Of that, planetary scientists are certain, which leaves them puzzling over what could be producing methane gas detected in the thin Martian air. Methane molecules are easily blown apart by ultraviolet light from the Sun, so any methane around must have been released recently.
Could the gas be burbling from something alive? Cows, after all, burp methane on Earth. Other creatures, including a class of micro-organisms that live without oxygen, also produce methane.
The National Aeronautics and Space Administration could get some answers soon. On the launching pad at Cape Canaveral in Florida is a spacecraft, the Mars Science Laboratory, that is scheduled to lift off on Saturday and 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.
“Based on evidence, what we do have is, unequivocally, the conditions for the emergence of life were present on Mars — period, end of story,” said Michael J. Mumma, a senior scientist for NASA at the Goddard Space Flight Center in Greenbelt, Md., 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.
In other words: we could all be descendants of Martians.
The possibility of Martians has long fueled the imagination of Earthlings, from the Edgar Rice Burroughs Barsoom novels to the canals Percival Lowell deluded himself into seeing through his telescope to “War of the Worlds.”
Other times, the pendulum swung back the other way. 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, equipped with sophisticated life chemistry experiments, analyzed the soil and found it devoid of the organic building blocks of life.
Mars, it appeared in 1976, was really most sincerely dead.
“Things looked so grim for exobiology on Mars,” said Christopher F. Chyba, a professor of astrophysical sciences at Princeton University. “We made this tremendous investment in two Viking landers. There was a backlash of the people who felt the biology was oversold and premature.”
NASA subsequently played down the notion of life on Mars and instead set out on a methodical campaign to explore the past geology and climate of Mars. Although Mars today looks dry and cold — dead — geological markings like gullies, dry lake beds and colossal canyons point to a liquid past. “Follow the water” became the mantra. NASA’s last two rovers, Spirit and Opportunity, found convincing evidence of environments that were habitable in the distant 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 dark, boiling waters near ocean-bottom volcanic vents, also made scientists less dismissive of the notion that life persists on Mars. In 1996, a team of NASA scientists announced that they had found fossilized microbes in a Martian meteorite that had landed in Antarctica. Those claims remain at least as controversial as the methane findings.
But short of photographing a cow or some other life form ambling among the rocks, Curiosity is not going to discover life. As with every NASA probe since the Viking landers, Curiosity is not carrying experiments designed to tell whether the building blocks of life ever came together to form life. If there are microbial Martians thriving in the soil, Curiosity will not see them.
“I don’t think we’ve put down enough groundwork,” said Michael A. Meyer, NASA’s lead scientist for Mars.
That is frustrating in particular 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 320 degrees Fahrenheit 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 Dr. Levin’s results — even though they matched exactly what would be expected for life — were like announcing the discovery of a brick house in the absence of bricks. The consensus was that the claim was mistaken.
A recent discovery, however, offers a possible explanation for how Dr. Levin could be right after all. 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. But heating organic molecules in the presence of perchlorates destroys them, so even if they were there, Viking’s experiment may have missed them.
Dr. Levin said a more sophisticated version of his experiment, weighing a couple of pounds and costing a few million dollars, could definitively validate or disprove the Viking results.
“But they won’t fly it,” Dr. Levin said. “Changing a paradigm is a tough thing. We’ve run this experiment thousands of times on Earth. It’s never given a false positive. It’s never given a false negative.”
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 research scientist at the Massachusetts Institute of Technology who is intrigued by the possibility that life on Earth could have started on Mars, has proposed an even more ambitious experiment: send a DNA sequencer to Mars. That, too, has yet to find a mission to fly on.
Definitive answers may have to wait until a mission that brings Mars rocks back for study.
But that may be a very long wait. The Obama administration, mindful of tight federal budgets, has yet to give the green light on the lander missions, scheduled for 2016 and 2018, and is considering canceling them. Curiosity may be the last spacecraft landing on Mars for many years.
“That would derail the whole search for life, either extinct or extant, on Mars,” Dr. Mumma said. “That would be a disaster.”