quinta-feira, 18 de dezembro de 2014
A Curiosity descobre "sopros" de metano em Marte | Curiosity rover discovers methane ‘spikes’on Mars
Nasa’s robotic rover, Curiosity, has detected fluctuating wafts of methane onMars, fuelling speculation that the gas may be coming from a form of life on the red planet.
An instrument on the six-wheeled robot measured mysterious spikes of methane that cannot easily be explained by geology or organic material transported to the planet by comets or asteroids.
“That we detect methane in the atmosphere on Mars is not an argument that we have found evidence of life on Mars, but it’s one of the few hypotheses that we can propose that we must consider,” John Grotzinger, a scientist on the Curiosity team, told the American Geophysical Union meeting in San Francisco. “Large organic molecules present in ancient rocks on Mars is also not an argument that there was once life on ancient Mars, but it is the kind of material you’d look for if life had ever originated on Mars.”
The instrument recorded a 10-fold increase in methane in the atmosphere around it and detected other organic molecules in powdered rock collected by the rover’s drill, the first definitive detection of organics in surface materials of Mars. These organics could either have formed on Mars or landed on Mars via meteorites.
“This temporary increase in methane, sharply up and then back down, tells us there must be some relatively localised source,” said Sushil Atreya, of the Curiosity science team at the University of Michigan in Ann Arbor. “There are many possible sources, biological or non-biological, such as interaction of water and rock.”
Mars satellites have detected plumes of methane from the surface before, but none as extraordinary as the sudden venting of the gas measured at Gale Crater, where evidence suggests water once flowed billions of years ago.
Curiosity, one of Nasa’s two Mars exploration rovers, landed in the 96-mile-wide crater in August 2012 and has been exploring the region since. Last year the US space agency reported that Gale contained the remains of an ancient freshwater lake that may have been a hospitable environment for life in the distant past.
The latest discovery, reported in the journal Science, followed studies of gas samples by Curiosity’s tunable laser spectrometer (TAS), which uses intense light to carry out chemical analysis. It revealed a low background level of methane which spiked over 60 Martian days.
In four sequential measurements, Curiosity showed the methane level soaring from about 0.69 parts per billion by volume (ppbv) to 7.2 ppbv. The spikes occurred within 200 to 300 metres of each other and less than a kilometre from where the lower readings were detected.
By the time Curiosity had travelled a further kilometre, the higher methane levels had disappeared. Researchers led by Chris Webster at Nasa’s Jet Propulsion Laboratory in Pasadena, California, wrote: “The persistence of the high methane values over 60 sols (Martian days) and their sudden drop 47 sols later is not consistent with a well-mixed event, but rather with a local production or venting that, once terminated, disperses quickly.”
The wind direction indicated a source to the north of the rover.
Life is the chief producer of methane on Earth, but many non-biological processes can generate the gas.
The low background level of methane detected by Curiosity could be explained by the Sun’s rays degrading organic material possibly deposited by meteors, the Nasa scientists said. But the spikes of methane required an additional source, which was unlikely to be a recent impact by a comet or asteroid. Such an object would have had to measure several metres across and would have left a large crater, no sign of which was visible.
The short timescale of the methane spikes did not suggest the gas was released from volcanic deposits trapped in ice, called clathrates. Nor did it appear to come from the release of gaseous methane that had become bound to the soil.
The Nasa authors were cautious about jumping to conclusions, but concluded that “methanogenesis” – the formation of methane by microbial bugs known as methanogens – may be one answer to the riddle.
“Our measurements spanning a full Mars year indicate that trace quantities of methane are being generated on Mars by more than one mechanism or a combination of proposed mechanisms – including methanogenesis either today or released from past reservoirs, or both.”
Monica Grady, a planetary scientist at Open University, said: “They have repeated their measurements over an extended period of time, and performed careful calibrations, so I’m sure the data are good.
“They consider several different origins, including destruction by UV radiation of in-falling organic material from interplanetary dust and meteorites, release from clathrates and even methane-generating bugs. Without additional data, such as knowing the carbon and hydrogen isotopic composition of the gas, it is difficult to assign a specific source. My own preference, based on nothing other than instinct, would be to go for clathrates rather than bugs.”
Gale Crater, on the Martian equator, was created when a large meteor struck the planet 3.5bn to 3.8bn years ago. At its centre is a high mountain, named Mount Sharp, that rises 18,000ft above the crater floor. Flowing water appears to have carved channels in the sides of the mountain and the crater walls.
Another major discovery by Curiosity was that of water bound in the fine-grained soil within the crater. Each cubic foot of Martian soil was found to contain around two pints of water, not freely accessible but attached to minerals.
The rover has reached the base of Mount Sharp and over the coming months will begin a slow ascent. Scientists are keen to explore the mountain because its sedimentary layers provide snapshots of Martian history.
The question of whether there is, or was, life on Mars may finally be answered by the European Space Agency’s ExoMars mission, which will land a 300kg rover there in 2019. It will be equipped with a two-metre drill and the ability to detect biomarkers of life. It will not be heading for Gale Crater, however. Because it will land with less precision than Curiosity, the crater and its mountain are considered too potentially hazardous.