A measurement of the 362 GHz absorption line of Mars atmospheric H 2O 2

Abstract

The 362.156 GHz absorption spectrum of H 2O 2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0–30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124–150). Our derived H 2O 2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037–1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes ( L s ) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HO x abundances above ∼10 km altitudes between the L s =112° period of the Encrenaz et al. upper limit measurement and the current L s =250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785–12590). The observed H 2O 2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037–1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315–325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H 2O 2. Observation of Mars H 2O 2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own.