Deimos (astronomy)

Deimos (astronomy), outermost of two small moons orbiting the planet Mars. Deimos orbits Mars at an average distance of 23,460 km (14,580 mi), completing an orbit once every 1.26 Earth days. The moon’s orbit is almost circular and is only slightly tilted relative to the Martian equator. Deimos rotates once in exactly the same amount of time that it completes one orbit, keeping one face toward Mars at all times, just as Earth’s moon shows only a single face as seen from Earth’s surface.

Deimos is irregular in shape. The moon’s longest radius is 7.5 km (4.7 mi) and its shortest radius is 5.2 km (3.2 mi). Deimos could easily fit inside a medium-sized crater on Earth’s moon. The tiny moon is made of dark, carbon-rich rocks that are common in the outer asteroid belt, a group of asteroids that orbits between the orbits of Jupiter and Mars. Deimos has a lower density than a completely rocky composition allows for, however, which means it probably also contains water ice.

Scientists are uncertain about the origins of the moons of Mars. Some scientists theorize that Deimos and the larger Martian moon, Phobos, are asteroids captured by Mars billions of years ago. At that time Mars had a thick, dense atmosphere. According to one theory, Deimos passed through the Martian atmosphere and was slowed enough that the gravity of Mars captured it. Other scientists point out that the densities of the two moons are low for asteroids and that asteroid-capture scenarios are statistically unlikely. Future missions to Mars may help solve the mystery.

Deimos is peppered with craters dug out by collisions with small asteroids. The two largest craters, Swift and Voltaire, are each about 3 km (about 1.8 mi) across. Deimos’s surface is generally smoother than that of Phobos, and is covered with a layer of loose rocky material called regolith. The United States Viking orbiters snapped detailed photographs of Deimos in the late 1970s. The photographs showed large boulders sitting on the moon’s surface. The moon is too small to hold any atmosphere and shows no evidence of geological activity.

Deimos was discovered by American astronomer Asaph Hall when Mars came near Earth in 1877. The moon is named for a character in Greek mythology, who was the son of the war god Ares and the goddess of love and beauty, Aphrodite. In Roman mythology, Deimos was an attendant of the war god Mars. Features on Deimos are generally named for authors who wrote about the Martian moons.