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, N-2, and Ar, and is vertically well mixed. Since N-2 and Ar do not condense even during the polar nights, the ratio of N-2 and Ar remains constant, and the concentrations of N-2 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 (approximate to 70 profiles of approximate to 20,000 profiles in the whole data set) at southern polar latitudes (90 degrees S-75 degrees S) during the autumn, winter, and spring seasons (Ls = 0 degrees-240 degrees). The rederived profiles indicated that use of the standard global composition overestimated the temperature by at least approximately 5 K at Ls = approximate to 120 degrees (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 supersaturation using radio occultation profiles.