- If life exists on Titan, Saturn's biggest moon, we could
soon know about it - as long as it's the methane-spewing variety. The chemical
signature of microbial life could be hidden in readings taken by the European
Space Agency's Huygens probe when it landed on Titan in January.
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- Titan's atmosphere is about 5 per cent methane, and Chris
McKay of NASA's Ames Research Center in Moffet Field, California, thinks
that some of it could be coming from methanogens, or methane-producing
microbes. Now he and Heather Smith of the International Space University
in Strasbourg, France, have worked out the likely diet of such organisms
on Titan.
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- They think the microbes would breathe hydrogen rather
than oxygen, and eat organic molecules drifting down from the upper atmosphere.
They considered three available substances: acetylene, ethane and more
complex organic gunk known as tholins. Ethane and tholins turn out to provide
little more than the minimum energy requirements of methanogenic bacteria
on Earth. The more tempting high-calorie option is acetylene, yielding
six times as much energy per mole as either ethane or tholins.
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- McKay and Smith calculate that if methanogens are thriving
on Titan, their breathing would deplete hydrogen levels near the surface
to one-thousandth that of the rest of the atmosphere. Detecting this difference
would be striking evidence for life, because no known non-biological process
on Titan could affect hydrogen concentrations as much.
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- One hope for testing their idea rests with the data from
an instrument on Huygens called the GCMS, which recorded Titan's chemical
make-up as the probe descended. It will take time to analyse the raw data,
partly because hydrogen's signal will have to be separated from those of
other molecules. "Eventually, I hope, we will have numbers for at
least upper limits for hydrogen," says Hasso Niemann of Goddard Space
Flight Center in Maryland, principal investigator of the GCMS.
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- Acetylene could be easier to analyse, McKay says, and
it too might betray life. "I would guess that there would be a similar
fall-off of acetylene if the microbes are eating it." The work is
to be published in the journal Icarus.
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- http://www.newscientistspace.com/article.ns?id=dn7716
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