The reported presence of methane in the atmosphere of Mars is of interest to many geologists and astrobiologists, as methane may indicate the presence of microbial life on Mars, or a geochemical process such as volcanism or hydrothermal activity. Since 2004, trace amounts of methane (range from 60 ppbv to under detection limit (< 0.05 ppbv)) have been reported in various missions and observational studies. The source of methane on Mars and the explanation for the enormous discrepancy in the observed methane concentrations are still unknown and are under study. Whenever methane is detected, it is rapidly removed from the atmosphere by an efficient, yet unknown process. History of detections Methane (CH4) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to ultraviolet (UV) radiation from the Sun and chemical reactions with other gases. Therefore, a persistent or episodic presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas. The first evidence of methane in the atmosphere was measured by ESA's Mars Express orbiter with an instrument called the Planetary Fourier Spectrometer. In March 2004, the Mars Express science team suggested the presence of methane in the atmosphere at a concentration of about 10 ppbv. This was confirmed soon after by three ground-based telescope teams, although large differences in the abundances were measured between observations taken in 2003 and 2006. This spatial and temporal variability of the gas suggests that the methane was locally concentrated and probably seasonal. It is estimated that Mars produces 270 tons of methane per year. In 2011, NASA scientists reported a comprehensive search using high-resolution infrared spectroscopy from high-altitude Earth ground-based observatories (VLT, Keck-2, NASA-IRTF) for trace species (including methane) on Mars, deriving sensitive upper limits for methane (< 7 ppbv), ethane (< 0.2 ppbv), methanol (< 19 ppbv) and others (H2CO, C2H2, C2H4, N2O, NH3, HCN, CH3Cl, HCl, HO2 – all with limits at ppbv levels). In August 2012, the Curiosity rover landed on Mars. The rover's instruments are capable of making precise abundance measurements, but cannot be used to distinguish between different isotopologues of methane and so it cannot determine if it is geophysical or biological in origin. However, the Trace Gas Orbiter (TGO) can measure these ratios and point to their origin. The first measurements with Curiosity Tunable Laser Spectrometer (TLS) in 2012 indicated that there was