Ever since the Viking spacecraft sent home images of water-cut valleys on Mars in the 1970s, most Mars scientists have assumed that during its first billion years or so, the planet was shrouded in a thick, warm atmosphere capable of frequent rain. It was an environment seemingly hospitable to some form of life. But lately, some planetary scientists have been advancing a far less life-friendly view. According to these cold-and-icy thinkers, Mars never had an atmosphere thick enough to drench the planet, even in its early days. There was water on the surface all right, but it was almost always tied up as ice. On this alternative Mars, the surface was icy, dry, and hostile for many millions of years at a time, and life would have struggled to gain a foothold.