Phobos

Phobos, the inner and larger of Mars’s two moons. It was discovered telescopically with its companion moon, Deimos, by the American astronomer Asaph Hall in 1877. Phobos is a small irregular rocky object with a crater-scarred, grooved surface. Hall named the moons for Phobos, the personification of panic, and his twin brother, Deimos, the personification of dread in Greek mythology. They were sons of Ares, the Greek god of war and the counterpart of the Roman god Mars, and Aphrodite, the Greek goddess of love.

Orbit

Phobos is a rough ellipsoid—a flattened egglike shape with unequal length, width, and height—and measures about 26 km (16 miles) across at its widest point. It revolves once around Mars every 7 hours 39 minutes at an exceptionally close mean distance—9,378 km (5,827 miles), making it the natural satellite closest to its planet in the solar system. Its orbit is nearly circular and is tilted at only 1° from the planet’s equatorial plane. The satellite orbits Mars faster than Mars rotates (one Martian rotation takes 24 hours 37 minutes); thus Phobos appears to move from west to east from a point of observation on the planet’s surface. The long axis of Phobos constantly points toward Mars; as with Earth’s Moon, it has a rotational period equal to its orbital period and so keeps the same face to the planet.

Surface

The heavily cratered surface of Phobos is covered with a very dark gray regolith (unconsolidated rocky debris) that reflects only about 6 percent of the light falling on it—about one-half that of the Moon’s surface. This fact and the satellite’s low mean density (1.9 grams per cubic cm) are consistent with the composition of carbonaceous chondrite meteorites, suggesting that Phobos may be a captured asteroid-like object. Remarkable linear grooves, typically 100 meters (330 feet) wide and 20 meters (65 feet) deep, cover much of the surface. There is strong evidence that they are associated with the formation of the largest crater on Phobos. This structure, known as Stickney, measures about 10 km (6 miles) across. Precise observations of Phobos’s position over the past century suggest that tidal forces from Mars are slowly pulling the satellite toward the planet. If such is the case, it will collide with Mars in the very distant future.

Discover the fascinating events that shaped the world, straight from our editors.

SIGN UP

By clicking “Sign Up”, you consent to the Britannica Privacy Policy and Terms of Use.

Britannica Quiz

Space Odyssey

Beginning with Mariner 9 in 1971, several spacecraft that have orbited Mars have also studied Phobos. In 2026, Japan launched the Martian Moons eXploration (MMX) mission, which will arrive at Mars in 2027, and orbit Mars in a similar orbit to Phobos. MMX will place a rover on Phobos’s surface, attempt to collect 10 grams (0.4 ounces) of surface material, and return that material to Earth in 2031.