When you take a look at Mars, you probably wouldn't think that it looks like a nice place to live. It's dry, it's dusty, and there's practically no atmosphere. But some scientists think that Mars may have once looked like a much nicer place to live, with a thicker atmosphere, cloudy skies, and possibly even liquid water flowing over the surface. So how did Mars transform from a warm, wet world to a cold, barren desert? NASA's MAVEN spacecraft will give us a clearer idea of how Mars lost its atmosphere (and thus its water), and scientists think that several processes have had an impact.