Typically, when a lake dries up entirely, it leaves piles of pure salt crystals behind. But the Sutton Island salts suggest the water also concentrated into brine. Based on a series of mud cracks at a location named “Old Soaker,” the team already knew the area had intermittent drier periods. He and his co-authors describe salts found across a 500-foot-tall (150-meter-tall) section of sedimentary rocks called “Sutton Island,” which Curiosity visited in 2017. “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?” “We went to Gale Crater because it preserves this unique record of a changing Mars,” said lead author William Rapin of Caltech. Now exposed on the mountain’s slopes, each layer reveals a different era of Martian history and holds clues about the prevailing environment at the time. After the sediment hardened, wind carved the layered rock into the towering Mount Sharp, which Curiosity is climbing today. Sediment carried by water and wind eventually filled in the crater floor, layer by layer. Gale Crater is the ancient remnant of a massive impact. It represents a stark difference from lower down the mountain, where Curiosity discovered evidence of persistent freshwater lakes. This latest clue may be a sign of findings to come as Curiosity heads toward a region called the “sulfate-bearing unit,” which is expected to have formed in an even drier environment. Scientists would like to understand how long this transition took and when exactly it occurred. The deposits serve as a watermark created by climate fluctuations as the Martian environment transitioned from a wetter one to the freezing desert it is today. The authors interpret rocks enriched in mineral salts discovered by the rover as evidence of shallow briny ponds that went through episodes of overflow and drying. That is the landscape described by Curiosity scientists in a Nature Geoscience paper published today. Watch history in fast forward, and you’d see these waterways overflow then dry up, a cycle that probably repeated itself numerous times over millions of years. Streams might have laced the crater’s walls, running toward its base. Imagine ponds dotting the floor of Gale Crater, the 100-mile-wide (150-kilometer-wide) ancient basin that Curiosity is exploring. If you could travel back in time 3.5 billion years, what would Mars look like? The picture is evolving among scientists working with NASA’s Curiosity rover.
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