Enlarge / Phobos, shown in this image, had once been thought to have been an asteroid captured by Mars. (credit: NASA APOD)
Our Solar System’s two innermost rocky planets have no moons.

Earth has an unusually large one, the product of a massive collision.

And Mars’ two moons, Phobos and Deimos, are… well, weird, looking like asteroids but not behaving like them. Now, a new paper suggests that the moons’ oddity could be explained by a cycle of ring building and destruction that started more than four billion years ago.
The idea solves a bunch of problems, it may create a brand new one, and, best of all, it has some testable consequences.
Phobos and Deimos don’t look especially odd on the surface.
In fact, their surfaces look a lot like a pretty mundane class of asteroids.
So it has been suggested that the two moons are simply asteroids that wandered into Mars’ gravitational field and got caught. Phobos appears to be transient; in about 70 million years, it’s expected to drop close enough to Mars to be torn apart by the planet’s gravity.
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