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Phobos(fō`bŏs), in astronomy, innermost moon, or natural satellite, of MarsMars,
in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth. Physical Characteristics
Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as
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Phobos(foh -bos) The innermost and larger of the two satellites of Mars, both of which were discovered in 1877 by Asaph Hall. Observations from Mars-orbiting spacecraft show it to be an irregular potato-shaped body, measuring 27 × 22 × 19 km with a mass of 1016 kg and a density twice that of water. It orbits above the equator of Mars at a distance of 9380 km from the center of the planet every 7.65 hours, keeping its long axis pointing toward Mars. The low density, together with a low albedo of 0.06, suggest that it resembles the water-rich carbonaceous chondrite meteorites, although it is difficult to see how such an object could have formed at the distance of Mars from the Sun. Possibly Phobos and Deimos, Mars' other small satellite, formed as more distant asteroids and were later captured by Mars in some way. The orbit of Phobos is decaying (contracting) because of tidal drag and the satellite may fall from orbit within another 100 million years.
Phobos has a regolith-covered cratered surface with the two largest craters, Stickney and Hall, having diameters of 10 and 6 km. A network of pitted grooves, typically 100–200 meters wide and 20–30 meters deep, radiate around the satellite from Stickney and are thought to have arisen from severe internal fracturing and heating caused by the impact event that formed the crater. Chains of small craters parallel to the orbit of Phobos possibly represent the secondary impacts of clumps of material swept up from orbit around Mars after being ejected from the satellite during earlier primary impacts. See Table 2, backmatter.
the innermost satellite of the planet Mars, located at a mean distance of 9,380 km from the planet. According to photographs obtained by the space probe Mariner 9 in 1971–72, Phobos is an irregularly shaped body pitted with numerous craters. It measures 26 km in length and 21 km in width. Its period of revolution around Mars is 7 hr 39 min, that is, less than the period of rotation of Mars about its axis. The albedo of Phobos’ surface is close to the albedo of the darkest areas of the moon (about 5 percent). Phobos was discovered by A. Hall in 1877.