MAVEN Team Blog

Rob Lillis is an Associate Research Physicist at the University of California Berkeley Space Sciences Laboratory, a member of the MAVEN Science team and the deputy lead for the Solar Energetic Particle instrument.

Why is the surface of Mars no longer habitable?

Sounds like a straightforward question, right? However, those nine words comprise one of the most vexing questions in planetary science. There is now overwhelming evidence that Mars was once a place where liquid water flowed on the surface and, thus, life as we know it could have thrived, at least episodically. Orbiters have identified branching networks of valleys that most likely were carved by rainwater or snowmelt. Rovers have driven through ancient streambeds and found minerals that can only be formed over many years underwater. However, such stable surface water requires an atmospheric surface pressure much higher than today’s ~7 millibars (<1% of Earth’s pressure) to prevent evaporation and cause greenhouse warming.

Where did this ancient atmosphere go? If it had all been absorbed back into the crust, abundant carbonate minerals should exist on or near the surface. However, extensive surveys of Mars from orbit have revealed very little carbonate, not nearly enough to account for all the carbon dioxide that has been lost. The only other explanation: The atmosphere escaped out to space over billions of years. But how did this happen? What physical processes drove the escape? How did they vary over time as solar radiation and the solar wind buffeted Mars’ atmosphere, which lacked the protection of a global magnetic field? And, most importantly, how much total atmosphere escaped over Mars’ history?