PFS/MEX observations of the condensing CO2 south polar cap of Mars

Abstract

The condensing CO 2 south polar cap of Mars and the mechanisms of the CO 2 ice accumulation have been studied through the analysis of spectra acquired by the Planetary Fourier Spectrometer (PFS) during the first two years of ESA's Mars Express (MEX) mission. This dataset spans more than half a martian year, from Ls ∼ 330 • to Ls ∼ 194 • , and includes the southern fall season which is found to be extremely important for the study of the residual south polar cap asymmetry. The cap expands symmetrically and with constant speed during the fall season. The maximum extension occurs sometime in the 80 • -90 • Ls range, when the cap edges are as low as −40 • latitude. Inside Hellas and Argyre basins, frost can be stable at lower latitudes due to the higher pressure values, causing the seasonal cap to be asymmetric. Within the seasonal range considered in this paper, the cap edge recession rate is approximately half the rate at which the cap edge expanded. The longitudinal asymmetries reduce during the cap retreat, and disappear around Ls ∼ 145 • . Two different mechanisms are responsible for CO 2 ice accumulation during the fall season, especially in the 50 • -70 • Ls range. Here, CO 2 condensation in the atmosphere, and thus precipitation, is allowed exclusively in the western hemisphere, and particularly in the longitudinal corridor of the perennial cap. In the eastern hemisphere, the cap consists mainly of CO 2 frost deposits, as a consequence of direct vapor deposition. The differences in the nature of the surface ice deposits are the main cause for the residual south polar cap asymmetry. Results from selected PFS orbits have also been compared with the results provided by the martian general circulation model (GCM) of the Laboratoire de Météorologie dynamique (LMD) in Paris, with the aim of putting the observations in the context of the global circulation. This first attempt of cross-validation between PFS measurements and the LMD GCM on the one hand confirms the interpretation of the observations, and on the other hand shows that the climate modeling during the southern polar night on Mars is extremely sensitive to the dynamical forcing.

A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results

“…The polar night is an environment driven by dynamical processes rather than radiative processes, revealing the subtle differences between the various dynamical cores and their related assumptions. Therefore, although the LMD‐MGCM and LMD Martian Mesoscale Model share similar physical parameterizations, the two models might yield different diagnostics in the polar night, as is the case between the finite difference LMD‐MGCM and the spectral Oxford Mars GCM (L. Montabone, personal communication, 2007) [see also Giuranna et al , 2008]. In addition to the influence of the chosen numerical method to integrate the dynamical equations, the diagnostics derived from the models are sensitive to the dynamical filtering used in the polar regions to overcome the pole singularity problem.…”

Albedo control of seasonal South Polar cap recession on Mars

“…GCM studies show that a topographic forcing by the Hellas basin creates an asymmetry in the mode of deposition. Precipitation events are more frequent for the anti-cryptic sector than for the cryptic sector (Colaprete et al, 2005;Giuranna et al, 2008). This suggests that the texture should be more granular (smaller grain sizes) for the former compared to the latter.…”

Role of stationary and transient waves in CO2 supersaturation during northern winter in the Martian atmosphere revealed by MGS radio occultation measurements

“…Because CO 2 supersaturation can lead to the generation of CO 2 (dry ice) clouds and snowfall, the longitudinal dependence of CO 2 supersaturation observed in the present study provides information about the nature of CO 2 condensation and deposition in the polar regions on Mars. It has been shown that the mechanism of polar ice cap formation has longitudinal dependence at the south pole [ Giuranna et al , ]. Future studies on the effects of snowfall due to CO 2 supersaturation at specific longitudes in the northern polar region will elucidate the role of such CO 2 snowfall in forming the northern seasonal ice cap.…”