

Mars Has Always Been a Cold, Cold Desert

Mars never had the thick, luxurious atmosphere required to make it a mini-Earth way back when.


It's almost a given now that, 3.9 billion years ago, Mars had lakes of water on its surface. But new research from a Caltech team, published today in Nature Communications, seems to point to one thing: Mars never had that much atmosphere, so those lakes were probably less like a tropical paradise and more like the desert regions of Antarctica. 

"I think it clarifies the picture in our head that maybe Mars was always cold and arid," Bethany Ehlmann, a professor of planetary science at Caltech and coauthor of the study, said. Ehlmann added that the likely scenario is that the atmosphere when there was liquid water on the surface was just warm enough and just thick enough to ensure that the water didn't evaporate away.

Ehlmann and co-authors Renyu Hu, David M. Kass, Bethany L. Ehlmann, and Yuk L. Yung focus in on tracking carbon in the early atmosphere of Mars in the paper, "Tracing the fate of carbon and the atmospheric evolution of Mars." The team, lead by Hu, focuses in on the conditions seen on the surface now, specifically the absence of a carbon "reservoir."

Essentially, in models in which Mars has a more Earth-like atmosphere, one large agent accounts for the loss of massive amounts of carbon. But if the atmosphere were never Earth-like under those conditions, it better explains the geology we see on the red planet now. Rather than a massive loss of carbon volatiles, there was a small amount of carbon compounds in the atmosphere that gradually dwindled away to create the present then atmosphere. This accounts for the smaller than expected amount of carbonate deposits left on the surface. That is to say, there is no missing carbon to find.

Ehlmann and her co-authors based their observations off the work of Curiosity, as well as other data collected from orbital measurements. It will later be cross referenced with data from the MAVEN mission, which characterizes the elements associated with the loss of Mars' atmosphere long ago. So far, the Caltech team's results line up with MAVEN data. The culprit behind the atmospheric loss seems to be the loss of the magnetic field, with the sun stripping away layers of the atmosphere shortly after.

But rather than a lush, Earth-like atmosphere, the air pressure at its densest never reached above one bar, or the air pressure on Earth at sea level, if it even reached one bar. The atmosphere of Mars 3.9 billion years ago was likely a dry place with seasonal lakes, making it something akin to a frigid Death Valley, or dry areas of Antarctica that experience seasonal melting, places where "there is running water on the surface, it's available seasonally, but otherwise it's dry," Ehlmann says – right up until they bloom back to shallow lakes again. That water likely came not from the air, but from underground. 

"I think a lot of the action on early Mars was underground," Ehlmann says. Underneath the surface, an underground system of aquifers and hydrothermal vents likely existed, which may still feed recurring slope lineae recently spotted seasonally on present-day Mars. There may have still been life on the planet – but much of it may have been underground. 

MAVEN still has plenty of mysteries to unravel about the planet, and indeed further data may bolster the hypothesis set forth by the CalTech team. But if you want to explain where the carbon reservoir on Mars went, the answer may be that there never was reservoir to find. 

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