Titan, one of Saturn's moons, is the only satellite with a major atmosphere. It is composed largely of nitrogen, argon and methane with traces of hydrogen. Additionally, traces of hydrocarbons (ethane, propane, ethlene, etc) nitrogen compounds (hydrogen cyanide, cyanogen, etc) and carbon monoxide and dioxide have been found.
One of the things Huygens will be doing is testing for signs of life on Titan:
They think the microbes would breathe hydrogen rather than oxygen, and eat organic molecules drifting down from the upper atmosphere. They considered three available substances: acetylene, ethane and more complex organic gunk known as tholins. Ethane and tholins turn out to provide little more than the minimum energy requirements of methanogenic bacteria on Earth. The more tempting high-calorie option is acetylene, yielding six times as much energy per mole as either ethane or tholins.
“The microbes might breathe hydrogen rather than oxygen, and eat molecules drifting down from the upper atmosphere”
McKay and Smith calculate that if methanogens are thriving on Titan, their breathing would deplete hydrogen levels near the surface to one-thousandth that of the rest of the atmosphere. Detecting this difference would be striking evidence for life, because no known non-biological process on Titan could affect hydrogen concentrations as much.