NASA’s Curiosity rover has found salt-rich sediments indicating that Gale Crater, which it has been exploring on Mars for seven years, once had briny lakes that formed as ancient climate change caused it to go through recurrent arid cycles.
The find reinforces satellite observations indicating that Mars became substantially more arid shortly after Gale Crater was formed, about 3.5 billion years ago.
Since it landed in 2012, Curiosity has been slowly ascending the lower slopes of the giant peak at the centre of the crater, looking at ever-younger sediments as it climbs.
So far, the rover has climbed several hundred metres in the course of 21 kilometres of travel, but how many years of stratigraphy that represents is uncertain.
“We are struggling with it a little bit, because the detailed [rock] dating techniques we use on Earth can’t be transported to Mars yet,” says William Rapin, a planetary scientist at California Institute of Technology, Pasadena, California. “But we believe it’s on the order of tens of thousands to millions of years.”
What these rock layers do reveal, however, is that during this interval, Martian conditions varied wildly. Using a laser instrument called the ChemCam laser-induced breakdown spectrometer, which vaporises the rock surfaces to allow their compositions to be analysed spectroscopically, Rapin says, “We are peeling back these layers in the history of the planet.”
At the lowest points in the crater, where Curiosity landed, he says, “what we saw was evidence of sediments from a freshwater lake.” But as Curiosity climbed higher, the sediments started containing salty layers, indicating that the lake that filled the bottom of Gale Crater 3.5 billion years ago periodically dried out enough for dissolved salts to precipitate out, just as sodium chloride precipitates from evaporating seawater.
Such salts, he and his colleagues write in the journal Nature Geoscience, provide “geochemical fingerprints” of ancient lake conditions.
In Gale Crater, these fingerprints tell a story of repeated oscillations between fresh water, a fingerprint of wet times, and extremely saline water, a fingerprint of ones so arid that they compare to Bolivia’s high Altiplano desert.
“That gives us a snapshot into how the climate evolved and fluctuated into something progressively more harsh,” Rapin says. ”We see these deposits as evidence of the water level changing and salt crystalising,” he says.
It’s an important find, he says, partly because it shows what type of environment life would have had to cope with at that time, if it ever existed on Mars.
The rapid fluctuations, he says, may have been caused by rapid changes in the angle of Mars’s rotation.
Today, its spin axis is tilted 25°, very similar to Earth’s 23.5° tilt.
But unlike Earth, Mars is prone to dramatic changes in tilt, with correspondingly dramatic changes in its seasons.
And, at the time Gale Crater’s lake was drying, it had also lost enough of its atmosphere to space due to erosion from the solar wind that it was in the process of changing from its warm and wet youth to its present cold, dry conditions.
“Mars is extremely interesting in studying what big factors that influence climate change can do to a planet,” Rapin says.
Meanwhile, he notes, Curiosity continues to climb, reaching newer and younger strata with every metre of ascent. “We’re going to learn more in the near future,” Rapin says.