The weather forecast on early Mars was pretty extreme: asteroids, with a chance of scalding rain. A new study of the Martian climate billions of years ago suggests that this rain would have been enough to erode craters on the surface.

Previous research indicates that asteroids and comets hitting Mars would have been large enough to vaporise ice on the planet’s surface, creating vast clouds of hot water that would fall as rain over tens of years. It is possible that Martian valley networks …