Martian oxygen density at the exobase deduced from O 1130.4-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 1130.4-nm triplet in the upper atmosphere of Mars. The atomic oxygen density at the exobase is found to be 1.2+1.2-0.5 × 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.