While it looks like a barren wasteland today, Mars was once much more friendly towards life, and one of the main goals of the Curiosity rover mission was to determine if the Red Planet has ever harbored living organisms. The project has already discovered that the rover's landing site, Gale Crater, was once home to an ancient lake, and now a team at Los Alamos National Laboratory has found "halos" in the bedrock, suggesting the area could have supported life long after the lake dried up.

Although liquid water does still exist on or near the surface of the Red Planet, it's thought that Mars hasn't been habitable for about 3.3 billion years – around the time that the Gale Crater lake dried up for good. But the Los Alamos findings suggest that the window for habitability could extend considerably later than that.

The team reached that conclusion by studying images taken in 2015 by Curiosity on the lower slope of Mount Sharp. Rising out of the center of Gale Crater, Mount Sharp was likely formed by sediments being deposited onto the lakebed over time, and as it emptied and refilled over tens of millions of years, different layers built up a time capsule of the area.

The older sedimentary bedrock at the bottom of the crater contains high concentrations of silica, which should be mostly absent in the younger rocks, higher up the mountain. But the images, taken some 20 to 30 m (66 to 98 ft) up Mount Sharp, clearly show lighter-colored bedrock, high in silica, surrounding fractures in the more recent rock. These halos indicate that the older rock was swept up into the younger ones in the form of dunes, which would only form after the lake had dried up. The cracks in the newer rock, meanwhile, indicate that groundwater still persisted at the same time.

The halos can be seen as the lighter sections of bedrock, around the fractures, in this image from 2015(Credit: NASA/JPL-Caltech)

"The concentration of silica is very high at the centerlines of these halos," says Jens Frydenvang, lead author of the paper. "What we're seeing is that silica appears to have migrated between very old sedimentary bedrock and into younger overlying rocks. Gale Crater once held a lake with water that we would even have been able to drink, but we still don't know how long this habitable environment endured. What this finding tells us is that, even when the lake eventually evaporated, substantial amounts of groundwater were present for much longer than we previously thought – thus further expanding the window for when life might have existed on Mars."

Considering these observations are being taken on another planet, it's hard to pin down estimates of how much longer Mars might have been habitable. However, it is possible to compare the ages of rock layers relative to each other and study their interactions.

"Exact dating of anything on Mars is very difficult," Frydenvang tells New Atlas. "Dating rock on Mars is based on crater counting, and crater counting that is calibrated based on what we saw on the moon and the measurements made on the lunar material that was brought back to Earth. Hence, all notions of 'years' you see have a considerable uncertainty that will only be better understood when we get useful samples returned from Mars."

As the first stage of a sample-return mission, NASA's Mars 2020 rover could help solve some of these mysteries, but until then, researchers reach their estimates based on how long geological processes usually take on Earth.

"There are a number of unknowns in this sequence of events," Frydenvang says. "Gale crater was likely buried, then exhumed, and then partly buried again in the time between (when) the lakes disappeared, and the groundwater responsible for the halos flowed through the bedrock. This would likely take millions of years based on 'Earth-experience', but things could behave differently on Mars."

The paper was published in the journal Geophysical Research Letters and the team discusses the findings in the video below.

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