CUTTING EDGE

Mars's largest moon may have risen from the ashes of a previous satellite

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The origin of Mars's two moons, Phobos and Deimos, has long been a matter of debate. The idea that they are captured asteroids has been popular in the past, but the fact that they both orbit close to Mars's equatorial plane means that most planetary scientists today accept it is much more likely they formed from a disc of rock and dust circling the planet. Such a circum-Martian disc was probably formed of ejecta from a giant impact into the Red Planet.

Phobos, the larger and inner of the two moons, is spiralling ever closer to Mars due to tidal effects and will probably be broken apart by these forces within a few tens of millions of years. But Phobos's past could have been even more dramatic.