Detection and comparison of Martian frost boundaries in OMEGA observations and LMD GCM simulations around the North Pole
Authors: Szantai Andre , Forget Francois , Appéré Thomas , Schmitt Bernard 
Published: July 2021

&lt;ul&gt;
&lt;li&gt;&lt;strong&gt; Introduction&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;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 frost.&lt;/p&gt;
&lt;p&gt;The CO2 and water cycle has been modeled by Martian global climate models (GCMs). At high latitudes, its main components are the permanent polar ice cap, humidity in the atmosphere (water vapor), clouds in abundance in winter (forming the polar hood), locally formed ice and frost. Other components, CO2 ice and frost, and dust, may also have an impact on the water cycle.&lt;/p&gt;
&lt;p&gt;Starting with Mariner-9, several satellites have observed (water) frost in particular at high latitudes and in the vicinity of polar caps. The OMEGA imaging spectrometer (on Mars Express) has made numerous observations of the northern high latitudes of Mars since 2004. Based on a series of observations from the end of autumn of MY 27 (Ls ~260&amp;#176;) to the end of spring of MY 28, App&amp;#233;r&amp;#233; et al. (2011) described the temporal evolution of H2O and CO2 deposits, constantly evolving northwards through sublimation and deposition of the corresponding ice/frost. This ends just before the summer solstice (around Ls ~70&amp;#176;) after the complete disappearance of CO2 ice. At high latitudes, the sublimation of frost then contributes to an abundant emission of water vapor.&lt;/p&gt;
&lt;p&gt;Current Martian GCMs include the water and CO2 cycles. In the past decade, the modeling of the water cycle has been improved in the LMD GCM (Navarro et al., 2014 ; Pottier et al. 2017 ; Vals, 2019).&lt;/p&gt;
&lt;p&gt;The LMD Martian GCM is able to reproduce the global and seasonal water cycle during the winter-spring seasons. However, it releases excessive humidity in the polar region. In order to improve the model, we examined and compared the southernmost position of water frost and its poleward progression on Martian GCM data and on spectral images from OMEGA.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt; Data and Method&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;In OMEGA data, frost can be detected by an absorption band of H2O at 1.5 mm (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 limit of frost (&amp;#8220;crocus&amp;#8221;-type line).&lt;/p&gt;
&lt;p&gt;In the model simulation, we use the surface water ice content provided by the LMD GCM (Forget et al., 1999) in order to detect the frost dissipation. Water ice content values (in kg/m2) have been calculated on a regular grid (5.625&amp;#176; longitude x 3.75&amp;#176; latitude) 4 times every sol (at 0, 6, 12 and 18 h LT) over one Martian year. Starting at the end of the northern autumn (Ls ~ 260&amp;#176;), the evolution of the water ice content can be examined at every grid point.&lt;/p&gt;
&lt;p&gt;In most cases, all these pixels are observed at the same local time. We calculate an average GCM frost dissipation time Ls&lt;sub&gt;fd_GCM&lt;/sub&gt; from the 4 closest GCM neighbor grid points, weighted by the distance between each GCM grid point and the OMEGA frost limit. Then the time interval between the dissipation of frost in OMEGA water ice absorption depth profile and in the collocated (interpolated) water ice disappearance on the GCM can be determined.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt; Results&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;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&lt;sub&gt;fd&lt;/sub&gt; = Ls&lt;sub&gt;fd_OMEGA&lt;/sub&gt; - Ls&lt;sub&gt;fd_GCM&lt;/sub&gt; is positive (respectively negative), the model is late (in advance) w.r.t. observation. We will present results of the evolution of the frost time dissipation during the winter-spring season.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt; Conclusion and prospects&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;Determining the time interval between the frost dissipation line observed by OMEGA and by a GCM can help to understand the behavior of the model at high latitudes, to detect its limits and to improve it. On the other hand, the systematic presence of a non-negligible time interval could reveal a systematic bias, which can be related to the detection thresholds used for the detection of frost.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Thomas App&amp;#233;r&amp;#233; provided a part of the OMEGA water ice imagery and ancillary data. The other part of OMEGA data was extracted from the database on the PSUP portal (http://psup.ias.u-psud.fr/ ).&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;- App&amp;#233;r&amp;#233;, T., Schmitt, B., Langevin, Y., Dout&amp;#233;, S., Pommerol, A., Forget, F., Spiga, A., Gondet, B., and Bibring, J.-P. (2011). Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express. Journal of Geophysical Research (Planets), 116(E15):5001.&lt;/p&gt;
&lt;p&gt;- Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S. R., Read, P. L., and Huot, J.-P. (1999). Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res., 104:24,155&amp;#8211;24,176.&lt;/p&gt;
&lt;p&gt;- Langevin, Y., Bibring, J.-P., Montmessin, F., Forget, F., Vincendon, M., Dout&amp;#233;, S., Poulet, F., and Gondet, B. (2007). Observations of the south seasonal cap of Mars during recession in 2004-2006 by the OMEGA visible/near-infrared imaging spectrometer on board Mars Express. J. Geophys. Res., 112:E08S12.&lt;/p&gt;
&lt;p&gt;- Navarro, T., Madeleine, J.-B., Forget, F., Spiga, A., Millour, E., Montmessin, F., and M&amp;#228;&amp;#228;tt&amp;#228;nen, A. (2014). Global Climate Modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds. Journal of Geophysical Research (Planets), 119:1479&amp;#8211;1495.&lt;/p&gt;
&lt;p&gt;- Pottier, A., Forget, F., Montmessin, F., Navarro, T., Spiga, A., Millour, E., Szantai, A., and Madeleine, J.-B. (2017). Unraveling the martian water cycle with high-resolution global climate simulations. Icarus, 291:82&amp;#8211;106.&lt;/p&gt;
&lt;p&gt;- Vals, M. (2019). Mod&amp;#233;lisation num&amp;#233;rique des cycles de l&amp;#8217;eau et des poussi&amp;#232;res de la plan&amp;#232;te Mars et de leurs couplages. PhD thesis. Universit&amp;#233; Pierre et Marie Curie, Paris, Fr.&lt;/p&gt;