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 Topping the list is evidence of water and basic elements that teeny organisms could feed on, scientists said Tuesday.
"We have found a habitable environment that is 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," said chief scientist John Grotzinger of the California Institute of Technology.
The discovery comes seven months after Curiosity touched down in an ancient crater. Last month, it flexed its robotic arm to drill into a fine-grained, veiny rock and then tested the powder in its onboard labs.
Curiosity is the first spacecraft sent to Mars that could collect a sample from deep inside a rock, and scientist said they hit pay dirt with that first rock.
Mars today is a hostile, frigid desert, constantly bombarded by radiation. Previous missions have found that the planet was more tropical billions of years ago. And now scientists have their first evidence of a habitable environment outside of Earth.
This was an environment where microbes "could have lived in and maybe even prospered in," Grotzinger said.
The car-size rover made a dramatic "seven-minutes-of-terror" landing last August near the planet's equator. As high-tech as Curiosity is, it lacks the tools to detect actual microbes, living or extinct. It can only use its chemistry lab to examine Martian rocks to determine the kind of environment they might have lived in.
The analysis revealed the rock that Curiosity bore into contained a chemical soup of sulfur, hydrogen, oxygen, nitrogen, phosphorus and simple carbon
-- essential chemical ingredients for life. Also present were clay and sulfate minerals, signs that the rock formed in a watery environment.
NASA rovers Opportunity and Spirit -- before it fell silent -- also uncovered evidence of a wet Martian past elsewhere on the planet, but scientists think the water would have been too acidic for microbes.
The ancient water at Curiosity's pit stop -- possibly a former lake bed -- appears to be neutral and not too salty. It previously found a hint of the site's watery past
-- an old streambed that the six-wheel rover crossed to get to the flat bedrock.
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 Curiosity has yet to turn up evidence of complex carbon compounds, fundamental to all living things. Scientists said a priority is to search for a place where organics might be preserved.
The drilled rock isn't far from Curiosity's landing spot in Gale Crater; the rover is ultimately headed to a mountain in the crater's middle. Images from space spied signs of clay layers at the base of the mountain
-- a good spot to hunt for the elusive organics.
It has been slow going as engineers learn to handle the rover, which is far more tech-savvy than anything that has landed before on Earth's planetary neighbor.
Over the years, Mars spacecraft in orbit and on the surface have beamed back a wealth of information about the planet's geology. Scientists have also been able to study rocks from Mars that have occasionally landed on Earth.
The latest news comes during a lull in the two-year, $2.5 billion mission. Curiosity has been prevented from doing science experiments as engineers troubleshoot a computer problem.
Scientists still plan to drive toward the mountain, but not until Curiosity drills into another rock at its current location. Since flight controllers on Earth will be out of touch with Mars spacecraft for most of next month due to a planetary alignment, the second drilling won't get under way until May.
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NASA:
http://www.nasa.gov/mission_pages/msl [Associated
Press; By ALICIA CHANG]
Follow Alicia Chang at
http://twitter.com/SciWriAlicia.
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