Aeolian bedforms formed by ice sublimation and vapor condensation on Louth crater ice, Mars.
Authors: Collet Aurore , Carpy Sabrina , Bordiec Maï , Massé Marion , Bourgeois Olivier , Conway Susan 
Published: September 2022

&lt;p&gt;&lt;strong&gt;Introduction:&lt;/strong&gt; Louth Crater is a 36 km diameter located at 70 &amp;#176;N, 103.2 &amp;#176;E (Fig. 1) less than 1000 km from the Martian North Polar Cap. &amp;#160;At the center of Louth crater a perennial water ice cap ~10 km in diameter, ~250 m in width (Fig. 1) that undergoes phase changes (condensation / sublimation cycles) during the Martian year [1 - 4]. Some periodic structure at the surface of its perennial water ice cap have been observed [2]. The presence of smaller ice undulations superimposed on this periodic structure, comparable to the sublimation waves on the Martian North Polar Cap [5] could be formed during sublimation and condensation period of the water ice. &amp;#160;The characterization of this ice waves could make it possible to specify environmental conditions favorable to their formation. The first part of the study consists of identifying these ice waves using orbital topography and imaging data, and then associating these results with data from the Martian Database [6-7] and with the scaling laws inherent to the formation of sublimation and condensation waves.&lt;strong&gt; &lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Fig. 1:&lt;/strong&gt; Louth crater in summer. CTX product J02_045439_2504_XN_70N256W Ls = 133.2&amp;#176;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Geomorphological analysis:&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Methods:&lt;/em&gt; Digital Elevation Model (DEM) at ~100 m/pixel and MOLA elevation data at ~128 m/pixel have been coupled with imagery data from HRSC ~10 m/pixel, CTX at ~6 m/pixel and HiRISE images for more precise areas for up to 25 cm/pixel. Data were georeferenced in ESRI&amp;#8217;s ArcMap GIS software to produce geomorphological map and analyzed to evaluate ice waves shape and spatial organization.&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Observations:&lt;/em&gt; Louth presents two units (Fig. 2). (a) Lower unit with dark stucco texture and stratifications (Fig. 3a). (b) Fresh ice overlies this older, stratified structure (Fig 3.a). This fresh ice is distributed in a non-uniform manner as shown by the kilometrics waves of about 560 m wavelength on which are superimposed decametrics waves of about 55 m wavelength which are both perpendicular to the prevailing wind (Fig 3.b). The crests of both wave populations show similar NW-SE. The main wind direction was assessed from barkhane field on the sand mound.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Fig. 2:&lt;/strong&gt; Louth&amp;#8217;s geomorphological map in summer. CTX product J02_045439_2504_XN_70N256W Ls = 133.2&amp;#176;.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Fig. 3:&lt;/strong&gt;&amp;#160; Boxes from Fig. 2. (a) East side of the Louth crater ice cap. Green lines represents stratifications, HiRISE product EPS_045439_2505, Ls = 133.2&amp;#176;. (b) decametric waves on the ice waves. Black dashes represent leeside foot of each kilometric waves. Blue lines emphasize decametric waves, same HiRISE product from (a).&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Transport hypothesis:&lt;/strong&gt; Preliminary experiments studied the threshold velocity necessary to initiate a transport of ice grains in Martian Simulation Wind Tunnel [8]. These experiments were carried out for grains from a few hundred micrometers to 2 mm in diameter under atmospheric pressures of 40 to 1000 mbar. To transport ice particles under near-Martian condition is hard because the threshold velocity is ten times higher than Earth. The wind speeds estimated from the Martian Database [6-7] in Louth crater are much lower than those needed to transport ice grains at Martian pressure. We support the hypothesis of diffusion by sublimation/condensation rather than the transport of icy particles by the wind.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Mass transfer hypothesis: &lt;/strong&gt;In a recent theoretical model [5] scaling laws explain the formation of sublimation waves and have been validated on terrestrial bedforms and on the Martian North Polar Cap. These waves are periodic and oriented in accordance with a turbulent boundary layer flow that diffuse the sublimated vapor. These laws, which are superimposed on the two ice waves studied, make these waves suitable geomorphological markers for climatic predictions. This model has been adapted to study similar waves created by condensation [9] and results indicate that condensation waves would be larger with one order gap than sublimation waves.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Discussion: &lt;/strong&gt;From the wind tunnel experimentation of ice particles transport in Martian-Like environment [8], wind necessary to initiate transport of ice particles is unlikely in Louth. Friction velocities of 3 m.s&lt;sup&gt;-1&lt;/sup&gt; would be required, compared to 0.6 m.s&lt;sup&gt;-1&lt;/sup&gt; deduced from scaling laws, to transport the most mobilizable 0.3 mm ice particles [8]. Wind speed in Louth is too weak for initiating particles transport. From the scaling laws, we extract values of friction velocity and flow velocity at a given altitude that can be compared with those extracted from the Martian Data Base over two Martian periods: a period of sublimation (during the summer in the northern hemisphere) at the origin of the formation of decametric waves and a period of condensation at the end of the summer, beginning of the autumn favorable for the kilometric waves formation. Scaling laws are in agreement with the predicted ratio velocity between sublimation and condensation period from the Martian Data Base values. From the Martian Data Base, independent velocity values are larger. This can be explained by the gridding of the database which takes place on a global scale larger than the crater.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Conclusion:&lt;/strong&gt; The observations indicate kilometric bedforms on the ice cap of Louth Crater, on which decametric bedforms appear with an order of magnitude gap. Comparison with numerical results suggests that the kilometric bedforms are formed by condensation and the smaller ones by sublimation. These condensation and sublimation waves are suitable markers to constrain mesoscale climate modelling in small, complex regions such as this type of crater, where topographic and/or seasonal effects can affect climate data. These waves can also be used as geomarkers on other planetary bodies where climatic conditions are not well constrained.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgments:&lt;/strong&gt; We acknowledge PNP (Plan National de Plan&amp;#233;tologie) for founding the project &amp;#8220;sublimation and condensation waves&amp;#8221;. We used Planetary Data System (PDS) for observational data.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt; [1] Conway et al. (2012) &lt;em&gt;Icarus&lt;/em&gt;. [2] Brown et al. (2008) &lt;em&gt;Icarus&lt;/em&gt;. [3] Hofstader &amp; Murray (1990) &lt;em&gt;Icarus&lt;/em&gt;. [4] App&amp;#233;r&amp;#233; et al. (2011) &lt;em&gt;Journal of Geophysical Research.&lt;/em&gt; [5]&amp;#160;Bordiec M. et al. (2020) &lt;em&gt;Earth-Science Reviews, 211,&lt;/em&gt; 103350.[6] Forget et al. (1999). [7] Millour et al. (2018). [8] Herny et al. (2020). &lt;em&gt;7th Mars Polar Science Conf&lt;/em&gt;.&amp;#160; Abstract #2099. [9] Carpy S. et al. (2022). &lt;em&gt;EGU&lt;/em&gt;, Abstract #5998.&lt;/p&gt;