no methane —or less than 5 ppb— at the landing site, later calculated to a baseline of 0.3 to 0.7 ppbv. In 2013, NASA scientists again reported no detection of methane beyond a baseline. But 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 ppbv, implying that Mars is episodically producing or releasing methane from an unknown source. Before and after, readings averaged around one-tenth that level. On 7 June 2018, NASA announced the confirmation of a cyclical seasonal variation in the background level of atmospheric methane. The largest concentration of methane detected in situ by the Curiosity rover showed a spike to 21 ppbv, during an event in late June 2019. The Mars Express orbiter happened to be performing spot tracking in that area 20 hours before Curiosity methane detection, as well as 24 and 48 hours after the detection, and the TGO was performing atmospheric observations at around the same time but at a higher latitude. The Indian Mars Orbiter Mission, which entered orbit around Mars on 24 September 2014, is equipped with a Fabry–Pérot interferometer to measure atmospheric methane, but after entering Mars orbit it was determined that it was not capable of detecting methane, so the instrument was repurposed as an albedo mapper. As of April 2019, the TGO showed that the concentration of methane is under the detectable level (< 0.05 ppbv). The Perseverance rover (landed Feb 2021) and the Rosalind Franklin rover (due 2023) will not be equipped to analyze the atmospheric methane nor its isotopes, so the proposed Mars sample-return mission in the mid-2030s seems the earliest a sample could be analyzed to differentiate a geological from a biological origin. Potential sources Geophysical 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. The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface but may exist within