Martian oxygen density at the exobase deduced from O I 130.4‐nm observations by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars on Mars Express

Abstract

Several observations of the O I 130.4‐nm triplet have been analyzed to determine the oxygen density in the Martian upper atmosphere using a three‐dimensional Monte Carlo radiative transfer model describing each line of the triplet. Solar resonant scattering is the dominant source of excitation of the O I 130.4‐nm triplet in the upper atmosphere of Mars. The atomic oxygen density at the exobase is found to be 1.2−0.5+1.2 × 107 cm−3 for solar zenith angles between 20° and 55° and to decrease by a factor of 2 for solar zenith angles between 55° and 90°. Although the major contribution to the observed brightness is produced above the exobase, it is possible to extrapolate the density profile below the exobase and to estimate the [O]/[CO2] mixing ratio as 0.6–1.2% at 135 km. The major source of uncertainty comes from the uncertainty in the absolute calibration, as expected for an optically thick emission, and also, to a lesser degree, from the temperature at the exobase. The profiles are better reproduced by a large exospheric temperature (>300 K), which may suggest the presence of a hot oxygen population.