If life exists on Saturn's hydrogen- and methane-rich moon Titan, there will be two sure signs: hydrogen depletion near the lunar surface, and less acetylene than we'd expect. The Cassini probe has found both. So...could there be life on Titan?
Because Titan is so cold, it has lakes of methane and ethane where there would be lakes of liquid water on Earth. Still, the presence of such lakes at all suggests Titan is potentially complex enough to support some exotic form of life, and in 2005 two scientists theorized microbes could in fact live in these lakes if they were able to breathe hydrogen gas and consume the organic hydrocarbon acetylene. This process is a bit like certain kinds of anaerobic respiration used by bacteria and microbes on Earth, which also use hydrogen in place of oxygen.
If such microbes did exist, then it's predicted that their consumption of hydrogen and acetylene would cause there to be less hydrogen than otherwise expected near the surface of the lakes, as well as almost no acetylene at all on Titan. NASA now reports that the Cassini spacecraft has now confirmed both of these predictions are true, which definitely opens up the possibility that Titan really is home to microbial life. The lack of acetylene is particularly intriguing, because ultraviolet rays bombarding the moon's atmosphere should trigger its near-constant production.
The hydrogen gas is also produced in the atmosphere by ultraviolet-induced chemical reactions. Once created, the hydrogen either flies off into space or sinks down to the surface. The problem is that the hydrogen isn't accumulating near the surface, which means some second chemical process is converting the gas into something else. One definite possibility is that microbes are using the gas for respiratory purposes, although scientists are quick to point out that's not the only candidate.
For instance, hydrogen and carbon could be coming together to form methane on Titan's surface, a process that would not require a biological agent. There certainly is plenty of methane, but the moon's temperature is seemingly too low for those sorts of chemical reactions to occur as quickly as they would have to in order to make all the hydrogen disappear.
As for the disappearing acetylene, it could be converting into benzene, a hydrocarbon that is quite plentiful on Titan's surface. Still, that explanation has the same problem - the acetylene is disappearing way too quickly for that to be the whole story, so either some of the acetylene is being used in respiration or there's some undiscovered catalyst that's speeding up the conversion process.
Ultimately, these signs are some of the most convincing indicators yet that there really is life elsewhere in the Solar System, although the only way to actually prove it either way will be to send something to the surface of Titan that can find some of these microbes. Until then, they will have to remain strictly hypothetical.