Phobos. Mars’ innermost moon.
This tiny little spud of a moon is almost certainly a main-belt asteroid that became captured by Mars’ gravitational field, although there are still those who favor the theory that its debris that spalled off of Mars in some past impact. (Mars certainly has an abundance of very big craters to choose from as suspects, such as the terrifyingly large Hellas Basin.) And just as it was not always orbiting Mars, neither will it remain orbiting forever. Phobos is doomed. It orbits too low to be stable, completing several orbits for each Martian day, which means that to an observer on the Martian surface, watching the moon is more like watching a particularly large satellite scoot by overhead.
One thing astronomers have debated is whether Phobos will fall intact onto the surface of Mars, perhaps creating a new super basin, or whether tidal forces will shatter the moon first, spreading it out into a ring of debris around the red planet, and some recent work gives the latter option a great deal more oomph.
Phobos is tide-locked, which is to say that like our own Moon and most other moons in the solar system, it always presents the same face towards Mars. The technical term is that it rotates synchronously (at the same rate as it revolves around Mars), so one day on Phobos is equal to one orbit. This means one end of the moon faces forwards all the time, and that turns out to be the bit in the lower right corner of this image — the massive Stickney Crater.
The other thing you notice right away is all the grooves. Striations that run nearly the full length of the moon appearing to originate broadly from Stickney. For a long time, they were though to be crater chains produced by the impact that created Stickney — splash damage, if you will. Then, as astronomers realized they didn’t really point adequately at Stickney, so a new theory developed: perhaps Phobos has gone through debris clouds before, possibly after one of the big Martian impacts. But that theory didn’t quite work out either. Now they finally have another explanation, and hopefully it will be the last they need.
The mystery of the crater chains is revealed: they’re not crater chains at all. They’re stress fractures. That’s right, Phobos is being gradually torn apart by Mars’ gravity. As it sinks ever lower in its orbit, the stresses grow more intense, and one day they will excede the structural strength of rock itself and Phobos will shatter and spread itself into a ring around Mars. The long, strange grooves along the length of Phobos are just the first visible sign of this process, which has already been underway for a long time. So ultimately, Phobos is doomed even faster now, with the breakup estimated to occur in 30-50 million years from now.
So there’s still time to put a base on Phobos, but we probably ought to hurry.