A Measurement of the 362 GHz Absorption Line of Mars Atmospheric H₂O₂

The 362.156 GHz absorption spectrum of H₂O₂ 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). Our derived H₂O₂ abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ~10 km altitudes between the Ls = 112° period of the Encrenaz et al. upper limit measurement and the current Ls = 250° period of detection (Clancy and Nair, 1996). The observed H₂O₂ line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002) or heterogeneous (Krasnopolsky, 2003) chemistry, which have been advocated partly on the basis of infrared (8 µm) non-detections for Mars H₂O₂. Our observation of Mars H₂O₂ also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own.

Figure 1: A schematic of the Mars JCMT observing geometry for the September 4, 2003 spectral line H₂O₂ measurement. The solid outer circle represents the apparent Mars disk, as resolved by the FWHM primary diffraction beam (diagonally marked center circle) of the JCMT telescope at the observing frequency (362.156 GHz). The center local time and latitude, as well as the local times/latitudes of the beam FWHM limits, are indicated.

Photodissociation of water vapor is the fundamental source for Mars atmospheric H₂O₂, such that photochemical model H₂O₂ abundances scale roughly with input model water vapor densities. The vertical column of Mars water vapor varies by >100% versus latitude and Ls (Jakosky and Farmer, 1982; Smith, 2002), which requires that comparisons of model and observed H₂O₂ abundances be adjusted for equivalent water vapor conditions. Our measurement of 18±4 ppbv for the 0-30 km H₂O₂ mixing ratio corresponds to water vapor column densities of 15-20 pr-?m for the 50S-20N latitude range and southern summer season (Ls.= 254°) of observation (Smith, 2002).

Figure 2: The observed 362 GHz H₂O₂ absorption spectrum of Mars (solid line), which has been smoothed to an effective spectral resolution of 1 MHz. The line center offset of -2.45 MHz from zero is the Doppler shift associated with a Mars-Earth relative velocity at the (one week) post opposition period of observation. Synthetic spectra with equivalent 1 MHz spectral resolutions are calculated for Mars H₂O₂ abundances consistent with the Encrenaz et al. (2002) upper limit of 6 ppbv (square symbols) and a best-fit mixing ratio of 18 ppbv (asterisk symbols).

This discovery of H₂O₂ in the Martian atmosphere appeared in the March issue of Icarus (2004, Icarus, v168, pp116-121) and was issued as a press release on March 1 2004. The release was picked up by a significant number of newspapers, as well as CBS radio. See here for links to some of the press coverage.

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