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 11 km (7 mi). It orbits 6,000 km (3,700 mi) 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 −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) 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 2 centimetres (0.79 in) 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.) 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 26 by 23 by 18 kilometres (16 mi × 14 mi × 11 mi), 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 −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) 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 9 km (5.6 mi) 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 30 metres (98 ft) deep, 100 to 200 metres (330 to 660 ft) wide, and up to 20 kilometres (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 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.
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