October 14, 2014

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

Chaufray et Al.
Journal of Geophysical Research, Volume 114, Issue E2, CiteID E02006

Summary: 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 x 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.