text: The presence of methane in the atmosphere of Mars is an area of ongoing research and a highly contentious subject. Because of its tendency to be destroyed in the atmosphere by photochemistry, the presence of excess methane on a planet can indicate that there must be an active source. With life being the strongest source of methane on Earth, observing a disequilibrium in the methane abundance on another planet could be a viable biosignature.
  Since 2004, there have been several detections of methane in the Mars atmosphere by a variety of instruments onboard orbiters and ground-based landers on the Martian surface as well as Earth-based telescopes. These missions reported values anywhere between a background level ranging between 0.24 and 0.65 parts per billion by volume (p.p.b.v.) to as much as 45 ± 10 p.p.b.v.
  However, recent measurements using the ACS and NOMAD instruments on board the ESA-Roscosmos ExoMars Trace Gas Orbiter have failed to detect any methane over a range of latitudes and longitudes on both Martian hemispheres. These highly sensitive instruments were able to put an upper bound on the overall methane abundance at 0.05 p.p.b.v. This nondetection is a major contradiction to what was previously observed with less sensitive instruments and will remain a strong argument in the ongoing debate over the presence of methane in the Martian atmosphere.
  Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time. Neither its fast appearance nor disappearance can be explained yet. To rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed, as the isotopic proportions of carbon-12 to carbon-14 in methane could distinguish between a biogenic and non-biogenic origin, similarly to the use of the δ13C standard for recognizing biogenic methane on Earth.