The topography of Mars indicates that water flowed at one point on the cold, dry planet. Scientists have long assumed that ancient Mars had a dense atmosphere that was rich with greenhouse gases like carbon dioxide and water vapor. After all, a warmer, wetter Mars with a strong greenhouse gas effect would explain the presence of liquid water on the surface. 

 

Now, new analysis shows that -- at the time when rivers flowed -- the atmospheric pressure of early Mars wasn’t sufficient to warm the surface of the planet to above freezing. The paleopressure was simply too low to sustain stable liquid water in the long term. 

 

One way to tell atmosphere thickness is by looking at craters. Whether a meteoroid of a given size will survive transit through the Martian atmosphere and form a crater -- without breaking apart -- depends on the density of the air that it has to penetrate. So the size of the smallest craters in river deposits offers information about the atmospheric pressure near the time the rivers flowed. In Earth’s atmosphere, for example, small objects entering at high speeds break up as they pass through, without surviving intact enough to form giant pockmarks. 

 

Using images captured by the Mars Reconnaissance Orbiter, a team led by Caltech’s Edwin Kite (now of Princeton) identified hundreds of small craters embedded within river deposits of the Aeolis Dorsa region near Gale Crater on Mars that date back to about 3.6 billion years ago. They compared the size distribution of the craters to numerical simulations over a range of atmospheric pressures. 

 

Around 10 percent of the craters they studied were 50 meters in diameter or less, and they found that a pressure of 0.9 bar explained those crater formations best -- that’s 150 times today’s value. Their findings suggest that while the atmospheric pressure of early Mars was much greater than it is today, it’s still less than what’s required to warm the Martian surface above freezing. 

 

Instead, they think intermittent, short-lived surface warming -- by gases released during volcanic eruptions and asteroid impacts or from the planet’s orbital changes -- may have permitted liquid water on the Martian surface temporarily.

 

“It’s clear that Mars was wet, but it’s not so clear how it was warm,” says Sanjoy Som of NASA Ames Research Center. And Martians? “The longevity of stable liquid water on the ancient Martian surface may prove to be a key factor in considering whether life could have taken hold early in the planet’s history,” Som writes in an accompanying paper. 

 

The work was published in Nature Geoscience this week. 

 

[Via Nature]

 

Image: NASA/JPL-Caltech/ASU