Roughly ellipsoidal in shape, Deimos measures about 15 km (9 miles) in its longest dimension. It revolves once around Mars every 30 hours 18 minutes at a mean distance of 23,459 km (14,577 miles) in a circular orbit that lies within 2° of Mars's equatorial plane. The satellite's long axis is always directed 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.

In spite of its tiny gravity, only about a thousandth that of Earth, Deimos has retained considerable amounts of fine regolith (unconsolidated rocky debris) on its surface. It thus appears smoother than Phobos because its craters lie partially buried under this loose material. The largest crater, located near the satellite's south pole, is about 2.5 km (1.6 miles) wide. The surface of Deimos is gray and very dark; its reflectance is only 7 percent—about half that of the Moon's surface. This fact and the satellite's low mean density (less than two grams per cubic centimetre) indicate a carbonaceous composition and suggest that Deimos may be a captured asteroid-like (asteroid) object.