A stringent upper limit of the abundance in the Martian atmosphere

¹ LESIA, Observatoire de Paris, 92195 Meudon, France
² Department of Astronomy, University of Texas at Austin, RLM 15.308, C-1400, Austin, TX 78712-1083, USA
³ The University of Michigan, Ann Arbor, MI 48109-2143, USA
⁴ Department of Physics, One Shields Ave., University of California, Davis, CA 95616, USA

Corresponding author: Th. Encrenaz, therese.encrenaz@obspm.fr

Received: 31 July 2002
Accepted: 24 September 2002

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 ν ₆ 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 (10–40 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  cm^-2 in the first case,  cm^-2 in the second case, and  cm^-2 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. ([CITE]) 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.