Mars Reconnaissance Orbiter color image of Mars' moon Deimos
Viking spacecraft image of Deimos a showing cratered surface
Mars Reconnaissance Orbiter color image of Mars' moon Deimos

Mars Reconnaissance Orbiter color image of Mars' moon Deimos
(NASA/JPL)

Viking spacecraft image of Deimos a showing cratered surface
(NASA/JPL)

Mars Reconnaissance Orbiter color image of Mars' moon Deimos
(NASA/JPL)

Chariot of Terror

Deimos [DEE-mos] is the outermost moon of Mars and is the smaller of the two. With a length of only 9.9 miles (11 km), It is also one of the smallest known moons in the Solar System. Deimos was named after Terror, another of the charioteers of the Roman god, Mars. In Greek mythology, Deimos represents dread, and is one of the sons of Ares (Mars) and Aphrodite (Venus). Deimos was discovered on August 12, 1877 by American astronomer Asaph Hall while observing Mars at the U.S. Naval Observatory. Hall discovered Phobos at the same time while searching for Martian moons.

Viking spacecraft close-up image of the northern quarter of Deimos
Viking image of Deimos showing a cratered surface
Viking spacecraft close-up image of deimos sowing craters

Viking spacecraft close-up image of the northern quarter of Deimos
(NASA/JPL)

Viking image of Deimos showing a cratered surface
(NASA/JPL)

Viking spacecraft close-up image of deimos sowing craters
(NASA/JPL)

The Smallest Moon

If we thought Phobos was a small moon, Deimos is even smaller. At 9.9 miles (11 km) in length, it is little more than a pockmarked chunk of rock in orbit around the red planet. At a distance of 14,573 miles (23,460 km), Deimos is more than twice as far from Mars than Phobos. It appears as little more than a moderately bright spot in the Martian sky. Because of the greater distance, Deimos takes over 30 hours to make one complete orbit of Mars. Like Phobos, its irregular shape bears more of a resemblance to the large asteroids in the asteroid belt between Mars and Jupiter. Research suggests that Deimos is composed of many of the same materials as the asteroid belt objects. It is possible that is was an asteroid at one time, but was captured by the gravity of Mars. Some astronomers believe that the two Martian moons may be leftover fragments of a larger object that was destroyed by a collision. We may never know for sure.

 

Features of Deimos

Like Phobos, Deimos is a rather unremarkable object. It, too, is essentially just a large rock potted with several craters. Deimos does have a much smoother appearance and fewer craters than its cousin, Phobos. The largest of these craters is two miles in diameter, about 1/5 the size of the moon itself. Unlike Phobos, Deimos shows no evidence of surface fracturing. This may be because it is twice as far from Mars as Phobos, and not subject to the same strong gravitational forces. Like Phobos, Deimos is believed to be composed of a mixture of ice and carbon-rich rock like that found in the C-type asteroids in the outer asteroid belt. Deimos has no atmosphere and no magnetic field.

 

A Different Fate

Because of its greater distance from Mars, Deimos faces a different fate than its sibling. It is not destined to crash into Mars like Phobos. Rather, Deimos is slowly moving farther away from Mars. Just like Earth's moon, it is believed that Deimos will eventually leave the orbit of its parent planet, never to be see again.

 

 

Statistics for Deimos

Discovered by

Year of Discovery

Diameter

Mean Distance from Mars

Rotational Period

Orbital Period

Orbital Eccentricity

Orbital Inclination

Main Atmospheric Component

Apparent Magnitude

......

......

......

......

......

......

......

......

......

......

Asaph Hall

1877

6.2 x 7.5 x 9.9 miles (15 x 12.2 x 11 km)

14,573 miles (23,460 km)

30.35 hours

30.35 hours

0

1.8 degrees

none

12.4

 

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