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Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23, 2008.
Discovered by Asaph Hall
Discovery date August 18, 1877
Epoch J2000
Periapsis 9235.6 km
Apoapsis 9518.8 km
Semi-major axis 9377.2 km[1]
Eccentricity 0.0151
Orbital period 0.31891023 d
(7 h 39.2 min)
Average orbital speed 2.138 km/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[2]
Mean radius 11.1 km[3]
(0.0021 Earths)
Surface area ~6100 km²
(11.9 µEarths)
Volume 5680 km³[4]
(5.0 nEarths)
Mass 1.072 × 1016 kg[5]
(1.8 nEarths)
Mean density 1.887 g/cm³[4]
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 (at longest axis' tips)
Axial tilt
Albedo 0.071[3]
Temperature ~233 K
Apparent magnitude 11.3[6]

Phobos (pronounced /ˈfoʊbəs/ FOE-bəs, or as Greek Φόβος) (systematic designation: Mars I) is the larger and closer of the two moons of Mars, the other being 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.



Asaph Hall, discoverer of Phobos.

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 16:06 Washington mean time).[7][8][9] Hall also discovered Deimos, Mars' other moon. 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, where Ares summons Dread (Deimos) and Fear (Phobos).[10][11]

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,[12] and is apparently of composition similar to carbonaceous chondrite material.[13] Phobos' density is too low to be solid rock, however, and it is known to have significant porosity.[14][15][16] 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,[17][18] but ice below the regolith is not ruled out.[19]

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.[20] 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.[21]

Phobos is highly non-spherical, with dimensions of 27 × 22 × 18 km.[2] Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles.[citation needed]

Phobos is heavily cratered.[22] The most prominent surface feature is Stickney crater, named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name. Similar to Mimas's crater Herschel, albeit on a smaller scale, the impact that created Stickney must have nearly shattered Phobos.[23] 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 theorize 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.[24]

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.[25]


"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.[26] 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 km across but less than 6 cm thick.[26][27] In a February 1960 letter to the journal Astronautics,[28] S. Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, came out in support of Shklovsky's theory, stating:

[Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet.
My conclusion there is, and here I back Shklovsky, that 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.[28]

Subsequently, however, the existence of the acceleration that had caused the claims was called into doubt,[29] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.[30] Singer's critique was justified when earlier studies were later 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.[31] 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,[4] which is inconsistent with a hollow shell. In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one, and the "hollow Phobos" speculations have been relegated to the status of a historical curiosity.

However, mapping by the Mars Express probe and subsequent volume calculations do suggest the possible presence of vast caverns within the moon and indicate that it is not a solid chunk of rock but a porous body instead.[32]

Named geological features

color MRO view of Stickney Crater
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.[33] The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler. Several craters have been named.[34]

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)
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)
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's north pole

Phobos' 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' phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos' synodic period) to run their course, a mere 13 seconds longer than Phobos' 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 kilometers 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. Phobos is not large enough to cover the Sun's disk, and so cannot cause a total eclipse. Several of these transits have been photographed by the Mars Rover Opportunity. During the transits, Phobos' shadow is cast on the surface of Mars, which has been photographed by several spacecraft.

Future destruction

Because Phobos' orbital period is shorter than a Martian day, tidal deceleration is decreasing its orbital radius at the rate of about 20 meters per century. In an estimated 11 million years it will either impact the surface of Mars or, more likely, break up into a planetary ring.[35] 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). At this distance Phobos will likely begin to break up and form a ring system which will continue to spiral slowly into Mars.[36]


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

The origin of the Martian moons is still controversial.[37] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedos and densities very similar to those of C- or D-type asteroids.[12] Based on this similarity, one hypothesis is that both moons may have been captured into Martian orbit from the main asteroid belt.[38][39] 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 likely by a combination of atmospheric drag and tidal forces,[40] although it is not clear that sufficient time is available for this to occur for Deimos.[37] Capture also requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.[37] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.[39]

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.[41] 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.[42]


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[43] and Mars Reconnaissance Orbiter in 2007 and 2008. 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, and the second returned documented, unusual data and images but failed shortly before beginning its detailed examination of the moon's surface.

Future missions

The Russian Space Agency is planning to launch a sample return mission to Phobos in 2011, called Phobos-Grunt. The return capsule will include a life science experiment of The Planetary Society, called Living Interplanetary Flight Experiment, or LIFE.[44] A second contributor to this mission is China with a surveying satellite called "Yinghuo-1" that will be released in the orbit of Mars, and a soil grinding and sieving system for the scientific payload of the Phobos lander.[45][46][47]

Astrium in the UK is also planning a sample return mission.[48]

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

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 the "Phobos Monolith," a bright object near Stickney which casts a prominent shadow.[49][50] The PRIME mission would be composed of an orbiter and lander, and each would carry 4 instruments designed to study various aspects of Phobos' geology.[51] At present, PRIME does not have a projected launch date. 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…”[52]

Phobos has also been proposed as an early target for a manned mission to Mars,[53] since a landing on Phobos would be considerably less difficult (and hence, much less 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 in-situ 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), while a lander intended for Phobos could be based on equipment designed for lunar and asteroid landings.

See also


  1. ^ NASA Celestia
  2. ^ a b "Mars: Moons: Phobos". NASA Solar System Exploration. 2003-09-30. Retrieved 2008-08-18. 
  3. ^ a b "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2006-07-13. Retrieved 2008-01-29. 
  4. ^ a b c "Mars Express closes in on the origin of Mars' larger moon". DLR. 2008-10-16. Retrieved 2008-10-16. 
  5. ^ a b use a spherical radius of 11.1 km; volume of a sphere * density of 1.877 g/cm³ yields a mass (m=d*v) of 1.07 × 1016 kg and an escape velocity (sqrt((2*g*m)/r)) of 11.3 m/s (40 km/h)
  6. ^ "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2007-09-28. 
  7. ^ "Notes: The Satellites of Mars". The Observatory 1 (6): 181–185. September 20, 1877. Retrieved 2009-02-04. 
  8. ^ Hall, A. (October 17, 1877, signed September 21, 1877). Observations of the Satellites of Mars. 91. Astronomische Nachrichten. pp. 11/12–13/14. Retrieved 2009-02-04. 
  9. ^ Morley, T. A. (February 1989). "A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982". Astronomy and Astrophysics Supplement Series (ISSN 0365-0138) 77 (2): 209–226.  (Table II, p. 220: first observation of Phobos on 1877-08-18.38498)
  10. ^ Madan, H. G. (October 4, 1877, signed September 29, 1877). "Letters to the Editor: The Satellites of Mars". Nature 16 (414): 475. doi:10.1038/016475b0. 
  11. ^ Hall, A. (March 14, 1878, signed February 7, 1878). "Names of the Satellites of Mars". Astronomische Nachrichten 92 (2187): 47–48. doi:10.1002/asna.18780920304. 
  12. ^ a b "New Views of Martian Moons". 
  13. ^ Lewis, J. S. (2004). Physics and Chemistry of the Solar System. Elsevier Academic Press. pp. 425. ISBN 0-12-446744-X. 
  14. ^ "Porosity of Small Bodies and a Reassesment of Ida's Density". "When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2..." 
  15. ^ "Close Inspection for Phobos". "It is light, with a density less than twice that of water, and orbits just 5989 km above the Martian surface." 
  16. ^ Busch, M. W.; et al. (2007). "Arecibo Radar Observations of Phobos and Deimos". Icarus 186 (2): 581–584. doi:10.1016/j.icarus.2006.11.003. 
  17. ^ Murchie, S. L., Erard, S., Langevin, Y., Britt, D. T., Bibring, J. P., and Mustard, J. F., "Disk-resolved Spectral Reflectance Properties of Phobos from 0.3-3.2 microns: Preliminary Integrated Results from PhobosH 2," in Abstracts of the Lunar and Planetary Science Conference, volume 22, page 943, (1991)
  18. ^ Rivkin, A. S.; et al. (March 2002). "Near-Infrared Spectrophotometry of Phobos and Deimos". Icarus 156 (1): 64. doi:10.1006/icar.2001.6767. 
  19. ^ Fanale, Fraser P., "Water regime of Phobos" (1991).
  20. ^ Showalter, M. R.; Hamilton, D. P.; and Nicholson, P. D.. "A Deep Search for Martian Dust Rings and Inner Moons Using the Hubble Space Telescope" (PDF). Planetary and Space Science, Vol. 54 (2006), pp. 844–854. 
  21. ^ "Forgotten Moons: Phobos and Deimos Eat Mars' Dust". 
  22. ^ "Phobos". 
  23. ^ "Stickney Crater-Phobos". "One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only 28 by 20 kilometers (17 by 12 miles), the moon must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact." 
  24. ^ Murray, J. B.; et al.. "New Evidence on the Origin of Phobos’ Parallel Grooves from HRSC Mars Express" (PDF). 37th Annual Lunar and Planetary Science Conference, March 2006. 
  25. ^ "The Kaidun Meteorite: Where Did It Come From?" (PDF). "The currently available data on the lithologic composition of the Kaidun meteorite– primarily the composition of the main portion of the meteorite, corresponding to CR2 carbonaceous chondrites and the presence of clasts of deeply differentiated rock – provide weighty support for considering the meteorite’s parent body to be a carbonaceous chondrite satellite of a large differentiated planet. The only possible candidates in the modern solar system are Phobos and Deimos, the moons of Mars." 
  26. ^ a b Shklovsky, I. S.; The Universe, Life, and Mind, Academy of Sciences USSR, Moscow, 1962
  27. ^ Öpik, E. J. (September 1964). "Is Phobos Artificial?". Irish Astronomical Journal, Vol. 6. pp. 281–283. Retrieved 2009-02-04. 
  28. ^ a b Singer, S. F.; Astronautics, February 1960
  29. ^ Öpik, E. J. (March 1963, signed September 1962). "News and Comments: Phobos, Nature of Acceleration". Irish Astronomical Journal, Vol. 6. pp. 40. Retrieved 2009-02-04. 
  30. ^ Singer, S. F. (1967). "On the Origin of the Martian Satellites Phobos and Deimos". Seventh International Space Science Symposium held 10-18 May 1966 in Vienna, North-Holland Publishing Company. 
  31. ^ "More on the Moons of Mars". Singer, S. F., Astronautics, February 1960. American Astronautical Society. Page 16
  32. ^ "Cheap Flights to Phobos" by Stuart Clark, in New Scientist magazine, 30th January 2010.
  33. ^ Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Categories
  34. ^ Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Craters
  35. ^ Sharma, B. K. (2008-05-10). "Theoretical Formulation of the Phobos, moon of Mars, rate of altitudinal loss". 
  36. ^ Holsapple, K. A. (December 2001). "Equilibrium Configurations of Solid Cohesionless Bodies". Icarus 154 (2): 432–448. doi:10.1006/icar.2001.6683. Retrieved 2007-11-12. 
  37. ^ a b c Burns, J. A. "Contradictory Clues as to the Origin of the Martian Moons," in Mars, H. H. Kieffer et al., eds., U. Arizona Press, Tucson, 1992
  38. ^ "Close Inspection for Phobos". "One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids." 
  39. ^ a b Landis, G. A. "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001; abstract.
  40. ^ Cazenave, A.; Dobrovolskis, A.; Lago, B. (1980). "Orbital history of the Martian satellites with inferences on their origin". Icarus 44 (3): 730–744. doi:10.1016/0019-1035(80)90140-2. 
  41. ^ Martin Pätzold and Olivier Witasse (2010-03-04). "Phobos Flyby Success". ESA. Retrieved 2010-03-04. 
  42. ^ Craddock, R. A.; (1994); The Origin of Phobos and Deimos, Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14-18 March 1994, p. 293
  43. ^ "Closest Phobos flyby gathers data". Retrieved March 7, 2010. 
  44. ^ "Projects LIFE Experiment: Phobos". The Planetary Society. 
  45. ^ "Russia, China Could Sign Moon Exploration Pact in 2006". RIA Novosti. 
  46. ^ "HK triumphs with out of this world invention". Hong Kong Trader. May 1, 2007. 
  47. ^ "PolyU-made space tool sets for Mars again". Hong Kong Polytechnic University. 
  48. ^ Amos, J.; Martian Moon ’Could be Key Test’, BBC News (February 9, 2007)
  49. ^ Optech press release, "Canadian Mission Concept to Mysterious Mars moon Phobos to Feature Unique Rock-Dock Maneuver," May 3, 2007.
  50. ^ PRIME: Phobos Reconnaissance & International Mars Exploration, Mars Institute website, accessed July 27, 2009.
  51. ^ Mullen, Leslie (30 April 2009). "New Missions Target Mars Moon Phobos". Astrobiology Magazine ( Retrieved 2009-09-05. 
  52. ^ "Buzz Aldrin Reveals Existence of Monolith on Mars Moon". C-Span. July 22, 2009. 
  53. ^ Landis, Geoffrey A. "Footsteps to Mars: an Incremental Approach to Mars Exploration," Journal of the British Interplanetary Society, Vol. 48, pp. 367-342 (1995); presented at Case for Mars V, Boulder CO, 26-29 May 1993; appears in From Imagination to Reality: Mars Exploration Studies, R. Zubrin, ed., AAS Science and Technology Series Volume 91 pp. 339-350 (1997). (text available as Footsteps to Mars (PDF)

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