Martian moons Phobos and Deimos may have once had a lunar sibling. The discovery of a pair of elliptical craters on Mars’ surface have led astronomers to speculate that a small moon may have held an unsteady orbit around the planet, only to be dragged down by Mars’ gravity and destroyed. And one thing is for certain: the Red Planet will kill again.The Martian craters are unusual in size and alignment, offering puzzling clues as to the object that caused them:
Observations of the Martian surface, just north of Olympus Mons, show two oval-shaped craters...Usually impact craters are approximately circular, so the elongated craters indicate the impactor(s) entered the atmosphere at a very shallow angle. This isn't the only strange characteristic of these two craters. They lie 12.5 km (7.8 miles) apart and they are almost exactly aligned from east to west …
One theory is that a small moonlet, roughly 1.5 km in diameter, once orbited Mars and, after sinking into the planet’s atmosphere, split in two, creating the closely aligned depressions. Another possibility is that a binary asteroid hit the surface at an extremely shallow angle, as has happened on our own moon. But even if Mars hasn’t already demolished one orbital body, it has designs on another:
Regardless, whether a third moon or binary asteroid hit Mars, it will be of little comfort to Phobos. The moon (with a mean radius of 11 km) 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. Either way, Phobos is a doomed moon.
So, in several million years, Deimos will be lunar top dog. But if I were it, I’d watch my back. Could Strange Mars Craters be from a Fallen Third Moon? [Universe Today]