A stringent upper limit to SO₂in the Martian atmosphere

Abstract

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 SO 2 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 SO 2 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 SO 2 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 SO 2 sources in the observed area is consistent with a homogeneous distribution being expected around equinox for noncondensible species with a lifetime longer than the global mixing time. In view of the typically large SO 2 /CH 4 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.

HDO and SO2thermal mapping on Venus: evidence for strong SO2 variability

“…Similar observations were performed on Mars in October 2009 and led to a stringent upper limit (0.3 ppb) for SO 2 on Mars (Encrenaz et al 2011a). Venus was observed over three nights in 3 spectral ranges, 1350-1358 cm −1 (Jan. 10), 1362-1370 cm −1 (Jan. 11) and 1344-1352 cm −1 (Jan. 12).…”

NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance

“…No sulfur-bearing species has been detected to date in the Martian atmosphere. Both Encrenaz et al (2011) and Nakagawa et al (2009) reported an upper limit of 2 ppb of SO 2 , while recent groundbased observations of the Tharsis region revealed upper limits of 1.1 ppb . HCl is probably the most abundant reservoir of chlorine on Mars.…”

The Sample Analysis at Mars Investigation and Instrument Suite

“…CH 2 O 3× 10 −9 High resolution ground based spectroscopy (Krasnopolsky et al 1997) SO 2 3 × 10 −10 Ground based infrared spectroscopy (Encrenaz et al 2011a(Encrenaz et al , 2011b a δ 13 C VPDB = −2.5 ± 4.3 per mil that was interpreted as an atmospheric contribution from ongoing volcanic outgassing and carbonate formation. Impact erosion is yet another process that could remove an early massive atmosphere (Melosh and Vickery 1989, Owen and Barnun 1995a, 1995b.…”

“…Farquhar et al 2000;Greenwood 2000;McCubbin et al 2009), no sulfurous gases have been detected in the martian atmosphere. Recent ground-based spectroscopic observations have placed an upper limit of 0.3 ppb of SO 2 in the current atmosphere (Encrenaz et al 2011a(Encrenaz et al , 2011b. Nevertheless, SAM will search for trace atmospheric constituents such as SO 2 and H 2 S. The SAM QMS is expected to measure the major isotopes of SO 2 evolved from solid samples containing S such as sulfates if these dissociate in the 950-1100…”