IF LIFE exists on Titan, Saturn鈥檚 biggest moon, we could soon know about it 鈥� as long as it鈥檚 the methane-spewing variety. The chemical signature of microbial life could be hidden in readings taken by the European Space Agency鈥檚 Huygens probe when it landed on Titan in January.
Titan鈥檚 atmosphere is about 5 per cent methane, and Chris McKay of NASA鈥檚 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.
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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聯The microbes might breathe hydrogen rather than oxygen, and eat molecules drifting down from the upper atmosphere聰
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.
One hope for testing their idea rests with the data from an instrument on Huygens called the GCMS, which recorded Titan鈥檚 chemical make-up as the probe descended. It will take time to analyse the raw data, partly because hydrogen鈥檚 signal will have to be separated from those of other molecules. 鈥淓ventually, I hope, we will have numbers for at least upper limits for hydrogen,鈥� says Hasso Niemann of Goddard Space Flight Center in Maryland, principal investigator of the GCMS.
聯Hydrogen levels near the surface could betray life, because no known non-biological process could affect them that much聰
Acetylene could be easier to analyse, McKay says, and it too might betray life. 鈥淚 would guess that there would be a similar fall-off of acetylene if the microbes are eating it.鈥� The work is to be published in the journal Icarus.
