Scientists say they have uncovered firm evidence that Mars's biggest moon, Phobos, is made from rocks blasted off the Martian surface in a catastrophic event. The origin of Mars's satellites Phobos and Deimos is a long-standing puzzle. It has been suggested that both moons could be asteroids that formed in the main asteroid belt and were then captured by Mars's gravity. The latest evidence has been presented at a major conference in Rome. The new work supports other scenarios. Material blasted off Mars's surface by a colliding space rock could have clumped together to form the Phobos moon.