The surface of Mars is completely hostile to life as we know it. Martian deserts are blasted by radiation from the sun and space. The air is so thin, cold, and dry, if liquid water were present on the surface, it would freeze and boil at the same time. But there is evidence, like vast, dried up riverbeds, that Mars once was a warm and wet world that could have supported life. Are the best times over, at least for life, on Mars?