The planet Mars is orbited by two irregularly shaped moons, neither larger that 15 kilometers across. The smaller of the two, named Phobos after the Greek god of terror, measures only about 11 kilometers across and is closer to its primary planet than any other moon in our solar system. It is so close to the surface of Mars, in fact, that it orbits the planet twice daily. What has led astronomers, writers, and science fiction enthusiasts to so much speculation regarding Phobos is its peculiar size and shape, making it look like a sinister extra terrestrial skull drifting through the void of space.
Discovered in 1877, Phobos has long stood out amongst non-planetary astronomical bodies due, in addition to its odd shape, to the non-reflective quality of its surface. This has led astronomers to speculate that it may have originally been an interstellar asteroid that was caught in Mars’ gravitational orbit. The porous nature of its composition suggests that Phobos’ mass is startlingly low, an observation which led to a widely-held theory, popular in the 1950s and 1960s, that Phobos is nothing more than a hollow metal shell, perhaps serving as a Martian space station. This reasoning has been called into serious question by more recent observations.
When studying Phobos from afar, much attention has been paid to its series of odd and quite large craters. Several of these are named after characters from Jonathan Swift’s 1726 novel Gulliver’s Travels, which hypothesized that Mars has two moons many years before this was discovered to be the truth. The largest crater, dubbed Stickney after the wife of the astronomer who first identified Phobos, measures 9 kilometers across.
What makes Phobos an object of particular interest, however, is not its curiously porous composition. Phobos stands out in the night sky because of the undefinably ghastly quality of its appearance. Whether imagined as the rotted skull of an immense species long extinct, a ghostly bit of refuse from a distant, wicked solar system, or a malicious eye socket gazing at us from a mere 35 million miles away, Phobos is aptly named. It seems fitting, then, that Phobos itself is not long for this world. Due to its low orbit, Phobos is expected to, in about 11 million years, crash into the surface of Mars, crushing this celestial wraith into bits of dust.
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