This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Titan, Saturn's largest moon and the second-biggest natural satellite in the solar system, is an unquestionably interesting place. It's a world with a thick atmosphere and with lakes, fog and rainfall—only with liquid hydrocarbons rather than liquid water.
Titan would be even more interesting if a speculation made five years ago proved out: that the moon could be teeming with extraterrestrial life. Titan-based (Titanate? Titanic?) life could dwell in liquid methane and breathe gaseous hydrogen, just as so much Earthly life dwells in liquid water and breathes gaseous oxygen. Such organisms would consume hydrocarbons such as acetylene near Titan's surface, so their presence might be recognizable by a dearth of acetylene and hydrogen at the surface, Chris McKay and Heather Smith noted in Icarus in 2005.
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A pair of new studies provide evidence for such depletions of acetylene and hydrogen, stirring up a sudden frenzy of public interest in Titan, including a characteristically bombastic treatment from the British press. The Telegraph grabbed readers' attention with the headline, "Titan: Nasa scientists discover evidence 'that alien life exists on Saturn's moon'." On Twitter, Carolyn Porco, lead scientist on the imaging team for NASA's Cassini spacecraft, bemoaned the headline and reported that she "just got an e-mail from someone 'applying' to be among those folks we send to Titan."
Now McKay, of the NASA Ames Research Center in Moffett Field, Calif., has seen fit to clear the air himself, making plain that extraterrestrial life on Titan is but one possible explanation of many—and far from the most likely.
The astrobiological significance of the lack of acetylene at Titan's surface, as measured by Cassini, rests on the finding that hydrogen seems to be disappearing at the surface as well, McKay wrote on the Cassini imaging team's Web site. "This is the key that suggests that these depletions are not just due to a lack of production but are due to some kind of chemical reaction at the surface." (Ethane, another possible fuel for methanogenic life on Titan, is also depleted.)
Nevertheless, the key finding that hydrogen is vanishing at ground level on Titan needs to be confirmed before jumping to any conclusions. The most plausible explanation for the new results, according to McKay? "The determination that there is a strong flux of hydrogen into the surface is mistaken." Other possible mechanisms for the presumed hydrogen loss include atmospheric processes that transport hydrogen out of the upper atmosphere, or nonbiological chemistry at the surface, driven by some unknown catalyst.
The existence of methane-based life churning through hydrocarbons and gaseous hydrogen is the fourth most likely explanation out of four, according to McKay. "This is a still a long way from 'evidence of life'," he wrote. "However, it is extremely interesting."
Photo of sunlight glinting off a Titan lake: NASA/JPL/University of Arizona/DLR