Since it landed on Mars in 2012 NASA's Curiosity rover has been exploring Gale Crater, a vast, dry ancient lake bed with a mountain at its center. Now, Curiosity has found sediments containing sulfate salt in the crater, which suggest it once held salty lakes.
A range of salts have been found on Mars in different locations. Researchers interpret these as evidence of ancient brines. Salty water formations seemed to increase on the Martian surface as the planet transitioned to an arid climate 3.5 billion years ago.
The new detection of sulfate salts came from sedimentary rocks dated to between 3.3 and 3.7 billion years ago. Curiosity has analyzed other, older rocks on Mars and didn't detect these salts in them.
Researchers believe the salt is evidence of the crater's lake evaporating in the Red Planet's arid environment. They also believe studying younger rocks in the future could shed more light on how the Martian surface dried out.
Gale Crater was created by an ancient impact, then filled in with sediment layers over time. Mount Sharp, the mountain at its center, was created by wind erosion of the hardened sediment layers. Those same layers act like chapters in the history of Mars and provide clues to its environment at each point in time.
At the time the sulfate salts were present, lakes were probably scattered across the crater floor, fed by streams in the crater walls. Over time, the streams would overflow and then evaporate, trapped in a repetitive cycle taking place over millions of years.
The findings were published Monday in the journal Nature Geoscience .
"We went to Gale Crater because it preserves this unique record of a changing Mars," said William Rapin, lead study author at the California Institute of Technology. "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?"
The sulfate salts represent a different finding from Curiosity's evidence of freshwater lakes in the foundation of Mount Sharp back in 2015. These new salts were actually found in a section of rocks called "Sutton Island." Curiosity visited the 500-foot-tall formation in 2017. At the time, they noticed mud cracks, delineating arid periods. But the salts also provide new evidence for brines.
Pure salt is usually left behind when a lake dries and disappears, but these are mineral salts mixed in with sediment. The researchers believe this means that the salts crystallized in a wet environment beneath the briny ponds.
One comparison on Earth is the Altiplano, a high plateau in South America where saline lakes are fed by mountain streams and rivers and influenced by climate shifts.
"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 evidence over the years is contributing to a picture of Mars' past climate, which went through wet and dry cycles.
"As we climb Mount Sharp, we see an overall trend from a wet landscape to a drier one," said Ashwin Vasavada, Curiosity Project Scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
"But that trend didn't necessarily occur in a linear fashion. More likely, it was messy, including drier periods, like what we're seeing at Sutton Island, followed by wetter periods."
Earlier in its exploration, Curiosity found flat sediment layers. Now, it's exploring larger rocks that could have formed due to wind and water forces.
"Finding inclined layers represents a major change, where the landscape isn't completely underwater anymore," said Chris Fedo, University of Tennessee researcher of sedimentary layers. "We may have left the era of deep lakes behind."