Mars' liquid water may have had an atmospheric "escape route"

At a time when Carbon dioxide levels in the Martian atmosphere were between 10 and 100 times less than required for surface temperatures to be warm enough for liquid water, Gale Crater was probably like a glacial lake surrounded by partially or seasonally frozen large chunks of ice, Fairén added.

The CO2 level in Mars' primitive atmosphere 3.5 billion years ago was too low for sediments, such as those found by NASA's Curiosity exploration vehicle in areas like the Gale Crater on the planet's equator, to be deposited.

In a new Martian mystery, the rover Curiosity detected no evidence of carbonate within the red planet's rocks. It did not detect the presence of any carbonate minerals in the bedrock samples it analysed. Pretty much all Mars scientists agree that billions of years ago the planet had flowing rivers and lakes on its surface. There is life virtually wherever there is water on Earth, so these findings raise the possibility that Mars was once a home to life, and might host it still.

"We've been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined", Thomas Bristow, of NASA's Ames Research Center in Moffett Field, California, said.

Other greenhouse gases such as sulfur-dioxide could have been responsible for maintaining liquid water but would be hard to detect, as Bristow explained to Space.com: "The downside of all these other greenhouse gases is that they tend to be quite reactive, so when you put them in the atmosphere, they don't hang out an especially long time".

Scientists can't quite reconcile the carbon dioxide amounts on Mars today from epochs gone by.

Paul Niles, a planetary scientist at NASA's Johnson Space Center who wasn't involved in the research, said one explanation could be that Mars was icy overall with brief repeated warm periods in which water melted and formed the suggestive features of rivers and lakes.

Bristow was the lead author of a study on the topic published in the journal Proceedings of the National Academy of Sciences. Clues such as isotope ratios in today's Martian atmosphere indicate the planet once held a much denser atmosphere than it does now. An atmosphere with plenty of carbon dioxide would be the simplest answer, Dr. Bristow said, but "it doesn't seem that easy solution will work in this case".

For the past two decades, researchers have used spectrometers on Mars orbiters to search for carbonate that could have resulted from an early era of more abundant carbon dioxide. One successful model proposes a thick carbon-dioxide atmosphere that also contains molecular hydrogen.

The paper suggests that perhaps layers of ice over Martian lakes could have kept carbonates from forming, but Curiosity has found no geological evidence of an ice layer in Gale Crater.

If the lakes were not frozen, the puzzle is made more challenging by the new analysis of what the lack of a carbonate detection by Curiosity implies about the ancient Martian atmosphere.

Additionally, the new NASA study found only tens of millibars (one one-thousandth of sea-level air pressure on Earth) of carbon dioxide present when the lake at Gale Crater existed. "That's not what we see in the rock records", he says. Carbon dioxide is a gas capable of generating a powerful "greenhouse effect" and, therefore, of heating the planet.

  • Tracy Ferguson