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Methane (CH4) is a compound with molecules each consisting of one carbon and four hydrogens atoms. It is generally unstable, breaking down with time, so its presence on a world suggests it is being produced continually. It can be generated abiotically, but on Earth, it is continually produced by biological processes, thus it constitutes a biosignature: not a certain sign of life, but evidence of possible life. It is observed on some solar system moons. It has been detected on Mars (making subsurface life plausible but abiotic sources are also plausible: e.g., a reaction of water with olivine in rocks), but presents a mystery regarding the varying amounts detected: both whether the detections are correct, and if so, how to account for the drastic changes in amount detected: the observations showed the vast majority disappearing within a year, yet the expected means by which Mars would lose it requires hundreds of years.
If a methane molecule contains a deuterium atom, in which case the molecule can be referred to as deuterated methane or CH3D, then methods of tracing isotopes (e.g., detecting the differences in spectra and/or mass) can be used to detect details of chemical reactions. This has been done using spectrography of Titan's atmosphere with Keck Observatory.