NASA's Mars rover Curiosity, dispatched to look for the chemical ingredients and environments for microbial life, has found hints of carbon, though whether this building block for life on Earth has played a similar role on Mars is unknown, scientists have said.
"Just finding carbon somewhere doesn't mean that it has anything to do with life, or the finding of a habitable environment," lead scientist John Grotzinger of the California Institute of Technology, told reporters at an American Geophysical Union conference in San Francisco on Monday.
"If you have organic carbon and you don't have any water, you don't have a habitable environment," he said.
Even with carbon and water, life needs other chemicals, such as sulfur, oxygen, phosphorous and nitrogen, to form and evolve.
"It's not unexpected that this sand pile would not be rich in organics. It's been exposed to the harsh Martian environment," said planetary scientist Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"It's really going to be an exciting hunt over the course of this mission to find early environments that might be protected from this surface Mars environment and see what we can add to the carbon story."
The rover, which touched down in August on the floor of a 150km-wide impact crater near the Martian equator, has already turned up evidence that its landing site was once covered in water.
Scientists do not know if the carbon compounds in the soil are contamination from Earth - arrived on the surface of Mars via comets or asteroids - or if they are indigenous and came from geologic or biological activities on Mars.
"It tells us that we have a lead into a measurement of one of the important ingredients that adds to a habitable environment," Grotzinger said. "We still have a lot of work to do to qualify and characterise what it is."
The rover is expected to reach a richer slice of Martian history next year when it begins examining layers of sediment in a mountain rising from the floor of the crater.
The $2bn Curiosity mission, which is slated to last two years, is NASA's first astrobiology mission since the 1970s