The torrential downpours and flash floods that carved the gigantic river valleys on Mars may have resulted from a ferocious asteroid bombardments billions of years ago. The valleys indicate a wet past, but researchers have struggled to explain how Mars could ever have been warm enough to sustain rainfall that could gouge the Martian valleys. But, according to Teresa Segura, at the University of Colorado, Boulder, and her colleagues, the frequent asteroid impacts that showered Mars 3.5 billion years ago could have warmed the planet for thousands of years at a time and created conditions for heavy rainfall.