Gale Crater, the region being explored by NASA's Curiosity rover, isn't the only place on Mars where ancient microbes may have thrived.
New evidence from NASA's senior robotic Mars scout, Opportunity, shows life-friendly water once mixed with telltale, clay-bearing rocks that now lie on the broken rim of Endeavour Crater, an ancient 14-mile wide basin on the other side of the planet from Gale.
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"If I were to go Mars early in time and wanted to do a well, I'd do it there," planetary scientist Ray Arvidson, with Washington University in St. Louis, told Discovery News.
"It's like drinking water," he said, as opposed to the "acidic goo" Opportunity found at a previous site.
"This would have been a niche for whatever life at the time existed," Arvidson said.
The finding dovetails with similar discoveries made by newcomer Curiosity, which, unlike Opportunity, is outfitted with a drill, onboard chemistry lab, and other instruments to hunt for potential life-friendly habitats. Opportunity's mission -- to find signs of past water -- was more basic.
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"You've got the same kind of clay minerals on a completely different part of the planet, and in a much older -- relatively speaking, hundreds of millions of years older -- succession of rocks," geologist John Grotzinger, with the California Institute of Technology, told Discovery News.
Curiosity, which arrived in August 2012, already completed the primary goal if its mission. Analysis of samples drilled out from inside a slab of once water-covered bedrock shows that Mars did indeed have the right conditions and chemistry to support life.
Curiosity scientists now are focused on a more ambitious challenge to find places where organic carbon may be shielded from the radiation-rich and highly oxidizing environment of present-day Mars.
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Likewise, the Opportunity science team is stepping up its game. Once Martian spring arrives at Endeavour Crater and there is more power for the rover's solar arrays, Opportunity will head toward what lead scientist Steve Squyres calls "the mother lode" of clays, based on chemical data collected by instruments on NASA's Mars Reconnaissance Orbiter and Europe's Mars Express satellite.
Scientists have learned to be patient -- it took Opportunity nearly three years to reach the rim of Endeavour Crater -- and count their blessings. Opportunity, along with a now-dormant sister rover, Spirit, were only designed to last 90 days. Saturday marks Opportunity's 10-year anniversary on Mars.
"It's the hardest working rover in show business," Grotzinger said.
The research is published in this week's Science.