Elevation changes also indicate where Mars may have once had a vast northern ocean. (Credit: Mars Orbiter Laser Altimeter Science Team/NASA)

Three and a half billion years ago, an asteroid slammed into Mars. The cataclysm wasn’t terribly unusual for this period in the solar system’s history, but the fallout would leave its mark. The asteroid carved out an enormous crater. It also sent a wall of water a thousand feet high hurtling around the young Red Planet, which was much more blue at the t