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Photograph of Mars, NASA and Kinetikon Pictures; of Ray Walston, Everett Collection
Mars casts a long shadow in both science and pop culture, inspiring novels and TV shows, including "My Favorite Martian."
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In findings that are as scientifically significant as they are crushing to the popular imagination, NASA reported Thursday that its Mars rover, Curiosity, has deflated hopes that life could be thriving on Mars today.
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/NASA, via Associated Press
A view of Gale Crater near Mars’s equator. The panorama comprises nearly 900 images taken by Curiosity.
Nasa/ Jpl-Caltech/Malin Space/European Pressphoto Agency
The Darwin site, with rocks of particular interest.
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The conclusion, published in thejournal Science, comes from the fact that Curiosity has been looking for methane, a gas that is considered a possible calling card of microbes, and has so far found none of it. While the absence of methane does not rule out the possibility of present-day life on Mars — there are plenty of microbes, on Earth at least, that do not produce methane — it does return the idea to the realm of pure speculation without any hopeful data to back it up.
The history of human fascination with the possibility of life on Mars is rich, encompassing myriad works of science fiction, Percival Lowell’s quixotic efforts to map what turned out to be imaginary canals, Orson Welles’s panic-inducing 1938 “War of the Worlds” radio play, and of course Bugs Bunny’s nemesis, Marvin the Martian.
But Marvin apparently did not emit enough methane for Curiosity’s sensitive instruments to find him.
“You don’t have direct evidence that there is microbial process going on,” said Sushil K. Atreya, a professor of atmospheric and space science at the University of Michigan and a member of the science team.
But NASA scientists are going strictly by their data, and they are leery about drawing broader implications to the question once posed by David Bowie, “Is there life on Mars?” John P. Grotzinger, the project scientist for the Curiosity mission, would go only so far as to say that the lack 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. Atreya said. “It just isn’t there.”
Curiosity, which has been trundling across the planet for a little over a year, made measurements from Martian spring to late summer, coming up empty for methane.
Scientists have long thought that Mars, warm and wet in its early years, could have been hospitable for life, and the new findings do not mean that it was not. But that was about three and a half billion years ago. Methane molecules break apart over a few centuries — victims of the Sun’s ultraviolet light and of chemical reactions in the atmosphere — so any methane in the air from primordial times would have disappeared long ago.
That is why reports of huge plumes of methane rising over Mars in 2003 fueled fresh hopes for Martian microbes. Those findings, based on data from telescopes on Earth and a 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, but the gas is also produced without living organisms, in hydrothermal vents. Either possibility would be a surprising result for Mars.
After the 2003 methane readings, “a lot people got excited and started working on it,” said Christopher R. Webster of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and the lead author of the paper in Science. “It was a very important result, because of the magnitude of methane.” The fresh data from Curiosity brings the earlier claims into question.
Not everyone is daunted. Robert Zubrin, president of the Mars Society, a nonprofit group dedicated to the planet’s exploration and settlement, said he was still convinced that Martian life was waiting to be discovered in underground aquifers.
“If it had found methane, that would have been killer,” Dr. Zubrin said, referring to Curiosity. “Yes, it’s disappointing in that we didn’t get a pony for Christmas. But it doesn’t mean there aren’t ponies out there.”
One of the scientists who found the methane plumes in 2003, Michael J. Mumma, of NASA’s Goddard Space Flight Center in Greenbelt, Md., said in an interview this week he was certain that his earlier measurements were still valid. He said he now believed that methane on Mars was episodic — released in large plumes and then quickly destroyed. He suggested, half-jokingly, that there could be huge colonies of methane-eating microbes on Mars that eliminated the gas from the air.
Dr. Mumma acknowledged that he could not identify any phenomena that would explain why methane plumes spurted out that year but not more recently, or how methane could be 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 chemicals on 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 — or microbes — on Mars.
Kim Stanley Robinson, a science fiction author, wrote three novels in the 1990s about the colonization 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 the possibility of indigenous Martian microbes.
“In my Mars trilogy, I assumed what everyone assumed back then, which was that it was a dead rock,” Mr. Robinson said by e-mail on Thursday. “Actually, it would be very problematic to write that book today.”
These days there are plans, on paper, to send humans to Mars in roughly Mr. Robinson’s time frame. One of them, a private effort called Mars One, which has yet to prove it has the technology to achieve its goals, has nevertheless attracted hundreds of thousands of people to 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 the solar system. Some scientists have even suggested that all life on Earth could be descended from Martian microbes that were carried here embedded within meteorites. As the surface of Mars turned cold and dry and most of the air dispersed to space, microbes could have migrated underground and persisted, the thinking goes.
To pursue the methane mystery, Curiosity was outfitted with an instrument that can measure minute quantities of methane and other gases. The first measurements by Curiosity last fall showed a definite signal from methane. “When we saw it for it for the first 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. Once the 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 that down to 1.3 parts per billion and expect to improve their precision by at least another factor of 10 in the coming months.
As exciting as it is to see the beautiful full-color pictures of the Martian landscape that Curiosity sends back, it is the tantalizing prospect of creatures living on a neighboring planet that fuels public interest the most, space enthusiasts say.
“That’s the mythology,” said Seth Shostak, an astronomer with the Seti Institute in Mountain View, Calif., which searches for intelligent life in the universe. “Mars is about life, not geology, as interesting as that is. That’s the triumph of hope over measurement, and maybe it is.”
In a month, India is to launch a Mars orbiter that also has a methane-measuring instrument to look for the gas from orbit. “They may be disappointed when they try to create maps of methane,” Dr. Webster said.
Via : nytimes.com
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