Determination and comparison of Martian seasonal frost boundaries in LMD PCM simulations and in OMEGA observations around the poles

<p class="western">This study focuses on the water cycle around the seasonal polar caps during the winter/spring seasons, and more precisely on the progression and retreat of CO2 and H2O frosts derived from the Martian Planetary Climate Model (PCM) of the LMD and from the OMEGA imaging spectrometer onboard Mars Express.</p> <p class="western">In a previous study, Appéré et al. (2011) used a series of OMEGA observations from the end of autumn of MY 27 (Ls ~260°) to the end of spring of MY 28 to describe the temporal evolution of H2O and CO2 ice deposits, constantly evolving northwards through sublimation and deposition of the corresponding ice/frosts. This ends just before the summer solstice (around Ls ~70°) after the complete disappearance of CO2 ice. At high latitudes, the sublimation of H2O frost then contributes to an abundant emission of water vapor.</p> <p class="western">The LMD Martian PCM (Forget et al., 1999) reproduces the global and seasonal water and CO2 cycles during the winter-spring seasons. In the previous version (v. 5.3), it releases excessive humidity in the polar regions. We compare the southernmost position of frosts and their poleward progression on the old (v. 5.3) and new (v. 6.1) versions of Martian PCM data, and on data from OMEGA spectral images.</p> <p class="western">In OMEGA data, water and CO2 frosts can be detected by absorption bands at 1.5 μm, respectively at 1.43 μm (Langevin et al., 2007). Similarly, when the depth of the absorption band falls below a chosen value, the frost is considered as having disappeared. On one orbit-segment image, the southernmost pixels form a more or less continuous line corresponding to the frost boundary (“crocus-line” type).</p> <p class="western">We use the surface ice content (in v. 5.3) or directly the seasonal frost amount (v. 6.1) in the model simulations, in order to detect the frost dissipation. Water (resp. CO2) ice content values (in kg/m2) are calculated on a regular grid (5.625° longitude x 3.75° latitude) every 2 hours over one standard (climatological) Martian year.</p> <p class="western">In most cases, all the OMEGA pixels of an image are observed at the same local time. We calculate an average GCM frost dissipation time Ls<sub>fd_GCM</sub> from the 4 closest GCM neighbor grid points, weighted by the distance between each GCM grid point and the OMEGA frost line. Then the time interval between the dissipation of frost in OMEGA water (CO2) ice absorption depth profile and in the collocated (interpolated) water ice disappearance on the GCM can be determined. When the frost time dissipation interval ΔLs<sub>fd</sub> = Ls<sub>fd_OMEGA</sub> - Ls<sub>fd_GCM</sub> is positive (respectively negative), the model is late (in advance) w.r.t. observations. Preliminary results show that CO2 frost dissipates later in the PCM v. 5.3 dataset than on OMEGA data near the North Pole.</p> <p class="western">We will also present other comparisons of the evolution of the frost time dissipation.</p>