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The National Aeronautics and Space Administration could get some answers soon. At Cape Canaveral in Florida a
spacecraft, the Mars Science Laboratory, lifted off on November 26 and will reach Mars next August. It will deliver
an S.U.V.-size rover named Curiosity that carries an instrument that can detect methane in the air, and if it does, it will unleash new excitement about the prospect of life on Mars.
"The conditions for the emergence of life were present on Mars - period, end of story," said Michael J. Mumma, a scientist for NASA at the Goddard Space Flight Center in Greenbelt, Maryland, who led one of three teams that have made still-controversial claims of detecting methane in Mars's atmosphere. "So life certainly could have arisen there."
Because Mars is smaller than Earth, it cooled faster, and it probably would have been hospitable for life earlier. That raises the intriguing possibility that pieces of Mars containing microbes were blasted into space by asteroid impacts and later landed on Earth, seeding life here.
The possibility of Martians has long fueled the imagination of Earthlings. But Mariner 4, the first space probe to whiz past Mars, in 1965, sent back pictures not of verdant forests, but of barren rocks.
And NASA's two Viking landers in 1976 analyzed the soil and found it devoid of the organic building blocks of life.
"There was a backlash of the people who felt the biology was oversold and premature," said Christopher F. Chyba, an astrophysicist at Princeton University. Geological markings on Mars like gullies, dry lake beds and colossal canyons point to a liquid past. NASA's last two rovers found convincing evidence of environments that were habitable in the past. Curiosity will go further, looking for carbon-based molecules, including methane, that are the building blocks of life.
Recent orbital images show that water might still occasionally flow on the surface of Mars. New knowledge about life on Earth and how it can thrive in seemingly hostile environments, like the boiling waters near ocean-bottom volcanic vents, also made scientists less dismissive of the notion that life persists on Mars. In 1996, scientists announced they had found fossilized microbes in a Martian meteorite that had landed in Antarctica. Those claims remain controversial.
But Curiosity is not carrying experiments designed to tell whether the building blocks of life ever came together to form life.
That is frustrating for Gilbert V. Levin, who believes that his experiment on the Vikings 35 years ago, designed to detect life, did indeed detect life.
Drops of a nutrient solution containing radioactive carbon-14 were added to Martian soil, and a stream of radioactive carbon dioxide was detected rising out of the soil. That is what would be expected from micro-organisms eating the food.
To rule out the possibility that a nonbiological chemical process was generating the carbon dioxide, other samples were heated to 160 degrees Celsius to sterilize them. No radioactive carbon dioxide was seen rising from those when the nutrient drops were added, fitting with the hypothesis that the heat had killed the Martian microbes. If a nonbiological process were at play, the radioactive carbon dioxide should have been seen after the sterilization as well.
But other Viking experiments had failed to measure any organic molecules, so the consensus was that the claim was mistaken.
But in 2008, NASA's Phoenix lander found chemicals known as perchlorates in the Martian soil. Viking's organic molecule detector heated the soil to release organics. Heating organic molecules in the presence of perchlorates destroys them, so if they were there, Viking's experiment may have missed them.
Dr. Levin said a more sophisticated version of his experiment could definitively validate or disprove the Viking results.
But the two lander missions that are to follow Curiosity - collaborations between NASA and the European Space
Agency - do not have a version of Dr. Levin's experiment planned. Christopher E. Carr, a scientist at the Massachusetts Institute of Technology, has proposed an even more ambitious experiment: send a DNA sequencer to Mars.
The Obama administration, mindful of tight budgets, has yet to approve the missions, and is considering canceling them.
"That would derail the whole search for life, either extinct or extant, on Mars," Dr. Mumma said. "That would be a disaster."
