Bruce Jakosky: We've never sent a spacecraft to Mars just to explore its upper atmosphere. If you imagine Mars as having had a thicker, warmer atmosphere earlier in its history, where did the atmosphere go? Where did the water go? And there are only two places it can go--the atmosphere can go down or it can go up and be lost to space. MAVEN is the Mars Atmosphere and Volatile Evolution Mission. We're trying to understand basically why the climate changed on Mars--why Mars appears to have gone from an environment that was habitable--to microorganisms, at least--to one that is the cold, dry, uninhabitable environment we see today. By looking at the nature of the upper atmosphere today, and how gases can be lost out of the atmosphere to space today, we learn about the processes that control the atmosphere, and we're gonna have a good understanding of what the history of the atmosphere has been.