The lakes on Earth turn salty on drying out and the same incident happened when the Curiosity Mars climbed to identify the younger rocks. It found some of the salts which were left behind gathering insights on life could have prospered, rather than the mere survival on Mars. Gale crater was selected in part as it provides the possibility to investigate sedimentary rocks of different ages layered on top of each other. Curiosity has found periodic clay-bearing deposits containing 30-50 percent calcium sulfate by weight as reported in a Nature Geoscience paper.
All the rocks are 3.3-3.7 billion years old dating to the Hesperian period. Likewise, rich deposits have not been found in the older rocks of the crater. According to Dr. William Rapin of the California Institute of Technology and co-authors, the salts are present due to the percolation in the rocks by the waters of the bygone lakes which were very salty. Older rocks were much less salty although they were also exposed to the waters. Curiosity might detect more recent examples even though the younger ones were never touched by water.
Like a desert lake on Earth, the waters of the Gale crater evaporated, leaving a saltier residue, but it was an intermittent process on Mars that lasted 400 million years. The rocks have been subjected to forces of weathering over this vast time period even without water, and the calcium sulfate-enhanced portions are more resistant to erosion, producing mini versions of the formations in places such as Monument Valley, where harder rocks extend above the terrain.
Curiosity found a 10-meter (33-feet) slope containing 26-36 percent magnesium sulfate, in the 150 meters (500 feet) of calcium sulfate-enriched layers. Researchers believe that before the deposition of more soluble salts, it precipitated out first.
The paper mentions that their outcomes do not compromise the life search in the Gale crater. Terrestrial magnesium sulfate-rich and hypersaline lakes are known to sustain halotolerant biota while the preservation of biosignatures may be supported by crystallization of sulfate salts.
The occasional bursts of salty water are observed even today hence it is not unique to Gale crater in having such salts. As the planet dried, sulfate deposits have been identified by Martian orbiters across several places on Mars and it is the first instance where a rover has been operated its instruments over these samples. The periodic bursts of sulfate salts found by Curiosity showed Gale crater had many rounds of drying with several wet periods rather than one single great drought.
Journal Reference: Nature Geoscience