Closeup of Phobos and the crater Stickney – NASA/JPL/University of Arizona
NASA recently released several very detailed and evocative photos of Mars largest moon, Phobos taken by the Mars Reconnaissance Orbiter. Named after the god of fear, Phobos revolves around Mars, the god of war. At 17×13 miles, the moon is too small for gravity to crush it into a sphere. Instead it’s an oblong, crater-pocked rock closely resembling an asteroid. It’s possible that sometime in the distant past, Mars’ gravity snatched Phobos for itself on a close encounter.
The little moon is dominated by one big crater, named Stickney, which is nearly six miles across. Notice all the grooves that appear to radiate from the crater. Astronomers believe these are stress cracks caused by the impact of such a large meteorite on a small moon.
From the surface of Mars, Phobos would only be about a third as big as our moon. That’s not the only difference. Because it orbits so close to Mars (5,286 miles), it zips around the planet in just under eight hours. During a Martian night, you’d watch Phobos rise in the west and set in the east about four hours later.
Phobos at left, and video of a partial eclipse of the sun by Phobos – NASA
During its frequent passes around the globe, Phobos routinely passes in front of the sun during the daytime, causing partial eclipses. All this Phobic fun will come to an end in 30-80 million years. The tug of Mars gravity on the low-orbiting moon is causing it to drop six feet closer to the planet per century. Eventually it will either collide with Mars, or break apart into lots of teeny tiny Phobos bits, and form a ring around the planet.