A new look at ancient craters on Mars finds five that are arrayed along an arc that’s part of a giant circle around the planet. The circle may have been Mars’ equator long ago. The craters might all have been formed when one giant asteroid broke apart, its fragments slamming into the planet at different times and locations around the then-equator, says Jafar Arkani-Hamed of McGill University in Montreal. If the analysis is right, it has implication for where water might lurk beneath the Martian surface today.