Mars' moon Phobos may actually be a comet — or at least part of one — that was gravitationally captured by the Red Planet long ago, a new preprint study based on previously unpublished photos suggests.
For years, researchers have puzzled over the origins of Phobos and its twin, Deimos. Some have theorized that the moons are former asteroids lured in by Mars' gravity, because their chemical composition is similar to that of certain rocks in the main asteroid belt between Mars and Jupiter. However, computer models simulating this capture process have not been able to replicate the pair's near-circular paths around Mars.
