Mars: Moons

In 1877, American astronomer Asaph Hall discovered and named the two moons of Mars, Phobos and Deimos, after the god's fierce dogs. These heavily-cratered dark pieces of rock may have been asteroids of the Asteroid Belt sucked into Mars's gravitational pull. They revolve about the planet in almost perfectly circular orbits. Their surfaces boast many carbon-rich rocks. Their low densities indicate that they may contain water ice. In fact, a Russian probe found a tiny amount of water vapor on Phobos. Neither moons have an atmosphere.

Phobos

The innermost moon of Mars at a mean distance of 9,380 km, Phobos sets the solar system record for the closest moon to its planet. In fact, it is so close that the gravitation pull of Mars is stronger than the moon's centripetal force. In 40 million years, Phobos will fall towards and crash into Mars. The highly-irregularly shaped planet is at most 27 km in diameter and 19 km at its shortest. It completes one rotation and revolution in 0.319 Earth days. This 1.08 x 10¹⁶ kg chunk of rock rises in the west and sets in the east usually twice each Martian day as observed on the Martian surface. Cracks cover the surface. A crater called Stickney is one very noticeable feature at 10 km in diameter. The moon is covered in dust up to 1 meter deep.

Deimos

This is the outermost moon of Mars at a mean distance of 23,460 km. The highly-irregularly shaped planet is at most 15 km in diameter and at least 11 km. It completes one rotation and revolution in 1.26 Earth days. This 1.8 x 10¹⁵ kg chunk of rock (pitted with impact craters) rises in the east and sets in the west. Two of the largest craters, Swift and Voltaire, measure 3 km in length. The moon is covered in regolith and does not have any measurable geological activity.

Copyright © 2000 by Gary Chan and Matthew McDermott. All rights reserved.