Estimation of changes in the composition of the Martian atmosphere caused by CO₂ condensation from GRS Ar measurements and its application to the rederivation of MGS radio occultation measurements

We propose a method to estimate seasonal changes in the composition of the Martian atmosphere, which is influenced by CO₂ condensation due to the polar nights at southern high latitudes. The method relies on measurements of the Ar concentration obtained by the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey. We assume that the Martian atmosphere is composed of CO₂, N₂, and Ar, and is vertically well mixed. Since N₂ and Ar do not condense even during the polar nights, the ratio of N₂ and Ar remains constant, and the concentrations of N₂ and CO₂ can be estimated from Ar measurements. Estimates of the atmospheric composition were utilized for the rederivation of temperature and pressure profiles in the Mars Global Surveyor (MGS) radio occultation measurements (∼70 profiles of ∼20,000 profiles in the whole data set) at southern polar latitudes (90°S-75°S) during the autumn, winter, and spring seasons (Ls = 0°-240°). The rederived profiles indicated that use of the standard global composition overestimated the temperature by at least approximately 5 K at Ls = ∼120° (midwinter), when the largest CO₂ depletion occurred and the CO₂ volume mixing ratio fell to 78%. The occurrence and degree of CO₂ supersaturation were several times higher and larger, respectively, in the rederived temperature profiles than in the original MGS profiles. This suggests that consideration of CO₂ depletion during southern polar nights is needed when studying CO₂ super saturation using radio occultation profiles.