Search for methane and upper limits to ethane and SO2 on Mars

The IRTF/CSHELL observations in February 2006 at LS = 10° and 63–93°W show ∼10 ppb of methane at 45°S to 7°N and ∼3 ppb outside this region that covers the deepest canyon Valles Marineris. Observations in December 2009 at LS = 20° and 0–30°W included spectra of the Moon at a similar airmass as a telluric calibrator. A technique for extraction of the martian methane line from a combination of the Mars and Moon spectra has been developed. The observations reveal no methane with an upper limit of 8 ppb. The results of both sessions agree with the observations by Mumma et al. (Mumma, M.J. et al. [2009]. Science 323, 1041–1045) at the same season in February 2006 and are smaller than those in the PFS and TES maps. Production and removal of the biological methane on Mars do not significantly change the redox state of the atmosphere and the balance of hydrogen. A search for ethane at 2977 cm−1 results in an upper limit of 0.2 ppb. However, this limit does not help to establish the origin of methane on Mars. Reanalysis of our search for SO2 using TEXES confirms the recently established upper limit of 0.3 ppb. Along with the lack of hot spots and gas vents with endogenic heat sources in the THEMIS observations, the very low upper limit to SO2 on Mars does not favor geological methane that is less abundant than SO2 in the outgassing from the terrestrial planets.

► Comparison with Moon spectra improves extraction of methane from Mars spectra.
► Methane is detected at 3–10 ppb in February 2006.
► Methane abundance is less than 8 ppb in December 2009.
► Abundance of ethane on Mars is less than 0.2 ppb.