First it was the overabundance of methane in certain areas of the Martian atmosphere, now a lack of certain chemicals close to the surface of Saturn’s largest moon Titan that is causing a bit of a stir in the world of astrobiology.
Titan is a cold world. So cold, water lies as permafrozen pebbles on channels suspected to be the bed of rivers through which liquid methane and ethane flood the plains during the rainy season. Robbed of any hope of a water based existence, it has been speculated that life could have thrived as a methane or ethane based concern. In 2005, it was suggested that microbes that breathed hydrogen and ‘ate’ acetylene, converting it to methane to gain energy, could exist on the surface. Now a Cassini eye view of the frigid world has shown these very chemicals to be less abundant on the surface than would be expected from looking at their abundances in the atmosphere.
There are other ways of removing hydrogen and acetylene (which, although likely to be constantly produced in the atmosphere through interactions with the ultraviolet radiation of the Sun, was totally missing within the sensitivity range of the Visual and Infrared Mapping Spectrometer). Hydrogen can be locked into the massive amounts of hydrocarbons known to exist on titan through interactions with carbon. Acetylene can also be converted into benzene without any biological activity – and benzene was observed by VIMS. However, both of these reactions require warmer temperatures or a catalyst to be present that haven’t yet been seen. So either Cassini has seen the signs of life or it has seen the signs of exotic low temperature hydocarbon chemistry, either way an interesting result.
As it happens, the Saturn orbitting probe is on its way back to Titan for a closer look at the Kraken Mare, the big lake on the small moon, following the disappearance of the hood of cloud that had been obscuring it from VIMS.