The Martian atmosphere today is too cold and thin for water to exist as liquid for long periods of time. Yet we often hear that Mars is an appealing place to look for evidence of past surface life. Why?

Mars contains abundant evidence of long-lived liquid surface water in its past. We infer that the atmosphere has undergone significant change, including removal of atmospheric particles to space. I’ll discuss some of the evidence for climate change on Mars, the mechanisms that strip the atmosphere away, and the newly arrived MAVEN spacecraft mission to measure atmospheric loss processes. I’ll connect MAVEN’s goals to two big picture questions about the connection between planetary magnetic fields and climate and the surface habitability of exoplanets.