The European Space Agency probe Mars Express has been orbiting the Red Planet for just over seven years now, returning vast amounts of information. It looks at Mars, of course, but also the two dinky and weird moons Phobos and Deimos. For example, a little while back it took this phenomenal shot of Phobos over the limb of Mars:
[Click to greatly barsoominate.]
That’s fantastic! Note how dark Phobos appears; it really is much less reflective than Mars. Its origin is unclear, but a popular idea is that it’s an asteroid Mars captured long ago. I’ve never been comfortable with this idea, since capturing an object is extremely difficult. An asteroid moving past a planet will just fly on by unless it is slowed considerably, and there aren’t many ways to do that. If it passes extremely close, the atmosphere of the planet might slow it sufficiently, but that results in a highly elliptical orbit that’s unlikely to last very long. Perhaps Phobos was a binary asteroid, and one of the two components absorbed the extra energy and was ejected, while the other settled into orbit and became Phobos. Maybe it got its start in some other way entirely.
We’ll hopefully learn more when the Russian probe (and lander with sample return!) Phobos-Grunt launches later this year. In the meantime, Mars Express is in an orbit that periodically brings it close to Phobos several times, and we’re entering a new season of passes right now. In fact, it just had a close encounter with Phobos, and word has it the flyby was a success! That means we’ll soon be getting more even interesting and beautiful images of this enigmatic little moon, so keep your eyes open for them.