Methane (CH₄) is chemically unstable in the current oxidizing atmosphere of Mars.
It would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases.
Therefore, a persistent presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas.

The ESA-Roscomos Trace Gas Orbiter, which has made the most sensitive measurements of methane in Mars' atmosphere with over 100 global soundings, has found no methane to a detection limit of 0.05 parts per billion (ppb).
However, there have been other reports of detection of methane by ground-based telescopes and Curiosity rover.
Trace amounts of methane, at the level of several ppb, were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003.
Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.

In 2014, NASA reported that the Curiosity rover detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014.
Four measurements taken over two months in this period averaged 7.2 ppb, implying that Mars is episodically producing or releasing methane from an unknown source.
Before and after that, readings averaged around one-tenth that level.
On 7 June 2018, NASA announced a cyclical seasonal variation in the background level of atmospheric methane.

The principal candidates for the origin of Mars' methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite formation, all of which produce H₂ that could then generate methane and other hydrocarbons via Fischer–Tropsch synthesis with CO and CO₂. It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.
Living microorganisms, such as methanogens, are another possible source, but no evidence for the presence of such organisms has been found on Mars.
There are some suspicions about the detection of methane, which suggests that it may instead be caused by the undocumented terrestrial contamination from the rovers or a misinterpretation of measurement raw data.
Methane (CH₄) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases. Therefore, a persistent presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas.
The ESA-Roscomos Trace Gas Orbiter, which has made the most sensitive measurements of methane in Mars' atmosphere with over 100 global soundings, has found no methane to a detection limit of 0.05 parts per billion (ppb). However, there have been other reports of detection of methane by ground-based telescopes and Curiosity rover. Trace amounts of methane, at the level of several ppb, were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003. Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.
In 2014, NASA reported that the Curiosity rover detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7.2 ppb, implying that Mars is episodically producing or releasing methane from an unknown source. Before and after that, readings averaged around one-tenth that level. On 7 June 2018, NASA announced a cyclical seasonal variation in the background level of atmospheric methane.
The principal candidates for the origin of Mars' methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite formation, all of which produce H₂ that could then generate methane and other hydrocarbons via Fischer–Tropsch synthesis with CO and CO₂. It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars. Living microorganisms, such as methanogens, are another possible source, but no evidence for the presence of such organisms has been found on Mars. There are some suspicions about the detection of methane, which suggests that it may instead be caused by the undocumented terrestrial contamination from the rovers or a misinterpretation of measurement raw data.
Atmosphere of Mars