Mars was not only awash with water, it also once had rainfall, according to a new French study. The evidence comes from infra-red imaging, which probed under dust deposited over the millions of years and found dense networks of dry valleys, whose branching bear the hallmarks of having been carved out by rain. The research, published in the US journal Science on Friday, could prompt a rewrite of the Martian history books, for it suggests the planet had a longer “summer” than anyone thought.