From Scientific American:
“If this water was around and you had been on the planet, you would have been able to drink it,” says Curiosity project scientist John Grotzinger, a planetary geologist at the California Institute of Technology in Pasadena. He and other NASA researchers announced the findings today at a news briefing in Washington DC. Previous missions to Mars have spotted clay minerals. And Curiosity itself had already found signs that liquid water once flowed across the surface. But the pinch of powder tested by Curiosity, from a rock nicknamed John Klein, is the first hard evidence of water-borne clays in a benign pH environment. “This is the only definitive habitable environment that we’ve described and recorded,” says David Blake, principal investigator for the rover’s Chemistry and Mineralogy instrument (CheMin) at NASA’s Ames Research Center in Moffett Field, California. An X-ray analysis by CheMin showed that the ground-up rock comprised mostly igneous minerals such as feldspar, pyroxene, olivine and magnetite. But at least 20% of the rock was made up of clay minerals, such as smectite, that form in the presence of water. The salts in the rock, such as halite, are of the sort that life tolerates, Blake says, unlike the iron salts found elsewhere on Mars by the rover Opportunity, which indicate an acidic environment. A second instrument on Curiosity, known as the Sample Analysis at Mars (SAM), heated a fraction of a gram of powder and analyzed the gases released. Water was released from the sample at relatively high temperatures, which is characteristic of clay minerals and a good confirmation of CheMin’s findings, says Paul Mahaffy, principal investigator for SAM at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
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