Two potential signatures of life on Saturn's moon Titan have been found by the Cassini spacecraft. But scientists are quick to point out that non-biological chemical reactions could also be behind the observations.
Titan is much too cold to support liquid water on its surface, but some scientists have suggested that exotic life-forms could live in the lakes of liquid methane or ethane that dot the moon's surface.
In 2005, Chris McKay of NASA's Ames Research Center in Moffett Field and Heather R Smith of the International Space University in Strasbourg, France, calculated that such microbes could eke out an existence by breathing in hydrogen gas and eating the organic molecule acetylene, creating methane in the process.
This would result in a lack of acetylene on Titan and a depletion of hydrogen close to the moon's surface, where the microbes would live, they said.
Now, measurements from the Cassini spacecraft have borne out these predictions, hinting that life may be present.
Hungry for hydrogen
Infrared spectra of Titan's surface taken with the Visual and Infrared Mapping Spectrometer (VIMS) showed no sign of acetylene, even though ultraviolet sunlight should constantly trigger its production in the moon's thick atmosphere. The VIMS study, led by Roger Clark of the US Geological Survey in Denver, Colorado, will appear in the Journal of Geophysical Research.
Cassini measurements also suggest hydrogen is disappearing near Titan's surface, according to a study to appear in Icarus by Darrell Strobel of Johns Hopkins University in Baltimore, Maryland.
Observations with the spacecraft's Ion and Neutral Mass Spectrometer and its Composite Infrared Spectrometer revealed that hydrogen produced by UV-triggered chemical reactions in the atmosphere is flowing both upwards and off into space as well as down towards the surface.
Yet the hydrogen is not accumulating near the surface, hinting that something may be consuming it there. The results reveal "very unusual and currently unexplained chemistry", McKay told New Scientist. "Certainly not proof of life, but very interesting."
It is possible that the hydrogen is combining with carbon in molecules on Titan's surface to make methane. But at the low temperatures prevalent on Titan, these reactions would normally occur too slowly to account for the disappearing hydrogen.
Similarly, non-biological chemical reactions could transform acetylene into benzene – a hydrocarbon that the VIMS instrument did observe on Titan's surface. But in that case, too, a catalyst would be needed to boost reaction rates enough to account for the dearth of acetylene.
"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," says Mark Allen of NASA's Jet Propulsion Laboratory in Pasadena, California. "We have a lot of work to do to rule out possible non-biological explanations."
Jonathan Lunine of the University of Arizona in Tucson, a member of Clark's team, agrees. But he says it may not be possible to distinguish between biological and non-biological explanations without additional missions to Titan. "The only way to know for sure would be to actually get hold of an organism and show that it is alive," he told New Scientist.