When lakes on Earth dry out they turn salty, and it makes sense that the same thing happened on Mars. As the Curiosity Mars rover climbs to find younger rocks, it has not only confirmed this, but found some of the salts left behind, a record of the last time life could have flourished, rather than merely survived, on Mars.
Gale crater, which Curiosity is exploring, was chosen in part because it offers the opportunity to study sedimentary rocks of different ages layered on top of each other. A paper in Nature Geoscience reports that among these Curiosity has found intermittent clay-bearing deposits containing 30-50 percent calcium sulfate by weight.
All these rocks date to the Hesperian period, making them 3.3-3.7 billion years old. Similarly, rich deposits have not been found in the crater’s older rocks.
Dr William Rapin of the California Institute of Technology and co-authors attribute the presence of these salts to infiltration of rocks by the waters of the crater’s long-gone lake when it was very salty. Older rocks were also exposed to the lake’s waters, but at the time these were much less salty. Anything much younger never knew the touch of water, although it is still possible Curiosity will find some more recent examples.
Like a desert lake on Earth, the waters of Gale crater evaporated, leaving an ever saltier residue, but on Mars, it appears this was an intermittent process lasting 400 million years.
Even without water, the rocks have been weathered over the vast span of time since then, and the calcium sulfate-enriched portions are more resistant to erosion, leading to miniature versions of the formations seen in places like Monument Valley, where harder rocks jut up above the terrain.
In the midst of the 150 meters (500 feet) of calcium sulfate-enriched strata Curiosity found a 10-meter (33-feet) slope with 26-36 percent magnesium sulfate, but little calcium. Calcium sulfate is less soluble than magnesium sulfate, and the authors think it precipitated out first, with more soluble salts deposited in the final drying stage.
“Our findings do not compromise the search for life in Gale crater; terrestrial magnesium sulfate-rich hypersaline lakes are known to accommodate halotolerant biota, and crystallization of sulfate salts may also aid preservation of biosignatures,” the paper notes.
Gale crater is not unique in having salts like these. Even today occasional bursts of salty water are seen. Martian orbiters have detected the spectra of sulfate deposits laid down across much of Mars as the planet dried out. However, this is the first time a rover has been able to run its instruments over such material. Moreover, the intermittent bursts of sulfate salts Curiosity found demonstrate Gale crater experienced several rounds of drying, with wet periods in between, rather than a single great drought that never ended.