A sensitive search for SO 2 in the martian atmosphere: Implications for seepage and origin of methane

Abstract

Mars was observed near the peak of the strongest SO 2 band at 1364–1373 cm −1 with resolving power of 77,000 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The observation covered the Tharsis volcano region which may be preferable to search for SO 2. The spectrum shows absorption lines of three CO 2 isotopomers and three H 2O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 12 ± 1.0 pr. μm , in agreement with the simultaneous MGS/TES observations of 14 pr. μm at the latitudes (50° S to 10° N) of our observation. Summing of spectral intervals at the expected positions of sixteen SO 2 lines puts a 2 σ upper limit on SO 2 of 1 ppb. SO 2 may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO 2 lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO 2 should be rather uniformly distributed across Mars. Seepage of SO 2 is less than 15,000 tons per year on Mars which is smaller than the volcanic production of SO 2 on the Earth by a factor of 700. Because CH 4/SO 2 is typically 10 −4–10 −3 in volcanic gases on the Earth, our results show seepage is unlikely to be the source of the recently discovered methane on Mars and therefore strengthen its biogenic origin.