"This was an ancient environment with … all the prerequisites to support life — a habitable environment," reported Michael Meyer, lead scientist with NASA's Mars Exploration Program at a news conference Tuesday.
The car-sized robotic rover analyzed a sample of rock that it had drilled out of an area in Mars's Gale Crater known as Yellowknife Bay — named after the capital of Canada's Northwest Territories because of the city’s link to the exploration of North America's oldest rocks.
The powdered sample, collected in February, proved to be a fine-grained mudstone containing clay minerals and other compounds that showed it was once a body of water that wasn't too acidic, oxidizing or salty.
John Grotzinger, a Curiosity project scientist at the California Institute of Technology, said it was "so benign and supportive of life that probably if this water was around and you had been on the planet, you would have been able to drink it."
Grotzinger estimated that the comfortable conditions likely occurred around three billion years ago — around the same time as life first arose on Earth.
In addition to clay, Curiosity's first drill sample contained sulphur compounds in different forms that could theoretically provide a source of chemical energy for microbes, as they do on Earth, the researchers reported.
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Among the other chemicals detected by Curiosity were carbon dioxide that could have come from reactions involving either carbonate rock or carbon. There was also a small quantity of the simple organic compounds chloromethane and dichloromethane, which are also based on carbon — an element necessary for building life as we know it.
The researchers are planning to drill another hole in May that will help confirm the results.
Grotzinger said the mission scientists are also hoping to undertake a more systematic search to get a bigger carbon signal.
However, the mission scientists emphasized that Curiosity is not properly equipped to detect life itself.
Curiosity landed on Mars on Aug. 6 and came across an ancient riverbed two months later. It arrived at Yellowknife Bay by following the riverbed a few hundred metres.
The rover's first drilling site was chosen with the help of a range of instruments on its arm, including one called the APXS (alpha-particle-X-ray spectrometer ) that was designed by Ralf Gellert at the University of Guelph in Ontario.
The rover's camera showed rocks with a white, fine-grained material in their cracks. Then the APXS revealed that the material contained sulphate and calcium, suggesting it was likely deposited by water on top of the existing bedrock, Gellert told CBC News in an interview.
That prompted the mission scientists to try drilling there.
Gellert noted that the rover has not yet reached Mount Sharp, a target area where instruments orbiting Mars had previously detected signs of water, and so the "great result" announced Tuesday was in fact, unexpected.
"The interesting thing for me personally is where we drove now is where the orbiters don't see actually much," he said. "You see way more from the ground, of course, than you see from above."