A stringent upper limit to SO2 in the Martian atmosphere

Surfur-bearing molecules have been found at the surface of Mars by the Viking lander, the Spirit and Opportunity rovers, and the OMEGA infrared spectrometer aboard Mars Express. However, no gaseous sulfur-bearing species have ever been detected in the Martian atmosphere. We search for SO₂ signatures in the thermal spectrum of Mars at 7.4 μm using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). Data were obtained on Oct. 12, 2009 (Ls = 353°), in the 1350-1360 cm^-1 range, with a spatial resolution of 1 arcsec (after convolution over three pixels along the N-S axis and two steps along the E-W axis) and a resolving power of 80 000. To improve the signal-to-noise ratio (S/N), we co-added the Martian spectrum around the positions of nine selected SO₂ transitions with a high S/N and no telluric contamination. From a mean spectrum, averaged over 35 pixels in the region of maximum continuum, we infer a 2σ upper limit of 0.3 ppb to the SO₂ mixing ratio, assuming that our instrumental errors are combined according to Gaussian statistics. Our upper limit is three times lower than the upper limit derived by Krasnopolsky (2005, Icarus, 178, 487), who used the same technique on previous TEXES data. In addition, we derive an upper limit of 2 ppb at each spatial pixel of the region observed by TEXES, which covers the longitude ranges 50 E-170 E for latitudes above 30 N, 100 E-170 E for latitudes between 0 and 30 N, and 110 E-170 E for latitudes between 15 S and 0. The non-detection of localized SO₂ sources in the observed area is consistent with a homogeneous distribution being expected around equinox for non-condensible species with a lifetime longer than the global mixing time. In view of the typically large SO₂/CH₄ ratio observed in terrestrial volcanoes, and assuming a comparable volcanic composition for Mars and the Earth, our result reaffirms that a volcanic origin is unlikely for any methane in the Martian atmosphere.