Scientists have a new idea about what happened to Mars' once-thick atmosphere – perhaps it was never there.
Ever since an increasingly sophisticated series of Mars orbiters, landers and rovers have returned evidence that the planet once had flowing rivers, lakes and possibly even oceans on its surface, scientists have wondered how that was possible. Temperatures on Mars are usually below freezing and the planet's thin, carbon dioxide atmosphere would cause surface water to quickly vaporize.
PHOTOS: Mind-Blowing Beauty of Mars' Dunes
The most popular explanation is that about 3 billion years ago, Mars had a thicker atmosphere, which would trap heat, keeping the planet warm enough for liquid surface water. Then over time, the planet lost its atmosphere, with carbon molecules either escaping to space or ending up as carbonates on the planet's surface.
NASA's ongoing MAVEN mission is designed to test the escape-to-space theory, but recent computer models of Mars' upper atmosphere, suggest the numbers may fall short, a paper published in Nature Communications shows.
Similarly, scientists so far have not found enough carbon-based chemistry on the planet's surface to account for a huge atmospheric transition.
"Neither mechanism, alone or coupled, fully accounts for the ‘missing' carbon dioxide, if a multi-bar early atmosphere is assumed," Renyu Hu, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., and colleagues write in the Nature Communications paper.
NEWS: Mystery Solved: Water DOES Flow on Mars
Their research takes another tack. Using atmospheric and mineralogical data collected by NASA's Curiosity rover and other probes, scientists found a variety of alternative scenarios that don't require Mars to have an early, thick atmosphere.
Rather, Mars might have had only a moderately dense atmosphere, equal or less than what is found on Earth.
"Such an atmosphere could have evolved into the current thin one, not only minus the ‘missing' carbon problem, but also in a way consistent with the observed ratio of carbon-13 to carbon-12, which differ only by how many neutrons are in each nucleus," NASA wrote in a summary of the research.
"Our paper shows that transitioning from a moderately dense atmosphere to the current thin one is entirely possible," Hu, a post-doctoral fellow and the lead author on the paper, said in the NASA summary.
ANALYSIS: Mars Water: Follow the Toxic Stream to Find Alien Life
"It is exciting that what we know about the Martian atmosphere can now be pieced together into a consistent picture of its evolution -- and this does not require a massive undetected carbon reservoir," he added.
A moderate atmosphere is still enough to account for the water-related chemistry found in Martian rocks and the geological evidence of past lakes, said planetary scientist Bethany Ehlmann, with JPL and the California Institute of Technology.
"A lot of the ‘action' for mineral formation may have taken place underground and via short-term interactions near the surface," Ehlmann wrote in an email to Discovery News. "Certainly, a lot of the lakes on Mars -- as on Earth -- are fed by groundwater in addition to overland flow."
"The big remaining question is whether ice/snow melt is enough ... or whether it absolutely had to rain to explain some of the morphologies," she added.
"Rain is really hard to get, even with a thick atmosphere based on the results of atmospheric models. The extent to which some of the complex valley networks might require rain is a subject of debate ... If it had to rain, perturbation of the atmosphere by release of volcanic gases, heating by impacts, or another gas in the atmosphere are needed," Ehlmann wrote.