Detection and comparison of Martian seasonal frost boundaries in OMEGA observations and LMD GCM simulations around the North Pole

This study focuses on the water cycle around the Northern seasonal polar cap from the end of autumn to the following spring season, and more precisely on the progression and retreat of CO₂ and H2O frosts observed by the Martian Global Climate Model (GCM) of the LMD and by the OMEGA imaging spectrometer onboard Mars Express.

Based on a series of OMEGA observations from the end of autumn of MY 27 (Ls ~260°) to the end of spring of MY 28, Appéré et al. (2011) described the temporal evolution of H₂O and CO₂ ice deposits, constantly evolving northwards through sublimation and deposition of the corresponding ice/frost. This ends just before the summer solstice (around Ls ~70°) after the complete disappearance of CO₂ ice. At high latitudes, the sublimation of frost then contributes to an abundant emission of water vapor.

The LMD Martian GCM is able to reproduce the global and seasonal water and CO₂ cycles during the winter-spring seasons. However, it releases excessive humidity in the polar region. In order to improve the model, we examine and compare the southernmost position of frosts and their poleward progression on Martian GCM data and on spectral images from OMEGA.

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).

In the model simulation, we use the surface ice contents provided by the LMD GCM (Forget et al., 1999) in order to detect the frost dissipation. Water (resp. CO₂) ice content values (in kg/m²) have been calculated on a regular grid (5.625° longitude x 3.75° latitude) 4 times per sol (at 0, 6, 12 and 18 h LT) over one Martian year. Starting at the end of the northern autumn (Ls ~ 260°), the evolution of the water (CO2) ice content can be examined at every grid point.

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_{fd_GCM} 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.

With a perfect GCM and well-chosen frost-detection thresholds on both datasets, the dissipation of frost should be simultaneous for collocated data in both datasets. Otherwise, when the frost time dissipation interval DLs_fd = Ls_{fd_OMEGA} - Ls_{fd_GCM} is positive (respectively negative), the model is late (in advance) w.r.t. observations. We will present results of the evolution of the frost time dissipation during the winter-spring season.