 2,000 kilometers (1,200 mi) to about 7,100 kilometers (4,400 mi). Newer calculations suggests this will happen in just 7.6 million years from now.[34] At this distance, Phobos will probably begin to break up and form a ring system which will continue to spiral slowly into Mars.[35]


Viking 1 image of Phobos, with Stickney Crater to the right

The origin of the Martian moons is still controversial.[36] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids.[13] Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.[37][38] 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 forces,[39] although it is not clear that sufficient time is available for this to occur for Deimos.[36] Capture also requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.[36] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.[38]

Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.[40]

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.[41] 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.[6] 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 meteorites, again pointing away from an asteroidal origin.[42] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,[43] similar to the prevailing theory for the origin of Earth's moon.


The Phobos monolith (right of center) as taken by the Mars Global Surveyor (MOC Image 55103) in 1998.

Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9 in 1971, followed by Viking 1 in 1977, Mars Global Surveyor in 1998 and 2003, Mars Express in 2004, 2008, and 2010,[44] and Mars Reconnaissance Orbiter in 2007 and 2008. On August 25, 2005, the Spirit Rover, 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.[45] 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. The return capsule includes a life science experiment of The Planetary Society, called Living Interplanetary Flight Experiment, or LIFE.[46] A second contributor to this mission is the China National Space Administration, 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.[47][48][49]

In 2007, the European aerospace subsidiary EADS Astrium was reported to have been developing a mission to Phobos as a technology demonstrator. Astrium is involved in developing a European Space Agency plan for a sample return mission to Mars, as part of the ESA's Aurora programme, 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.[50]

In 2007, the Canadian Space Agency funded a study by Optech and the Mars Institute 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", a bright object near Stickney which casts a prominent shadow[51][52][53] Astronaut Buzz Aldrin referred to this "monolith" in a July 22, 2009 interview with C-Span: "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...”[54] 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.[55] As of 30 April 2009 (2009 -04-30), 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.[56]

Phobos has been proposed as an early target for a manned mission to Mars. The tele-operation of robotic scouts on Mars by humans on Phobos could be conducted without significant time delay, and planetary protection concerns in early Mars exploration might be addressed by such an approach.[57] Phobos has also been proposed as an early target for a manned mission to Mars 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.[58] 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.[59]

"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 acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.[60] 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.[60][61] In a February 1960 letter to the journal Astronautics,[62] Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, 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.[62]

Subsequently, the systemic data errors that Singer predicted were found to exist, and the claim was called into doubt,[63] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.[64] 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.[65] The secular acceleration is now attributed to tidal effects,[63] 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.[4] Current observations are consistent with Phobos being a rubble pile.[4] 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.[66] The porosity 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.[6]


USS Phobos (AK-129) was a United States Navy Crater class cargo ship named after the Moon.

See also


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