How did Mars wind up with the skimpy atmosphere it has today?
Did ancient Mars have a thick atmosphere and cloudy skies? Did liquid water flow on its surface? It's hard to imagine either one on the dry, dusty Mars of today. But scientists think the conditions on Mars could have been quite different before the planet started to lose its atmosphere. Exactly how Mars' atmosphere thinned is still being investigated, but there are several possibilities. One is a process called sputtering, a kind of atomic billiards game in which high-energy particles from the sun collide with molecules in the atmosphere and knock them away. NASA's MAVEN mission, scheduled to launch in 2013, will study why and how quickly the atmosphere surrounding Mars turned into thin air. Watch the video to explore the idea that Mars' once-thick atmosphere sputtered away.