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The Curiosity rover's failure to detect methane on Mars is a blow to theories that the planet may still host some types of life, say mission scientists.
Telescopes and satellites have reported seeing small but significant volumes of the gas, but the six-wheeled robot can pick up no such trace.
On Earth, 95% of atmospheric methane is produced by microbial organisms.
Researchers have hung on to the hope that the molecule's signature at Mars might also indicate a life presence.
The inability of Curiosity's sophisticated instrumentation to make this detection is likely now to dent this optimism.
"Based on previous measurements, we were expecting to go there and find 10 parts per billion (ppbv) or more, and we were excited about finding it. So when you go to search for something and you don't find it, there's a sense of disappointment," said Dr Chris Webster, the principal investigator on Curiosity's Tuneable Laser Spectrometer (TLS).
The Nasa rover's search is reported online in a paper published by Science Magazine.
Curiosity has been sucking in Martian air and scanning its components since shortly after landing in August 2012.
From these tests, it has not been possible to discern any methane to within the present limits of the TLS's sensitivity.
This means that if the gas is there, it can constitute no more than 1.3ppbv of the atmosphere - equivalent to just over 10,000 tonnes of the gas.
This upper limit is about six times lower than the previous estimates of what should be present, based on the satellite and telescope observations.
Deep down
The number of 1.3ppbv is very low, and will put a question mark against the robustness of those earlier measurements.
The fact that Curiosity is working at ground level and in one location should not matter, as the Martian atmosphere is known to mix well over the course of half a year.
Methane at Mars could have a number of possible sources, of course - not just microbial activity.
It could be delivered by comets or asteroids, or produced internally by geological processes.
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Telescopes have reported relatively strong signals (red = 10ppbv) in the past
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But it is the link to life that has most intrigued planetary scientists.
Earth's atmosphere contains billions of tonnes of methane, the vast majority of it coming from microbes, such as the bacteria found in the digestive tracts of animals.
The speculation has been that some methane-producing bugs, or methanogens, could perhaps exist on Mars if they lived underground, away from the planet's harsh surface conditions.
This theory was bolstered by the previous observations making their detections in spring-time. It was suggested that the seasonal rise in temperatures was melting surface ices and allowing trapped methane to rise into the atmosphere in plumes.
But in Dr Webster's view, Curiosity's inability to detect appreciable amounts of methane now makes this scenario much less likely.
"This observation doesn't rule out the possibility of current microbial activity, [but] it lowers the probability certainly that methanogens are the source of that activity," he told the BBC's Science In Action Programme.
Or as team-member Prof Sushil Atreya, from the University of Michigan in Ann Arbor, put it: "There could still be other types of microbes on Mars. This just makes it harder for there to be microbes that kick out methane."
'Evolving story'
Dr Geronimo Villanueva is affiliated to the Catholic University of America and is based at Nasa's Goddard Space Flight Center.
He studies the Martian atmosphere using telescopes here on Earth. He cautioned that additional, much more precise measurements were needed from the rover before firm conclusions could be drawn.
"This is an evolving story as we get more numbers," he told BBC News.
"If Curiosity's statistics hold, it's important because it sets a new bound. Methane should last a long time in the atmosphere and the fact that the rover doesn't see it puts a big constraint on possible releases. But I would like to see more and better Curiosity results, and more orbiter results as well."
Dr Olivier Witasse is the project scientist on the European Space Agency's (Esa) Mars Express satellite, which made the very first claimed methane detection back in 2003.
He also said much more data was required.
"There is some indication from the Mars Express data - and it has not been published yet because it's a very complicated measurement - that the methane might peak at a certain altitude, at 25-40km. The Curiosity results are interesting but they have not yet settled the issue."
Esa has its ExoMars Trace Gas Orbiter launching in 2016, which will be able to make further methane searchers. And the Indian space agency (Isro) is due to despatch its Mangalyaan probe to the Red Planet later this year. This, too, has methane detection high on its list of objectives.
Curiosity itself will work to improve its readings, and will shortly deploy an "enrichment" process that will amplify any methane signal that might be present.
"We can lower that upper limit down to tens of parts per trillion, maybe 50 parts per trillion," said Dr Webster.
Quelle: BBC
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Low Upper Limit to Methane Abundance on Mars
By analogy with Earth, methane in the martian atmosphere is a potential signature of ongoing or past biological activity. During the last decade, Earth-based telescopic observations reported “plumes” of methane of tens of parts-per-billion by volume (ppbv), and those from Mars orbit showed localized patches, prompting speculation of sources from subsurface bacteria or non-biological sources. From in situ measurements made by the Tunable Laser Spectrometer (TLS) on Curiosity using a distinctive spectral pattern unique to methane, we here report no detection of atmospheric methane with a measured value of 0.18 ±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence level) that reduces the probability of current methanogenic microbial activity on Mars, and limits the recent contribution from extraplanetary and geologic sources.
Quelle: AAAS
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NASA Curiosity Rover Detects No Methane on Mars
Data from NASA's Curiosity rover has revealed the Martian environment lacks methane. This is a surprise to researchers because previous data reported by U.S. and international scientists indicated positive detections.
The roving laboratory performed extensive tests to search for traces of Martian methane. Whether the Martian atmosphere contains traces of the gas has been a question of high interest for years because methane could be a potential sign of life, although it also can be produced without biology.
"This important result will help direct our efforts to examine the possibility of life on Mars," said Michael Meyer, NASA's lead scientist for Mars exploration. "It reduces the probability of current methane-producing Martian microbes, but this addresses only one type of microbial metabolism. As we know, there are many types of terrestrial microbes that don't generate methane."
Curiosity analyzed samples of the Martian atmosphere for methane six times from October 2012 through June and detected none. Given the sensitivity of the instrument used, the Tunable Laser Spectrometer, and not detecting the gas, scientists calculate the amount of methane in the Martian atmosphere today must be no more than 1.3 parts per billion, which is about one-sixth as much as some earlier estimates. Details of the findings appear in the Thursday edition of Science Express.
"It would have been exciting to find methane, but we have high confidence in our measurements, and the progress in expanding knowledge is what's really important," said the report's lead author, Chris Webster of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We measured repeatedly from Martian spring to late summer, but with no detection of methane."
Webster is the lead scientist for spectrometer, which is part of Curiosity's Sample Analysis at Mars (SAM) laboratory. It can be tuned specifically for detection of trace methane. The laboratory also can concentrate any methane to increase the gas' ability to be detected. The rover team will use this method to check for methane at concentrations well below 1 part per billion.
Methane, the most abundant hydrocarbon in our solar system, has one carbon atom bound to four hydrogen atoms in each molecule. Previous reports of localized methane concentrations up to 45 parts per billion on Mars, which sparked interest in the possibility of a biological source on Mars, were based on observations from Earth and from orbit around Mars. However, the measurements from Curiosity are not consistent with such concentrations, even if the methane had dispersed globally.
"There's no known way for methane to disappear quickly from the atmosphere," said one of the paper's co-authors, Sushil Atreya of the University of Michigan. "Methane is persistent. It would last for hundreds of years in the Martian atmosphere. Without a way to take it out of the atmosphere quicker, our measurements indicate there cannot be much methane being put into the atmosphere by any mechanism, whether biology, geology, or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles."
The highest concentration of methane that could be present without being detected by Curiosity's measurements so far would amount to no more than 10 to 20 tons per year of methane entering the Martian atmosphere, Atreya estimated. That is about 50 million times less than the rate of methane entering Earth's atmosphere.
Curiosity landed inside Gale Crater on Mars in August 2012 and is investigating evidence about habitable environments there. JPL manages the mission and built the rover for NASA's Science Mission Directorate in Washington. The rover's Sample Analysis at Mars suite of instruments was developed at NASA's Goddard Space Flight Center in Greenbelt, Md., with instrument contributions from Goddard, JPL and the University of Paris in France.
Quelle: NASA
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