Thickness of the seasonal deposits by examining the shadow variations of the fallen ice blocks at Martian North Pole
Authors: Xiao Haifeng , Xiao Yuchi , Su Shu , Schmidt Frédéric , Lara Luisa M. , Gutierrez Pedro J. 
Published: March 2024

Due to its axial tilt of ~25&#176;, Mars has seasons. During its fall and winter, when temperature drops, there exist two depositional mechanisms of atmospheric CO2, that is, precipitation as snowfall and direct surface condensation in the form of frost (Hayne et al., 2012). Up to one third of the atmospheric CO2 exchanges with the polar surface through the seasonal deposition/sublimation process. Therefore, accurate measurements of the evolution of the seasonal polar caps can place crucial constraints on the Martian climate and volatile cycles.&#160;Recently, by reprocessing and co-registering the MOLA profiles, Xiao et al. (2022a, 2022b) derived both spatial and temporal thickness variations of the seasonal polar caps at grid elements of 0.5&#176; in latitude and 10&#176; in longitude. However, the MOLA-derived results can suffer from biases related to various processes, for example, pulse saturation due to high albedo of the seasonal deposits, non-Gaussian return pulses due to rough terrain and dynamic seasonal features, incomplete correction for the global temporal bias, and penetration of the laser pulses into the translucent slab ice. Furthermore, MOLA altimetric observations are limited to Mars Year 24 and 25 which prevents the detection of possible interannual variations in the CO2 seasonal transport.&#160;In this contribution, we will show how the shadow variations of fallen ice blocks at the bottom of steep scarps of the North Polar Layered Deposits (NPLDs) allow us to infer the thickness evolution of the seasonal deposits (Xiao et al., 2024). For this, we utilize the High Resolution Imaging Science Experiment (HiRISE/MRO) images with a spatial resolution of up to 0.25 m/pixel (McEwen et al., 2007). We successfully conduct an experiment at a steep scarp centered at (85.0&#176;N, 151.5&#176;E). We assume that no, or negligible, snowfall remains on top of the selected ice blocks, the frost ice layer is homogeneous around the ice blocks and their surroundings, and no significant moating is present. These assumptions enable us to separately determine the thickness of the snowfall and frost. We find that maximum thickness of the seasonal deposits at the study scarp in MY31 is 1.63&#177;0.22 m to which snowfall contributes 0.97&#177;0.13 m. Interestingly, our thickness values in the northern spring are up to 0.8 m lower than the existing MOLA results (Smith et al., 2001; Aharonson et al., 2004; Xiao et al., 2022a, 2022b). We attribute these differences mainly to the remaining biases in the MOLA heights. Furthermore, we demonstrate how the long time span of the HiRISE images (2008&#8212;2021; Mars Year 29&#8212;36) allows us to measure the interannual variations of the deposited CO2. Specifically, we observe that snowfall in the very early spring of Mars Year 36 is 0.36&#177;0.13 m thicker than that in Mars Year 31.&#160;&#160;Hayne et al. (2012). JGR: Planets, 117(E8).Xiao et al. (2022a). JGR: Planets, 127(7), e2022JE007196.Xiao et al. (2022b). JGR: Planets, 127(10), e2021JE007158.Xiao et al. (2024). JGR: Planets (In Revision).McEwen et al. (2007). JGR: Planets, 112(E5).Smith et al. (2001). Science, 294(5549), 2141-2146.Aharonson et al. (2004). JGR: Planets, 109(E5).