Life on Saturn moon? Well . . . maybe

Could Saturn’s cold, cloud-covered moon Titan host an exotic form of primitive life based on methane, in sharp contrast to the water-based life of Earth?

That’s one amazing possibility implied by two new related scientific studies based on data from NASA’s Saturn-orbiting Cassini spacecraft. But a leading astrobiologist from Tucson, who is a co-author on one of the two papers, says it’s best to be cautious about such a potentially profound conclusion.

Separate peer-reviewed papers, published online this week in the journal Icarus and the Journal of Geophysical Research, found chemical processes in the atmosphere and on the surface of Titan that could be explained by an extreme form of methane-based life. This surface life would “eat” the hydrogen that was found by one of the studies to be raining down from the moon’s dense atmosphere, which is even thicker than Earth’s.

Meanwhile, other observations of Titan by instruments aboard the Cassini spacecraft found a distinct lack of hydrogen on the surface, suggesting that it is being transformed or consumed by some unknown process that could be either physical or biological.

“It’s as if you have a hose and you’re squirting hydrogen onto the ground, but it’s disappearing,” said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on the hydrogen flow in the atmosphere.

“I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should ‘float’ to the top of the atmosphere and escape,” Strobel said in a NASA press release.

The surface of Titan is so cold (at minus 290 degrees Fahrenheit) that a chemical process involving an unknown mineral catalyst or some sort of un-Earthly biology would be needed to convert all of the hydrogen—and its close chemical cousin acetylene—
into the large amount of methane that is ubiquitous on the distant moon.

Planetary scientists are intrigued by the findings, because they so closely match the most common concepts for hypothetical methane-based life, nearly all of which involve the consumption of hydrogen in some form.

In response to an e-mail query, the University of Arizona’s Jonathan Lunine noted that while there is a clear lack of acetylene on Titan's surface, there is also a large amount of benzene, which is a known non-biological product of acetylene. “The results reported in the press release are intriguing, but the jury's still out” on the presence of life, said Lunine, who was recently elected to the prestigious National Academy of Sciences and is on a research sabbatical in Italy.

Mark Allen, a principal investigator with a team at the NASA Astrobiology Institute that studies Titan, stated that one non-biological possibility is that sunlight or cosmic rays are transforming the acetylene in the icy aerosols in the atmosphere into more complex
molecules before they fall to the ground and leave no signature of acetylene.

More data from Cassini, and from Earth-based telescopes, is clearly needed, the scientists agreed.

“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” Allen stated. “We have a lot of work to do.”