Phobos is the inner and larger of Mars's two moons. It is a small, irregular rocky object with a crater-scarred, grooved surface.

A roughly ellipsoidal body, Phobos measures 26.6 km (16.5 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)—in a nearly circular orbit that lies only 1° from the planet's equatorial plane. Phobos is so near to Mars that without internal strength it would be torn apart by gravitational (tidal) forces. Because the satellite's orbital period is less than the rotational period of Mars (24 hours 37 minutes), Phobos moves from west to east in the Martian sky. The long axis of Phobos constantly points toward Mars; as with Earth's Moon, it has a rotational period





mean distance from centre of planet (orbital radius)

23,459 km

9,378 km

orbital period (sidereal period)

1.262 44 Earth days

0.318 91 Earth days

mean orbital velocity

1.4 km/s

2.1 km/s

inclination of orbit to planet's equator



eccentricity of orbit



rotation period*



radial dimensions

7.5 x 6.1 x 5.2 km

13.3 x 11.1 x 9.3 km


525 km2

1,625 km2


1.8 x 1015 kg

1.08 x 1016 kg

mean density

1.8 grams/cm3

1.9 grams/cm3

escape velocity

6 metres/s

10 metres/s




*sync. = synchronous rotation; the rotation and orbital periods are the same.

*sync. = synchronous rotation; the rotation and orbital periods are the same.

Phobos, the inner and larger of the two moons of Mars, in a composite of photographs taken by the Viking 1 orbiter in October 1978 from a distance of about 600 km (370 miles). The most prominent feature is the impact crater Stickney, which is almost half as wide as the moon itself. Also visible are linear grooves that appear to be related to Stickney and chains of small craters. NASA/NSSDC

equal to its orbital period and so keeps the same face to the planet.

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 g/cm3[1.1 oz/ in3]) are consistent with the composition of carbonaceous chondrite meteorites, suggesting that Phobos may be a captured asteroid-like object. Remarkable linear grooves, typically 100 metres (330 feet) wide and 20 metres (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 may collide with Mars, possibly in less than 100 million years.

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