Phobos' days may be numbered

According to an article in IO9 Mars' two moons once had company. Astronomers studying two oddly elliptical craters on Mars' surface just north of Olympus Mons have theorized that Mars once had a third moon. This moon would have been small by any standards at 1.5 kilometers. The impact zone is two oval craters that lie 7.8 miles apart and they are almost exactly aligned from east to west. Most impact craters are round, so the oval craters indicate that 1 or 2 objects struck Mars at a very oblique angle. The double impact could indicate that the impactor broke up under gravitational shear or stresses encountered when striking the atmosphere. Of course it could also have been a binary asteroid which has president in other impacts on other moons in the solar system. Which brings us to diminutive Phobos. The moon is slowly dropping in altitude due to tidal forces. In about 11 million years it will either crash into Mars or be ripped apart through gravitational shear.

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