Phobos is a small, irregularly shaped object with amean radius of 11 km (7 mi). It orbits 6,000 km (3,700 mi) from the Martian surface, closer to itsprimary body than any other knownnatural satellite to a planet. It orbitsMars much faster than Mars rotates and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice eachMartian day. Phobos is one of the least reflective bodies in theSolar System, with analbedo of 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side. The notable surface feature is the largeimpact craterStickney, which takes up a substantial proportion of the moon's surface. The surface is also marked by many grooves, and there are numerous theories as to how these grooves were formed.
Images and models indicate that Phobos may be arubble pile held together by a thincrust that is being torn apart bytidal interactions. Phobos gets closer to Mars by about 2 centimetres (0.79 in) per year.
The names, originally spelledPhobus andDeimus respectively, were suggested by the British academicHenry Madan, a science master atEton College, who based them onGreek mythology, in whichPhobos is a companion to the god,Ares.[17][18]
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekphi (the initial of Phobos) combined with Mars's spear as the symbol of Phobos (). This symbol is not widely used.[19]
Deimos and Phobos as seen from Mars, compared inapparent size to theMoon as seen from Earth.
Phobos has dimensions of 26 by 23 by 18 kilometres (16 mi × 14 mi × 11 mi),[7] and retains too little mass to be rounded under its own gravity. Phobos does not have anatmosphere due to its low mass and low gravity.[20] It is one of the least reflective bodies in the Solar System, with an albedo of about 0.071.[21] Infrared spectra show that it has carbon-rich material found incarbonaceous chondrites, and its composition shows similarities to that of Mars's surface.[22] Phobos's density is too low to be solid rock, and it is known to have significantporosity.[23][24][25] These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surfaceregolith layer lacks hydration,[26][27] but ice below the regolith is not ruled out.[28][29] Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side.[30]
Unlike Deimos, Phobos is heavily cratered,[31] with one of the craters near the equator having a central peak despite the moon's small size.[32] The most prominent of these is the craterStickney, an impact crater 9 km (5.6 mi) in diameter, which takes up a substantial proportion of the moon's surface area. As with theSaturnian moonMimas's craterHerschel, the impact that created Stickney probably almost shattered Phobos.[33]
(top) False color image of the impact craterStickney imaged by theMars Reconnaissance Orbiter in March 2008; (bottom) Labeled Map of Phobos – Moon of Mars (U.S. Geological Survey)
Many grooves and streaks 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 theMars Express spacecraft revealed that the grooves are not radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspected that they had been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed ascrater 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.[34] In November 2018, based on computational probability analysis, astronomers concluded that the many grooves on Phobos were caused by boulders ejected from the asteroid impact that created Stickney crater. These boulders rolled in a predictable pattern on the surface of the moon.[35][36]
Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.[37] Images fromMars 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.[38]
The uniqueKaidun meteorite that fell on aSoviet military base inYemen in 1980 has been hypothesized to be a piece of Phobos, but this could not be verified because little is known about the exact composition of Phobos.[39][40]
In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.[41] Around 1958, Russian astrophysicistIosif Samuilovich Shklovsky, studying thesecularacceleration of Phobos's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.[42] 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 kilometers (9.9 mi) across but less than 6 centimetres (2.4 in) thick.[42][43] In a February 1960 letter to the journalAstronautics,[44]Fred Singer, then science advisor to U.S. PresidentDwight 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.[44]
Subsequently, the systematic data errors that Singer predicted were found to exist, the claim was called into doubt,[45] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.[46] Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 centimetres (2.0 in) per year for the rate of altitude loss, which was later revised to 1.8 centimetres (0.71 in) per year.[47] The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it to spiral inward.[48]
The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm3 (0.0682 lb/cu in).[49] Current observations are consistent with Phobos being arubble pile.[49] Images obtained by theViking probes in the 1970s showed a natural object, not an artificial one. Nevertheless, mapping by theMars Express probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body.[50] The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.[51]
There is one namedregio,Laputa Regio, and one namedplanitia,Lagado Planitia; both are named after places inGulliver's Travels (the fictionalLaputa, a flying island, andLagado, imaginary capital of the fictional nationBalnibarbi).[54] The only named ridge on Phobos isKepler Dorsum, named after the astronomerJohannes Kepler.[55]
Theorbital motion of Phobos has been intensively studied, making it "the best studiednatural satellite in the Solar System" in terms of orbits completed.[56] Its close orbit around Mars produces some distinct effects. With an altitude of 5,989 km (3,721 mi), Phobos orbits Mars below thesynchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates.[24] Therefore, from the point of view of an observer on the surface of Mars, 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). Because it is close to the surface and in anequatorial orbit, it cannot be seen above the horizon fromlatitudes greater than 70.4°. Its orbit is so low that itsangular 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; atzenith, it is 0.20°, one-third as wide as the fullMoon 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'ssynodic period) to run their course, a mere 13 seconds longer than Phobos'ssidereal period.
An observer situated on the Martian surface, in a position to observe Phobos, would see regulartransits of Phobos across the Sun. Several of these transits have been photographed by the Mars RoverOpportunity. During the transits, Phobos casts a shadow on the surface of Mars; this event has been photographed by several spacecraft. Phobos is not large enough to cover the Sun's disk, and so cannot cause atotal eclipse.[57]
Tidal deceleration is gradually decreasing the orbital radius of Phobos by approximately 2 m (6 ft 7 in) every 100 years,[58] and with decreasing orbital radius the likelihood of breakup due totidal forces increases, estimated in approximately 30–50 million years,[58][56] or about 43 million years in one study's estimate.[59]
Phobos's grooves were long thought to be fractures caused by the impact that formed the Stickney crater. Other modelling suggested since the 1970s support the idea that the grooves are more like "stretch marks" that occur when Phobos gets deformed by tidal forces, but in 2015 when the tidal forces were calculated and used in a new model, the stresses were too weak to fracture a solid moon of that size, unless Phobos is a rubble pile surrounded by a layer of powdery regolith about 100 m (330 ft) thick. Stress fractures calculated for this model line up with the grooves on Phobos. The model is supported with the discovery that some of the grooves are younger than others, implying that the process that produces the grooves is ongoing.[58][60][inconsistent]
Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically aMohr–Coulomb body), it will eventually break up due to tidal forces when it reaches approximately 2.1 Mars radii.[61] When Phobos is broken up, it will form aplanetary ring around Mars.[62] This predicted ring may last from 1 million to 100 million years. The fraction of the mass of Phobos that will form the ring depends on the unknown internal structure of Phobos. Loose, weakly bound material will form the ring. Components of Phobos with strong cohesion will escape tidal breakup and will enter the Martian atmosphere.[63]
An illustration of main-belt asteroid capture hypothesis
The origin of the Martian moons has been disputed.[64] Phobos and Deimos both have much in common with carbonaceousC-type asteroids, withspectra,albedo, anddensity very similar to those of C- or D-type asteroids.[65] Based on their similarity, one hypothesis is that both moons may be capturedmain-belt asteroids.[66][67] Since both moons have nearly circular orbits that lie almost exactly in Mars'sequatorial plane, a capture origin for them requires a mechanism for circularizing their initially highly eccentric orbits and adjusting their inclinations into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces.[68] However, it is not clear that sufficient time is available for this to occur for Deimos.[64] Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.[64]Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was abinary asteroid that separated under tidal forces.[67][69]
Phobos could be a second-generation Solar System object thatcoalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.[70]
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 largeplanetesimal.[71] 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's volume) is inconsistent with an asteroidal origin.[51] Observations of Phobos in thethermal infrared suggest a composition containing mainlyphyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes ofchondrite meteorites, again pointing away from an asteroidal origin.[72] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,[73] similar to theprevailing theory for the origin of Earth's moon.
Some areas of the surface are reddish in color, while others are bluish. The hypothesis is that gravity pull from Mars makes the reddish regolith move over the surface, exposing relatively fresh, unweathered and bluish material from the moon, while the regolith covering it over time has been weathered due to exposure of solar radiation. Because the blue rock differs from known Martian rock, it could contradict the theory that the moon is formed from leftover planetary material after the impact of a large object.[74]
In February 2021, Amirhossein Bagheri (ETH Zurich), Amir Khan (ETH Zurich),Michael Efroimsky (US Naval Observatory) and their colleagues proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data fromMars InSight Mission and other missions, they proposed that the moons are born from disruption of a common parent body around 1 to 2.7 billion years ago. The common progenitor of Phobos and Deimos was most probably hit by another object and shattered to form both moons.[75]
Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first wasMariner 7 in 1969, followed byMariner 9 in 1971,Viking 1 in 1977,Phobos 2 in 1989[76]Mars Global Surveyor in 1998 and 2003,Mars Express in 2004, 2008, 2010[77] and 2019, andMars Reconnaissance Orbiter in 2007 and 2008. On 25 August 2005, theSpirit 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, and was able to successfully photograph both Phobos and Deimos.[78]
TheSoviet Union undertook thePhobos program with two probes, both launched successfully in July 1988.Phobos 1 was shut down by an erroneous command from ground control issued in September 1988 and lost while still en route.Phobos 2 arrived at the Mars system in January 1989 and, after transmitting a small amount of data and imagery shortly before beginning its detailed examination of Phobos's surface, abruptly ceased transmission due either to failure of the onboard computer or of the radio transmitter, already operating on backup power. Other Mars missions collected more data, but no dedicatedsample return mission has been successfully performed.
TheRussian Space Agency launched a sample return mission to Phobos in November 2011, calledFobos-Grunt. The return capsule also included a life science experiment ofThe Planetary Society, calledLiving Interplanetary Flight Experiment, or LIFE.[79] A second contributor to this mission was theChina National Space Administration, which supplied a surveying satellite called "Yinghuo-1", which would have been released in the orbit of Mars, and a soil-grinding and sieving system for the scientific payload of the Phobos lander.[80][81] After achievingEarth orbit, theFobos-Grunt probe failed to initiate subsequent burns that would have sent it to Mars. Attempts to recover the probe were unsuccessful and it crashed back to Earth in January 2012.[82]
During the end of its 12 March 2025gravity assist from Mars, en route to65803 Didymos, theESA'sHera was able to observe Phobos retreating from the planet in its orbit at distances less than 13,000 km away.[84]
An artist's concept of Mars Moons eXploration spacecraft
TheJapanese Aerospace Exploration Agency (JAXA) unveiled on 9 June 2015 theMartian Moons Exploration (MMX), a sample return mission targeting Phobos.[85] MMX will land and collect samples from Phobos multiple times, along with conducting Deimos flyby observations and monitoring Mars's climate. By using acorer sampling mechanism, the spacecraft aims to retrieve a minimum 10 g amount of samples.[86] NASA, DLR, andCNES[87] are also participating in the project, and will provide scientific instruments[88][89] and a rover for the mission, namedIdefix. MMX is scheduled for launch in 2026, and will return samples to Earth in 2031.[86]
In 1997 and 1998, theAladdin mission was selected as a finalist in the NASADiscovery Program. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby (~1 km/s).[90] These samples would be returned to Earth for study three years later.[91][92] The Principal Investigator was Dr.Carle Pieters ofBrown University. The total mission cost, including launch vehicle and operations was $247.7 million.[93] Ultimately, the mission chosen to fly wasMESSENGER, a probe to Mercury.[94]
In 2007, the European aerospace subsidiaryEADS Astrium was reported to have been developing a mission to Phobos as atechnology demonstrator. Astrium was involved in developing aEuropean Space Agency plan for a sample return mission to Mars, as part of the ESA'sAurora programme, and sending a mission to Phobos with its low gravity was seen as a good opportunity for testing and proving the technologies required for an eventual sample return mission to Mars. The mission was envisioned to start in 2016, was to last for three years. The company planned to use a "mothership", which would be propelled by anion 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.[95]
Another concept of a sample return mission from Phobos and Deimos isOSIRIS-REx II, which would use heritage technology from the firstOSIRIS-REx mission.[101]
In 2014, a Discovery-class mission was proposed to place an orbiter in Mars orbit by 2021 to study Phobos and Deimos through a series of close flybys. The mission is calledPhobos And Deimos & Mars Environment (PADME).[103][104][105] Two other Phobos missions that were proposed for the Discovery 13 selection included aMerlin, which would flyby Deimos but actually orbit and land on Phobos, andPandora which would orbit both Deimos and Phobos.[106]
Russia plans to repeat Fobos-Grunt mission in the late 2020s, and the European Space Agency is assessing a sample-return mission for 2024 calledPhootprint.[107][108]
Phobos has been proposed as an early target for ahuman mission to Mars. Theteleoperation of robotic scouts on Mars by humans on Phobos could be conducted without significant time delay, andplanetary protection concerns in early Mars exploration might be addressed by such an approach.[109]
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 ofatmospheric entry and subsequent return to orbit without any support facilities, or would require the creation ofsupport facilities in-situ. A lander instead bound for Phobos could be based on equipment designed for lunar andasteroid landings.[110] Furthermore, due to Phobos's very weak gravity, thedelta-v required to land on Phobos and return is only 80% of that required for a trip to and from the surface of the Moon.[111]
It has been proposed that the sands of Phobos could serve as a valuable material foraerobraking during a Mars landing. A relatively small amount of chemical fuel brought from Earth could be used to lift a large amount of sand from the surface of Phobos to a transfer orbit. This sand could be released in front of a spacecraft during the descent maneuver causing a densification of the atmosphere just in front of the spacecraft.[112][113]
While human exploration of Phobos could serve as a catalyst for the human exploration of Mars, it could be scientifically valuable in its own right.[114]
First discussed in fiction in 1956 by Fontenay,[115] Phobos has been proposed as a future site forspace elevator construction. This would involve a pair of space elevators: one extending 6,000 km from the Mars-facing side to the edge of Mars's atmosphere, the other extending 6,000 km (3,700 mi) from the other side and away from Mars. A spacecraft launching from Mars's surface to the lower space elevator would only need a delta-v of 0.52 km/s (0.32 mi/s), as opposed to the over 3.6 km/s (2.2 mi/s) needed to launch to low Mars orbit. The spacecraft could be lifted up using electrical power and then released from the upper space elevator with a hyperbolic velocity of 2.6 km/s (1.6 mi/s), enough to reach Earth and a significant fraction of the velocity needed to reach theasteroid belt. The space elevators could also work in reverse to help spacecraft enter the Martian system. The great mass of Phobos means that any forces from space elevator operation would have minimal effect on its orbit. Additionally, materials from Phobos could be used for space industry.[116]
^ab"Close Inspection for Phobos".It is light, with a density less than twice that of water, and orbits just 5,989 kilometers (3,721 mi) above the Martian surface.
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^"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 mi), it 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.
^Ivanov, Andrei; Zolensky, Michael (2003)."The Kaidun Meteorite: Where Did It Come From?"(PDF).Lunar and Planetary Science.34.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.
^Efroimsky, Michael; Lainey, Valéry (29 December 2007). "Physics of bodily tides in terrestrial planets and the appropriate scales of dynamical evolution".Journal of Geophysical Research—Planets. Vol. 112. p. E12003.doi:10.1029/2007JE002908.
^Hurford, Terry A.; Asphaug, Erik; Spitale, Joseph; Hemingway, Douglas; et al.; "Surface Evolution from Orbital Decay on Phobos", Division of Planetary Sciences of the American Astronomical Society meeting #47, National Harbor, MD, November 2015
^Black, Benjamin A.; and Mittal, Tushar; (2015), "The demise of Phobos and development of a Martian ring system",Nature Geosci, advance online publication, doi:10.1038/ngeo2583
^abcBurns, Joseph A.; "Contradictory Clues as to the Origin of the Martian Moons" inMars, H. H. Kieffer et al., eds., University of Arizona Press, Tucson, AZ, 1992
^abLandis, Geoffrey A.; "Origin of Martian Moons from Binary Asteroid Dissociation",American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001,abstract
^Craddock, Robert 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
^Barraclough, Simon; Ratcliffe, Andrew; Buchwald, Robert; Scheer, Heloise; Chapuy, Marc; Garland, Martin (16 June 2014).Phootprint: A European Phobos Sample Return Mission(PDF). 11th International Planetary Probe Workshop. Airbus Defense and Space. Archived fromthe original(PDF) on 29 January 2016. Retrieved22 December 2015.
^Koschny, Detlef; Svedhem, Håkan; Rebuffat, Denis (2 August 2014). "Phootprint – A Phobos sample return mission study".ESA.40: B0.4–9–14.Bibcode:2014cosp...40E1592K.
^Landis, Geoffrey A.; "Footsteps to Mars: an Incremental Approach to Mars Exploration", inJournal of the British Interplanetary Society, vol. 48, pp. 367–342 (1995); presented at Case for Mars V, Boulder CO, 26–29 May 1993; appears inFrom Imagination to Reality: Mars Exploration Studies, R. Zubrin, ed.,AAS Science and Technology Series Volume 91, pp. 339–350 (1997). (text available asFootsteps to Mars)
^Lee, Pascal; Braham, Stephen; Mungas, Greg; Silver, Matt; Thomas, Peter C.; and West, Michael D. (2005), "Phobos: A Critical Link Between Moon and Mars Exploration",Report of the Space Resources Roundtable VII: LEAG Conference on Lunar Exploration, League City, TX 25–28 Oct 2005.LPI Contrib. 1318, p. 72.Bibcode:2005LPICo1287...56L
^Oberg, Jamie (20 May 2009)."Russia's Dark Horse Plan to Get to Mars".Discover. Archived fromthe original on 12 August 2022. Retrieved19 July 2021.The total delta-v required for a mission to land on Phobos and come back is startlingly low—only about 80 percent that of a round trip to the surface of Earth's moon. (That is in part because of Phobos's feeble gravity; a well-aimed pitch could launch a softball off its surface.)
^Arias, Francisco J. (2017).On the Use of the Sands of Phobos and Deimos as a Braking Technique for Landing Large Payloads on Mars. 53rd AIAA/SAE/ASEE Joint Propulsion Conference. Atlanta, GA.doi:10.2514/6.2017-4876.ISBN978-1-62410-511-1. AIAA 201–4876.
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