The long shallow grooves on Phobos are likely early signs of structural damage. Scientists now believe that Phobos is not solid but instead is a pile of rubble covered by a thick layer of dust which makes it look solid. Scientists expect the moon to be pulled apart in 30 to 50 million years thanks to Mars' gravity.
Phobos is the larger of Mars' two moons. Due to their low orbits and diminutive size, scientists have long wondered whether Mars' two moons, Deimos and Phobos, are real moons or just asteroids captured by Mars' gravity. Even so, Phobos orbits Mars at a distance of 6 000 km (3 700 miles) from the surface of the Red Planet. This makes Phobos closer to its host planet as compared to any other moon in the entire Solar System.
