The Moons of Mars: Phobos and Deimos
written by Jeff Harr on February 26, 2001


The world's first close-up photographs of Mars' moons were taken by Mariner 9 during its 1971 over flight. Rumored about for years by Kepler and others, they were finally discovered by Asaph Hall, and named after after the chariot horses of Mars, the Roman god of war. Phobos (fear) and Deimos (panic) were discovered using the telescope at the U.S. Naval Observatory, where Hall was an astronomer. Current theories explain them as captured asteroids from the belt between Mars and Jupiter, while others show evidence against this. These moons are most unlike our Eath's moon. First of all, they are extremely small in size, less than a few km in all dimensions. Also, they are roughly potato shaped, due to the fact that they don't possess enough gravity to keep them round.

The innermost and larger moon, Phobos, is a dark body that appears to be composed of C-type (blackish carbonaceous chondrite) surface materials. It is similar to the C-type asteroids that exist in the outer asteroid belt. It orbits Mars once every 7 hours, rising in the West and setting in the East. Scientists say that in 40 million years Phobos will crash into Mars' surface or be torn into tiny pieces which will continue to orbit as a ring. This is due to the 1.8cm a year that Phobos travels closer to Mars. The largest of Phobos' craters, Stickney, was named after the wife of Asaph Hall, and is 10 km in diameter, which is almost half of the average diameter of Phobos itself. The impact event that caused the formation of this crater is said to be responsible for the striations radiating around it. Probes were sent in 1988 to take detailed pictures, but the first of the Russian-made satellites lost its way, and the other returned a few photographs, but then was lost itself.

The second, smaller, and outermost moon of Mars is Deimos. It travels around Mars only once a Martian day, every 30 hours. It is similar in shape and appearance to Phobos, but is smaller and slightly less irregular in shape. It's largest crater is only 1/5th the size of Stickney. Deimos has a smoother appearance caused by the partial filling of some of its craters. Dust leaves the surface of the moon when something impacts because it doesn't have enough gravitational pull to retain it. However, the gravity from Mars keeps a ring of this debris around the planet in the same orbit as the moon. As Deimos revolves, it collects dust from this belt and fills in some of its rough spots. The regolith on Deimos in some spots can reach 50 meters deep.


Works Cited:

1) http://www.solarviews.com/eng/phobos.htm --