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. Atmospheric disequilibrium A disequilibrium in the abundances of gas species in an atmosphere can be interpreted as a biosignature. On Earth, life has greatly altered the atmosphere in a way that would be unlikely for any other processes to replicate. Therefore, a departure from equilibrium is evidence for a biosignature. For example, the abundance of methane in the Earth's atmosphere is orders of magnitude above the equilibrium value due to the constant methane flux that life on the surface emits. Depending on the host star, a disequilibrium in the methane abundance on another planet may indicate a biosignature. Agnostic biosignatures Because the only form of known life is that on Earth, the search for biosignatures is heavily influenced by the products that life produces on Earth. However, life that is different than life on Earth may still produce biosignatures that are detectable by humans, even though nothing is known about their specific biology. This form of biosignature is called an "agnostic biosignature" because it is independent of the form of life that produces it. It is widely agreed that all life–no matter how different it is from life on Earth–needs a source of energy to thrive. This must involve