Missing chemicals on Titan could signal life

Two new studies about the chemical makeup of Titan, Saturn’s hydrocarbon-shrouded moon, raise the possibility that methane-based bacteria might exist on its surface, munching on acetylene and hydrogen.

Newly reported deficits of acetylene and hydrogen could more readily be explained without invoking biology, says astrobiologist Chris McKay of NASA’s Ames Research Center in Moffett Field, Calif. The findings, he says, may nonetheless have implications for the possibility of life on Titan, which is believed to harbor methane lakes.

One of the new studies, posted online March 15 in Icarus, focuses on a computer simulation indicating that hydrogen molecules flow downward from Titan’s atmosphere but are somehow missing from the surface. Hydrogen molecules are generated in Titan’s atmosphere when ultraviolet light breaks down methane and acetylene molecules.

Using data from two spectrometers on the Cassini spacecraft, Darrell Strobel of Johns Hopkins University in Baltimore found that hydrogen molecules flow out of the atmosphere at a rate of 10,000 trillion trillion a second. But the analysis found no corresponding buildup at the surface. Strobel says it’s unlikely that hydrogen is sequestered in a cave or beneath the surface. And because Titan is so cold, a catalyst would be required to convert hydrogen molecules and acetylene back into methane, the researchers say.  

“These results are suggestive for exotic life, but by no means a clincher,” says Strobel. “What we want to do next is actually to measure something that may prove or disprove the abiotic and biological hypotheses,” he says – like the existence of such a catalyst.
A second paper, posted online on April 28 at the Journal of Geophysical Research, reports using another Cassini spectrometer to find a lack of acetylene. Both acetylene and benzene are expected to be produced when sunlight strikes the methane gas in Titan’s atmosphere, then fall to the surface. Roger Clark of the U.S. Geological Survey in Denver and colleagues found loads of benzene on the surface but no acetylene, even though acetylene is predicted to be the more abundant of the two compounds in the atmosphere.

Taken alone, the lack of acetylene, as well as a previously found deficit of ethane, wouldn’t make for much of a story, says McKay. The low abundances could have been due to a smaller than expected production of these compounds. 

But low acetylene and low ethane, plus low hydrogen, could equal life, McKay notes. Several researchers have suggested that hydrocarbons on Titan could be the basis for life, playing the same role that liquid water does on Earth.

Just as organisms on Earth combine molecular oxygen with organic compounds to get energy, organisms on Titan might react molecular hydrogen with organic materials such as acetylene. McKay and Heather Smith, now of Utah State University in Logan, proposed such a mechanism five years ago.  It’s therefore intriguing that both acetylene and hydrogen are missing on the surface, McKay says.

When combined with hydrogen, acetylene is a potentially huge source of metabolic energy, “large enough to drive a biosphere,” comments planetary scientist David Grinspoon of the Denver Museum of Nature & Science. And because the combination of acetylene and molecular hydrogen produces methane, it would also solve a major mystery on Titan. Methane is easily broken down by sunlight and destroyed, yet somehow the moon maintains a plentiful supply of it. Organisms that react hydrogen and acetylene might be the answer.

Yet although some scientists seem willing to speculate about life on Mars, “people are very cautious about saying anything suggesting openness to the possibility of extant life on Titan,” Grinspoon says. “We’re very hung up on the idea of needing liquid water” to support life, he says.

Still, cautions McKay, nonbiological explanations are more likely to account for the findings.

“It is important to note that each of the observations separately can be explained by abiotic processes,” adds Jonathan Lunine, a professor of physics at the University of Rome Tor Vergata who is on leave from the University of Arizona in Tucson and is a coauthor of the acetylene study. “However, the coupling of the two together is intriguing, and really calls for more detailed in situ measurements on a future mission that could test more sensitively for evidence of life, especially in the lakes and seas.”