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Moonday: The Face of Fear

Cry “Havoc!”, and let slip the dogs of war.
–William Shakespeare, “Julius Caesar”

Phobos, the largest of Mars' two moons, as seen by the Mars Reconnaissance Orbiter. The huge indentation at the right of the image is Stickney crater.

Earth’s Moon is not the largest in the Solar System (it’s the fifth largest overall), but it’s relatively large in comparison with its host planet. Ganymede and Titan are significantly bigger, but they orbit Jupiter and Saturn, respectively. Thus, even though Mars is closer to Earth than the outer planets, the moons of Mars weren’t discovered until long after the groundbreaking discoveries of the larger Jovian and Saturnian satellites by Galileo, Cassini, and others.

Mars' second moon Deimos, as seen by the Mars Reconnaissance Orbiter.

The moons of Mars are Phobos (fear) and Deimos (terror): the sons of the war god Mars (or Ares in Greek mythology), who sometimes took canine form to spread chaos on the battlefield. (See, I did come back to the Shakespeare quote!) Both moons are tiny compared to their host planet: Phobos is a scant 27 kilometers (about 17 miles) along its longest axis, while Deimos is only 15 kilometers (9 miles) across. The minuscule size and the fact that Phobos is one of the least reflective objects in the Solar System explains why the moons escaped the knowledge of science until American astronomer Asaph Hall (of the US Naval Observatory) found them in 1877.

Phobos compared to Earth. Deimos is even smaller (only about a pixel at this resolution).

Both Phobos and Deimos are irregular in shape, and resemble nothing more than asteroids. In fact, one formation scenario suggests they originally were asteroids, captured by Mars’ gravity. However, it’s still not entirely clear how the moons formed: I’ve read about a number of ideas, including the hypothesis that there used to be a lot of moonlets in orbit around Mars, which over time crashed into each other or were ejected into space, leaving only the two we still see. We know how much impacts shaped the early Solar System, so that kind of thing doesn’t seem far-fetched to me (noting as always that I’m not a planetary scientist!).

Impacts certainly affected Phobos: it is scarred with craters, including the huge Stickney crater, the mark of a collision that must have nearly shattered the moon into pieces. Not only that, Phobos is remarkably low density compared with asteroids of similar composition, meaning that it’s not solid rock. Possibly the heavy bombardment pulverized the moon, similar to the hypothesis about Saturn’s moon Hyperion.

In fact, Phobos is ultimately doomed. While our Moon is relatively far from Earth and slowly moving away (thanks to the complex interaction of tidal forces), Phobos orbits very close in to Mars, completing one orbit in less than 8 hours! That’s a lot shorter than the Martian day: if you lived on Mars, you’d see Phobos pass overhead twice per day. While this is cool and interesting in its own right, it also means the tidal forces acting on the moon are strong: the side closer to Mars experiences a significantly stronger gravitational pull than its far side, which has the effect of pulling it even closer to the planet. At some point in the future, the tidal force may become too large, breaking the moon apart. While it seems unlikely that Phobos will make a ring around Mars, I do hope the destruction happens during my lifetime—it would be a sight to behold, and teach us so much about the way moons break apart.

Because Phobos is small and not particularly massive, it has low surface gravity. That means even though it’s the same distance from us as Mars, it’s a lot easier to land on (not to mention having no atmosphere!). For that reason, Russia built a probe known as Phobos-Grunt, which contained a number of interesting experiments. Phobos-Grunt’s mission was to travel to the moon, collect samples, and return them to Earth. Of special note was the Planetary Society‘s LIFE project, carrying microbes to test whether they could survive the harsh environment of space and return safely to Earth. Unfortunately, the propulsion system on Phobos-Grunt failed shortly after launch last fall, and the probe crashed back to Earth in January 2012. The idea is still sound, however, and hopefully future missions will carry out the research program.

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