The red planet wasn’t always red. Billions of years ago, it was quite a bit bluer, and perhaps greener, too: Mars likely once harbored liquid water on its surface, and may have even sustained a smidgen of life in its rivers and pools. Those ancient oases have since disappeared. But researchers think the geologic remnants of one of them still exist in Gale Crater, the 96-mile-wide dry lake bed that NASA’s Curiosity rover has been rolling through for the past seven years.

Now, the salts in Gale’s sediments are helping to tell the story of how water may have vanished from the Martian surface. Curiosity’s latest measurements suggest that the crater’s lake evaporated in fits and starts, probably in lockstep with the intermittent aridity that struck the planet roughly 3.5 billion years ago. The findings, published today in the journal Nature Geoscience, support the notion this once warmer, wetter world transformed not all at once, but through a series of climatic fluctuations that eventually left behind the cold, dry Mars we see in our skies today. “This is important work,” says Elizabeth Rampe, a Mars geologist at NASA’s Johnson Space Center who is part of the team behind Curiosity, but was not involved in the study. “One of the reasons we went to Gale Crater was to try to better understand this time in Mars’ history...and this shows us that Mars’ climate was very dynamic.” Gale Crater, a rocky scar from a meteor that punched into Martian terrain between 3.8 and 3.5 billion years ago, is one of the best time capsules researchers have of the red planet’s tumultuous past. Shortly after the depression formed, rivers and streams, and later groundwater, began to fill it up, depositing rocks and minerals into its base over millions of years. Gale Crater, a 96-mile-wide basin that formed after a meteor impacted the Martian surface between 3.8 and 3.5 billion years ago. At its center is Mount Sharp, a three-mile-high peak piled high with layers of ancient sediment that preserve the basin's geologic history. Image Credit: NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS Eventually, Mars (which also lost much of its atmosphere around this time, exposing itself to the cosmic elements) settled into its modern desert form, purging Gale of its liquid center. Years of harsh winds then carved the basin out anew, leaving behind the three-mile-high Mount Sharp at its center—a peak stacked high with layers of sediment, each chronicling a chapter of the planet’s geologic history. Since 2014, Curiosity has been climbing Mount Sharp, using a combination of cameras, lasers, x-rays, and drills to read the rock record along the way. So far, the data the rover has beamed back suggest that many of Gale’s oldest sediments contain fine, silty clays, similar to what you’d find in a freshwater lake, says study author William Rapin, a planetary scientist at Caltech. The new findings are a bit younger, dating to roughly 3.5 billion years ago—around the time the planet began to trend toward a more arid climate, says Kathryn Stack, a Mars sedimentologist at NASA’s Jet Propulsion Laboratory who is a part of the team behind Curiosity, but was not involved in the study. “This is the place in the rock record where that switch might have flipped,” she says.

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Using Curiosity’s readings, Rapin and his team found that, during this time, clay gave way to other byproducts of water like sulfates and chlorides. Unlike clays, these compounds are hallmarks of salty liquids—the kind you might find in an evaporating lake, Rapin says. Sulfate salts have been detected elsewhere in Gale, crisscrossing the surface of the crater in the form of stark white veins. Researchers think these deposits are likely the ghosts of relatively recent groundwater flows that trickled between fractured rocks, long after the basin’s waters dried up and its sediments had compacted into their current form. The sulfates described in this paper, however, appeared to be interlaced with the rock matrix itself. That suggests these salts are much older, and were present around the time the water-borne sediments were first laid down, Rampe says. For these compounds to be left behind, the atmosphere would’ve needed to be arid enough to suck a significant amount of water out of Gale’s lake, she says. A self-portrait of NASA's Curiosity Mars rover on the lower slopes of Mount Sharp in Gale Crater, taken in June of 2018. Since 2012, the rover has been studying the contents of the crater to learn more about the red planet's past. Image Credit:

NASA/JPL-Caltech/MSSS The team’s analysis showed that the salts were interspersed throughout several hundred feet of rock, spanning tens of thousands to millions of years of deposition. Some layers of sediment brimmed with calcium sulfate, a compound that precipitates out of water fairly easily. Others, however, were rich in magnesium sulfate, which deposits only after things get ultra-saline, Rapin says. “That means there were bodies of water that evaporated to near completion,” he says. But the salts in the sediments didn’t follow a straightforward path. Instead, they alternated between briny and less briny—a haphazard pattern that points to multiple bouts of drying, rather than one continuous event that parched the lake all at once, says Eldar Noe Dobrea, a Mars researcher at the Planetary Science Institute who was not involved in the study. Gale’s waters probably ebbed and flowed before checking out for good, he says. “This shows how unstable the climate of Mars was at this time,” Rapin says. (Even in modern times, the planet is no stranger to flux: Unlike Earth, the red planet wobbles on its axis, launching it onto a climatic rollercoaster every few million years.) Mount Sharp, seen through the Mast Camera of NASA's Curiosity rover. The rover has been climbing Mount Sharp since 2014, assessing its layers of sediment, which contain a record of the waters that used to fill the basin of Gale Crater. Image Credit: NASA/JPL-Caltech But in the midst of all this change, water appears to have persisted at Gale for some time, Noe Dobrea points out. Even at its briniest extremes, he says, the lake was probably still habitable for salt-tolerant microbial life—examples of which abound here on Earth. Habitable, however, doesn’t mean inhabited. The data Curiosity turns up can only hint at the potential for life. We’ll learn more if and when we bring samples from the red planet to Earth, Rampe says—a process NASA intends to start with the upcoming Mars 2020 mission. In the meantime, Curiosity likely has more to tell us: Years past its original mission expiration date, the rover’s still going strong—and there’s plenty of Mount Sharp left to explore. The mere fact that we’ve put such successful rovers on another planet is already pretty exciting, Stack says. “Thanks to studies like this, we’re starting to understand Mars in ways that we understand our own Earth.”

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