Phobos () is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god of fear and panic, who is the twin brother of Deimos and son of Ares (Mars).
Phobos is a small, irregularly shaped object with a mean radius of . It orbits from the Martian surface, closer to its primary body than any other known natural satellite to a planet. It orbits Mars 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 each Martian day. Phobos is one of the least reflective bodies in the Solar System, with an albedo of 0.071. Surface temperatures range from about on the sunlit side to on the shadowed side. The notable surface feature is the large impact crater Stickney, 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. An example of this is caused by tidal forces from Mars.
Images and models indicate that Phobos may be a rubble pile held together by a thin crust that is being torn apart by tidal interactions. Phobos gets closer to Mars by about per year.
Discovery and etymology
Phobos was discovered by the American astronomer Asaph Hall on 18 August 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 17 August at 16:06 Washington Mean Time, meaning 18 August 04:06 in the modern convention.) (Table II, p. 220: first observation of Phobos on 18 August 1877.38498) Hall had discovered Deimos, Mars's other moon, a few days earlier. The discoveries were made using the world's largest refracting telescope, the 26-inch "Great Equatorial".
The names, originally spelled Phobus and Deimus respectively, were suggested by the British academic Henry Madan, a science master at Eton College, who based them on Greek mythology, in which Phobos is a companion to the god, Ares.
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek phi (the initial of Phobos) combined with Mars's spear as the symbol of Phobos (). This symbol is not widely used.
Physical characteristics
Phobos has dimensions of , and retains too little mass to be rounded under its own gravity. Phobos does not have an atmosphere due to its low mass and low gravity. It is one of the least reflective bodies in the Solar System, with an albedo of about 0.071. Infrared spectra show that it has carbon-rich material such as that found in carbonaceous chondrites, and its composition shows similarities to that of Mars's surface. Phobos's density is too low to be solid rock, and it is known to have significant porosity. 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, but ice below the regolith is not ruled out. Surface temperatures range from about on the sunlit side to on the shadowed side.
Unlike Deimos, Phobos is heavily cratered, with one of the craters near the equator having a central peak despite the moon's small size. The most prominent of these is the crater Stickney, an impact crater in diameter, which takes up a substantial proportion of the moon's surface area. As with the Saturnian moon Mimas's crater Herschel, the impact that created Stickney probably almost shattered Phobos.
Many grooves and streaks cover the oddly shaped surface. The grooves are typically less than deep, wide, and up to 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 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 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. 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.
Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. Images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 metres thick; it is hypothesised 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.
The unique Kaidun meteorite that fell on a Soviet military base in Yemen in 1980 has been hypothesised to be a piece of Phobos, but this could not be verified because little is known about the exact composition of Phobos.
Shklovsky's "Hollow Phobos" hypothesis 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's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.Shklovsky, Iosif Samuilovich; The Universe, Life, and Mind, Academy of Sciences USSR, Moscow, 1962 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 across but less than thick. In a February 1960 letter to the journal Astronautics,Singer, S. Fred; Astronautics, February 1960 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.
Subsequently, the systematic data errors that Singer predicted were found to exist, 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 per year for the rate of altitude loss, which was later revised to per year.Singer, S. Fred; "More on the Moons of Mars", Astronautics, February 1960. American Astronautical Society, page 16 The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it to spiral inward.
The density of Phobos has now been directly measured by spacecraft to be . Current observations are consistent with Phobos being a rubble pile. Images obtained by the Viking probes in the 1970s showed a natural object, not an artificial one. Nevertheless, mapping by the Mars 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.Clark, Stuart; "Cheap Flights to Phobos" in New Scientist magazine, 30 January 2010 The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.
Named geological features
Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels.Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Categories
Craters on Phobos Some craters have been named, and are listed in the following map and table.Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Craters
Crater Coordinates Diameter(km) ApprovalYear Eponym Ref Clustril 3.4 2006 Character in Lilliput who informed Flimnap that his wife had visited Gulliver privately in Jonathan Swift's novel Gulliver's Travels D'Arrest 2.1 1973 Heinrich Louis d'Arrest; German/Danish astronomer (1822–1875) Drunlo 4.2 2006 Character in Lilliput who informed Flimnap that his wife had visited Gulliver privately in Gulliver's Travels Flimnap 1.5 2006 Treasurer of Lilliput in Gulliver's Travels Grildrig 2.6 2006 Name given to Gulliver by the farmer's daughter Glumdalclitch in the giants' country Brobdingnag in Gulliver's Travels Gulliver 5.5 2006 Lemuel Gulliver; surgeon captain and voyager in Gulliver's Travels Hall 5.4 1973 Asaph Hall; American astronomer discoverer of Phobos and Deimos (1829–1907) Limtoc 2 2006 General in Lilliput who prepared articles of impeachment against Gulliver in Gulliver's Travels Öpik 2 2011 Ernst J. Öpik, Estonian astronomer (1893–1985) Reldresal 2.9 2006 Secretary for Private Affairs in Lilliput; Gulliver's friend in Gulliver's Travels Roche 2.3 1973 Édouard Roche; French astronomer (1820–1883) Sharpless 1.8 1973 Bevan Sharpless; American astronomer (1904–1950) Shklovsky 2 2011 Iosif Shklovsky, Soviet astronomer (1916–1985) Skyresh 1.5 2006 Skyresh Bolgolam; High Admiral of the Lilliput council who opposed Gulliver's plea for freedom and accused him of being a traitor in Gulliver's Travels Stickney 9 1973 Angeline Stickney (1830–1892); wife of American astronomer Asaph Hall (see Hall crater) Todd 2.6 1973 David Peck Todd; American astronomer (1855–1939) Wendell 1.7 1973 Oliver Wendell; American astronomer (1845–1912)
Other named features 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).Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Phobos The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler.
Orbital characteristics