Seasonal Variation of Oxygen in the Upper Atmosphere of Mars

We have inferred the abundance of O2 in the upper atmosphere of Mars using observations of stellar occultations by the SPICAM UV spectrograph on Mars Express. The distribution of O2 with altitude, latitude, and season is an important factor in the evolution and current stability of Mars's CO2-rich atmosphere. Interpreting the occultation data depends only on the ratio of attenuated to unattenuated spectra, so relative measurements yield absolute abundances. O2 is detected in the presence of the much more abundant CO2 by its absorption in the Schumann-Runge bands and continuum, which lie in the wavelength range 130--205~nm. A vertical inversion algorithm converts the column abundances measured directly in the occultation to volume abundances. We have concentrated on the Southern hemisphere during its winter season, when we expect that O2 will be easier to detect owing to the lower CO2 abundance. The measurements reveal the altitude profiles of O2 in the altitude range of ~70--120~km. The probable error is about 10--20% for most of this range, but uncertainties in the O2 absorption cross-sections at the relevant wavelengths lead to larger probable errors in O2 abundance over a range of about 15~km. We infer the altitude profiles of CO2 over the full altitude range from the same measurements, with much lower uncertainty. The 190 occultations analyzed to date cover discrete latitudes from 16S to 75S and span the range 90° < LS < 190°. We report results in terms of the mixing ratio of O2 relative to CO2 by number. The most reliable determinations of this value are in the altitude range ~100--120~km, but results at other altitudes are qualitatively similar. The ratio [O2]/[CO2] varies with season and latitude, from ~0.001 to ~ 0.03. The lower values are similar to the commonly-used value of 0.0012, but our new measurements show that often much higher values obtain.