A stringent upper limit of the H2O2 abundance in the Martian atmosphere

Abstract

Hydrogen peroxide H2O2 has been suggested as a possible oxidizer of the Martian surface. However, this minor species has never been detected. Photochemical models suggest that H2O2 and H2O abundances should be correlated. We have searched for H2O2 in the northern atmosphere of Mars, on Feb. 2-3, 2001 (Ls = 112 deg), at a time corresponding to maximum water vapor abundance in the northern hemisphere. The TEXES high-resolution grating spectrograph was used at the NASA/Infrared Telescope Facility (IRTF). Individual lines of the H2O2 ν6 band were searched for in the 1226-1235 cm-1 range (8.10-8.15 μm). Data were co-added for three different latitude sets: (1) full northern coverage (0-90 deg); (2) low northern latitudes (10-10 deg); (3) high northern latitudes (40-60 deg). From the absence of detectable H2O2 lines in each of the three co-added data sets, we infer an H2O2 2-σ upper limit of 9 × 1014 cm-2 in the first case, 1.2 × 1015 cm-2 in the second case, and 1.1 × 10-15 cm-12 in the third case. These numbers correspond to mean water vapor abundances of 30 pr-μm, 20 pr-μm and 40 pr-μm at the time of our observations. Our lowest upper limit is eight times lower than the value derived by Krasnopolsky et al. (1997) in the southern hemisphere in June 1988 (Ls = 222 deg); the mean water vapor abundance corresponding to their observation was 10 pr-μm. Our lowest upper limit is between 2.5 and 10 times lower than the values predicted by global photochemical models, also calculated for a mean H2O abundance of 10 pr-μm. In view of this, we have developed a new photochemical model which takes into account the actual geometry of the observations and the corresponding conditions of the water vapor abundance, dust and temperature in the Martian atmosphere, inferred from the MGS/TES data. Assuming an eddy diffusion coefficient of 107 cm2 s-1 in the lower atmosphere, the calculated H2O2 abundance is only a factor 1.5 greater than the observed upper limits.