9/20/13 10:58 AMLife on Mars? Well, Maybe Not - NYTimes.com

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/NASA, via Associated Press

A view of Gale Crater near Mars’sequator. The panorama comprisesnearly 900 images taken by Curiosity.

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Nasa/ Jpl-Caltech/Malin Space/EuropeanPressphoto Agency

The Darwin site, with rocks ofparticular interest.

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But Marvin apparently did not emit enough methane forCuriosity’s sensitive instruments to find him.

“You don’t have direct evidence that there is microbialprocess going on,” said Sushil K. Atreya, a professor ofatmospheric and space science at the University ofMichigan and a member of the science team.

But NASA scientists are going strictly by their data, andthey are leery about drawing broader implications to thequestion once posed by David Bowie, “Is there life onMars?” John P. Grotzinger, the project scientist for theCuriosity mission, would go only so far as to say that thelack of this gas “does diminish” the possibility of methane-exhaling creatures going about their business on Mars.

“It would have been great if we got methane,” Dr. Atreyasaid. “It just isn’t there.”

Curiosity, which has been trundling across the planet for alittle over a year, made measurements from Martian springto late summer, coming up empty for methane.

Scientists have long thought that Mars, warm and wet in itsearly years, could have been hospitable for life, and the newfindings do not mean that it was not. But that was aboutthree and a half billion years ago. Methane molecules breakapart over a few centuries — victims of the Sun’s ultravioletlight and of chemical reactions in the atmosphere — so anymethane in the air from primordial times would havedisappeared long ago.

That is why reports of huge plumes of methane rising overMars in 2003 fueled fresh hopes for Martian microbes.Those findings, based on data from telescopes on Earth anda spacecraft orbiting Mars, set off a surge of speculation

and scientific interest.

On Earth, most of the methane comes from micro-organisms known as methanogens, butthe gas is also produced without living organisms, in hydrothermal vents. Eitherpossibility would be a surprising result for Mars.

After the 2003 methane readings, “a lot people got excited and started working on it,” saidChristopher R. Webster of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and thelead author of the paper in Science. “It was a very important result, because of themagnitude of methane.” The fresh data from Curiosity brings the earlier claims intoquestion.

Not everyone is daunted. Robert Zubrin, president of the Mars Society, a nonprofit groupdedicated to the planet’s exploration and settlement, said he was still convinced thatMartian life was waiting to be discovered in underground aquifers.

“If it had found methane, that would have been killer,” Dr. Zubrin said, referring toCuriosity. “Yes, it’s disappointing in that we didn’t get a pony for Christmas. But it doesn’tmean there aren’t ponies out there.”

9/20/13 10:58 AMLife on Mars? Well, Maybe Not - NYTimes.com

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One of the scientists who found the methane plumes in 2003, Michael J. Mumma, ofNASA’s Goddard Space Flight Center in Greenbelt, Md., said in an interview this week hewas certain that his earlier measurements were still valid. He said he now believed thatmethane on Mars was episodic — released in large plumes and then quickly destroyed. Hesuggested, half-jokingly, that there could be huge colonies of methane-eating microbes onMars that eliminated the gas from the air.

Dr. Mumma acknowledged that he could not identify any phenomena that would explainwhy methane plumes spurted out that year but not more recently, or how methane couldbe destroyed much more quickly on Mars than on Earth.

“Mars may not be operating the same way,” he said. “It’s a puzzle.”

Dr. Atreya of the Curiosity team said he originally thought that highly reactive chemicalson the Martian surface could be destroying methane, as Dr. Mumma envisioned. But“that’s not panning out,” Dr. Atreya said.

A simpler explanation would be that there was never much in the way of methane — ormicrobes — on Mars.

Kim Stanley Robinson, a science fiction author, wrote three novels in the 1990s about thecolonization of Mars by people from Earth, which in his version of things begins in 2026.Except for one conversation between two scientists, he completely leaves out thepossibility of indigenous Martian microbes.

“In my Mars trilogy, I assumed what everyone assumed back then, which was that it was adead rock,” Mr. Robinson said by e-mail on Thursday. “Actually, it would be veryproblematic to write that book today.”

These days there are plans, on paper, to send humans to Mars in roughly Mr. Robinson’stime frame. One of them, a private effort called Mars One, which has yet to prove it has thetechnology to achieve its goals, has nevertheless attracted hundreds of thousands of peopleto apply for a one-way trip, which theoretically would arrive in 2023.

Mars is smaller than Earth and would have cooled off sooner after the formation of thesolar system. Some scientists have even suggested that all life on Earth could be descendedfrom Martian microbes that were carried here embedded within meteorites. As the surfaceof Mars turned cold and dry and most of the air dispersed to space, microbes could havemigrated underground and persisted, the thinking goes.

To pursue the methane mystery, Curiosity was outfitted with an instrument that canmeasure minute quantities of methane and other gases. The first measurements byCuriosity last fall showed a definite signal from methane. “When we saw it for it for thefirst time, we went ‘Oh, my gosh,’ ” Dr. Webster said.

But that turned out to be from residual air from Earth carried all the way to Mars. Oncethe Earth air was pumped away, the methane readings disappeared, too. Last November,the scientists reported an upper limit of 6 parts per billion. Now they have pushed thatdown to 1.3 parts per billion and expect to improve their precision by at least anotherfactor of 10 in the coming months.

As exciting as it is to see the beautiful full-color pictures of the Martian landscape thatCuriosity sends back, it is the tantalizing prospect of creatures living on a neighboringplanet that fuels public interest the most, space enthusiasts say.

“That’s the mythology,” said Seth Shostak, an astronomer with the Seti Institute in