Data collected by NASA's Curiosity rover suggests Mars once hosted dozens of shallow briny ponds that periodically overflowed and then dried.

Scientists on the Curiosity mission described their interpretation of the rover's Gale Crater observations -- and of the ancient Martian landscape -- in a new paper published Monday in the journal Nature Geoscience.

Soil and rock samples collected in 2017 from a region of Gale Crater called the "sulfate-bearing unit" have revealed a stark difference in geology, compared the samples collected at lower elevations.

Samples from the base of Mount Sharp showed Mars once hosted persistent freshwater lakes, but the latest analysis showed rock formation in the sulfate-bearing unit formed during a much drier climate.

The research has offered scientists a glimpse of a planet transitioning from its the water-rich past to the cold, arid landscape of present.

"We went to Gale Crater because it preserves this unique record of a changing Mars," lead study author William Rapin, a planetary scientist at the California Institute of Technology, said in a news update from NASA. "Understanding when and how the planet's climate started evolving is a piece of another puzzle: When and how long was Mars capable of supporting microbial life at the surface?"

With the help of Curiosity, Rapin and his research partners were able to identify large, intermittent layers of salt, including hydrated magnesium sulfate and calcium sulfate, within a deposit of sedimentary rocks known as "Sutton Island." Scientists knew from previously examined mud cracks that the region had experienced extended dry periods. But the salts identified by Rapin and company looked different than those typically produced by drying freshwater lakes.

When lakes are evaporated, pure salt crystals are usually left behind. But scientists found layers of mineral salts mixed with sediment within the Sutton Island deposit.

The findings suggest the salts were deposited in a wet environment, like one dotted by shallow, briny ponds. Scientists liken the region's ancient environment to the South America's Altiplano, where alpine lakes and streams from the surrounding mountains feed a series of saline lakes on an arid, high-altitude plateau. The lakes are very sensitive to changes in climate.

"During drier periods, the Altiplano lakes become shallower, and some can dry out completely," Rapin said. "The fact that they're vegetation-free even makes them look a little like Mars."

Curiosity's earliest observations revealed a landscape characterized by large, flat lake beds. But as the rover has explored higher and higher elevations, its instruments have revealed increasingly dynamic environs -- gradually inclined layers of unique minerals that could only have been formed by wind and streams.

"Finding inclined layers represents a major change, where the landscape isn't completely underwater anymore," said Curiosity researcher Chris Fedo. "We may have left the era of deep lakes behind."

The rover's climb up the slope of Mount Sharp has also helped scientists begin to see the gradual wet-to-dry shift that played out on Mars over the course of millions of years.

Curiosity is headed back to Sutton Island, and researchers hope the rover's observations will offer additional clues about the forces that shaped the region's unique sediment structures.

"We can't say whether we're seeing wind or river deposits yet in the clay-bearing unit, but we're comfortable saying is it's definitely not the same thing as what came before or what lies ahead," Fedo said.

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Pasadena CA (JPL) Aug 07, 2019
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