Deimos

Deimos is the tiny, irregularly shaped (about 16-km by 12-km by 10-km) outer moon of Mars, discovered in 1877 by the American astronomer Asaph Hall (1829–1907). Named after the god of panic in Greco-Roman mythology, Deimos is the smaller of the two natural Martian moons. The other moon is called Phobos (fear). Deimos orbits above the equator of the planet at a mean distance of 23,436 km. This tiny moon has an orbital period of 1.262 days (about 30 hours) and an orbital eccentricity of 0.0005. Because it makes one revolution about its shortest (10-km) axis in the same amount of time that it takes to make one orbit of Mars, astronomers say it is in a synchronous orbit—always keeping the same face toward the Red Planet. Deimos has a mass of about 1.8 × 10¹⁵ kg and an estimated mean density 1,750 kg/m3 (or 1.75 g/cm3). The acceleration of gravity on its surface is quite low, at only 0.005 m/s2.

Deimos is a dark body slightly red in coloration, and so is thought to be a captured asteroid with a composition similar to a Carbonaceous Chondrite. It is interlocked in synchronous rotation with Mars, such that its long axis is radially aligned with the planet. It shares these characteristics with its companion, Phobos.

Deimos is a dark celestial body with an albedo of 0.06. Due to the way it reflects the spectrum of sunlight, this tiny moon and its companion, Phobos, appear to possess C-type surface materials, similar to that of asteroids found in the outer portions of the asteroid belt. Like Phobos, Deimos is a lumpy, heavily cratered object. However, the impact craters on Deimos are generally smaller (less than 2.5 km in diameter) and lack the ridges and grooves observed on Phobos. Ejecta deposits are not seen on Deimos, possibly because this moon’s surface gravity is so low that the ejecta escaped to space. However, Deimos appears to have a thick regolith - perhaps as deep as 100 meters - formed over millennia as meteorites pulverized the moon’s surface.

The most widely held hypothesis concerning the origin of Deimos and Phobos is that the moons are captured asteroids. But this popular hypothesis raises some question. For example, some scientists think that it is highly improbable for Mars to gravitationally capture two different asteroids. They suggest that perhaps a larger asteroid was initially captured and then suffered a fragmenting collision, leaving Deimos and Phobos as the post collision products. Other scientists bypass the captured asteroid hypothesis altogether and speculate that the moons somehow evolved along with Mars. Expanded exploration of Mars this century should resolve this question.

Questions to Ponder

• How did Asaph Hall discover Phobos and Deimos?
• What is the atmospheric composition of Deimos?