Nasa’s
Curiosity rover measures fluctuations of gas not easily explained by geology or
organic material dropped by meteorites
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| The sun rises over Gale Crater on Mars where the Curiosity rover detected the methane fluctuations. Photograph: Stocktrek Images/Alamy |
Nasa’s
robotic rover, Curiosity, has detected fluctuating wafts of methane on Mars,
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.
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| A computer-generated image of the Curiosity rover. Photograph: Nasa/PA |
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
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.
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