Atmospheric Dust monitoring derived from Orbital Near-Infrared Imaging Spectroscopy and Implications for RSL Formation
Authors: Leseigneur Yann , Vincendon Mathieu 
Published: September 2022

&lt;p&gt;&lt;strong&gt;Introduction:&amp;#160;&lt;/strong&gt;Dust is a high albedo material, which is &lt;span lang=&quot;EN-US&quot;&gt;omnipresent on Mars&lt;/span&gt;: both on the surface and suspended in the atmosphere. These micrometer-sized particles modify heat balance while being highly mobile&lt;span lang=&quot;EN-US&quot;&gt;: it is&lt;/span&gt; a major contributor to current Martian atmosphere and surface dynamic. Information on dust spatial and time distribution was already provided from numerous orbital and surface studies (e.g., [1,2,3]). However, some characteristics of the dust cycle remain uncertain, which is notably related to the large range of spatial scales of the dust movements: from Recurring Slope Lineae (RSL) to global dust storms, and with important interannual variability ([4&lt;span lang=&quot;EN-US&quot;&gt;,&lt;/span&gt;5]). Further global monitoring of dust movements with high spatial sampling and over several years may help improving our Martian dust knowledge. Here we present a new method used to derived near-infrared atmospheric dust optical depth using OMEGA observations acquired between 2004 and 2010.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Data and method:&amp;#160;&lt;/strong&gt;We use the near-infrared (NIR) imaging spectrometer OMEGA [6] onboard Mars Express, which acquired about 8300 observations of the Martian surface over that time range, with a spatial sampling typically close to 1 km.&lt;/p&gt;
&lt;p&gt;Firstly, we quantify the prominent 2 &amp;#181;m atmospheric CO&lt;sub&gt;2&lt;/sub&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;absorption band &lt;span lang=&quot;EN-US&quot;&gt;which &lt;/span&gt;is sensitive to airborne dust that reduces the atmospheric optical paths of the photons. We have developed a model to predict the CO&lt;sub&gt;2&lt;/sub&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;optical depth under clear atmosphere conditions (notably depends on solar incidence angle, albedo and pressure). This allows to compare observed CO&lt;sub&gt;2&lt;/sub&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;optical depth to these predictions and to derive the difference &quot;&amp;#916;&amp;#964;&lt;sub&gt;CO2&lt;/sub&gt;&quot;&lt;span lang=&quot;EN-US&quot;&gt; (more details in [7])&lt;/span&gt;.&lt;/p&gt;
&lt;p&gt;&lt;span lang=&quot;EN-GB&quot;&gt;Then, we transform this &amp;#916;&amp;#964;&lt;sub&gt;CO2&lt;/sub&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;into dust optical depth by a calibration with the visible (VIS) dust optical depth (from 880 nm images) measured by the Mars Exploration Rovers (MER) [2]. Concomitant MER/OMEGA observations are used to characterize the relation between &amp;#916;&amp;#964;&lt;sub&gt;CO2&lt;/sub&gt; and the ground optical depth (notably depends on incidence angle and albedo). Then, we extrapolated this relation to the whole OMEGA dataset to compute a VIS-NIR dust optical depth &quot;&amp;#964;&lt;sub&gt;dust&lt;/sub&gt;&quot;.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Results and discussion:&amp;#160;&lt;/strong&gt;We produce latitude/solar longitude diagrams, such as in Figure 1 for the Martian year 28. We observed well-known dust cycle characteristics: low dust optical depth values (&lt;0.5) in the &quot;clear atmosphere period&quot; (solar longitude L&lt;sub&gt;s&lt;/sub&gt;=0-180&amp;#176;) and medium to high values (&gt;1) during the dusty season (L&lt;sub&gt;s&lt;/sub&gt;&gt;180&amp;#176;) such as the 2007 global dust storm&lt;span lang=&quot;EN-US&quot;&gt;&amp;#160;during L&lt;sub&gt;s&lt;/sub&gt;=260-310&amp;#176;&lt;/span&gt;. We also observed medium&lt;span lang=&quot;EN-US&quot;&gt;-&lt;/span&gt;high values at northern and southern latitudes during L&lt;sub&gt;s&lt;/sub&gt;=220-250&amp;#176;&amp;#160;that can correspond to dust storms identified in Hellas and Chryse [8]. Other main investigations were made with thermal&lt;span lang=&quot;EN-US&quot;&gt;-&lt;/span&gt;infrared (TIR) data [3,9]. We can notice an overall good agreement between our VIS-NIR study and TIR studies. But there are still differences that&lt;span lang=&quot;EN-US&quot;&gt; we&amp;#160;&lt;/span&gt;studied by computing the dust optical depth ratio between our VIS-NIR values derived at 2 &amp;#181;m (and converted to 880 nm using MER) and the thermal ones at 9 &amp;#181;m. This ratio distribution reaches a maximum at 2.4, which is in agreement with the value &lt;span lang=&quot;EN-US&quot;&gt;(&lt;/span&gt;2.6&lt;span lang=&quot;EN-US&quot;&gt;)&lt;/span&gt; considered in [3] and also with the values found locally with MER [2]. &lt;span lang=&quot;EN-US&quot;&gt;T&lt;/span&gt;his distribution is very large&lt;span lang=&quot;EN-US&quot;&gt;:&lt;/span&gt; 2.0 equivalent standard deviation, which may be related to the mean dust particle size variations according to [2] and to potential observational biases of each method (e.g., uncertainties in the vertical distribution of dust).&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;1081&quot; height=&quot;466&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Figure 1: VIS-NIR dust optical depth (derived from the OMEGA dataset: see text for details) as a function of solar longitude (L&lt;sub&gt;s&lt;/sub&gt;) and latitude for Martian year 28&lt;/em&gt;&lt;em&gt;&lt;span lang=&quot;EN-US&quot;&gt;.&lt;/span&gt;&lt;/em&gt;&lt;/p&gt;
&lt;p&gt;&lt;span lang=&quot;EN-US&quot;&gt;We also produce global seasonal maps of dust optical depth with a spatial coverage typically greater than 50% for most 60&amp;#176; wide intervals in solar longitude. We provide two seasonal maps of Martian year 29 in Figure 2 (L&lt;sub&gt;s&lt;/sub&gt;=180-240&amp;#176; and 300-360&amp;#176;). Firstly, we can notice some localized high values, such as in 355&amp;#176;E and 55&amp;#176;S on the L&lt;sub&gt;s&lt;/sub&gt;=180-240&amp;#176; map, that can be associated to a dust storm. Some main dust storm travel routes [8,10] can be identified, such as the Acidalia-Chryse route which is associated to medium-high dust optical depth values in these two maps. Activity is also observed at RSL sites in this area during both periods [11] represented in Figure 2: rapid disappearance events over L&lt;sub&gt;s&lt;/sub&gt;=180-240&amp;#176; suggesting dust deposition/horizontal dust transport [12], and RSL formation/lengthening over L&lt;sub&gt;s&lt;/sub&gt;=300-360&amp;#176; which suggest dust removal. This indicates that local surface movements may be related to large-scale atmospheric dust movements occurring at RSL location.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;643&quot; height=&quot;682&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Figure 2: Two seasonal atmospheric VIS-NIR dust optical depth maps of Mars during Martian year 29. Top: L&lt;sub&gt;s&lt;/sub&gt;=180-240&amp;#176;; bottom: L&lt;sub&gt;s&lt;/sub&gt;=300-360&amp;#176;.&lt;/em&gt;&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Conclusion:&amp;#160;&lt;/strong&gt;We have developed a new method to derived a VIS-NIR dust optical depth using near-infrared data (OMEGA). The method relies on CO&lt;sub&gt;2&lt;/sub&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;absorption band at 2 &amp;#181;m and on a calibration procedure that we implemented, based on MER &amp;#8220;ground truth&amp;#8221; measurements. We constructed latitude/solar longitude diagrams (Figure 1) and global seasonal maps (Figure 2). Overall, our results are in a good agreement with previous thermal-infrared studies, but with some differences noticed by a particular distribution of the dust optical depth ratio between VIS-NIR and thermal-infrared, with a maximum occurrence value of 2.4. Then, &lt;span lang=&quot;EN-US&quot;&gt;p&lt;/span&gt;reliminary results indicate that large scale atmospheric dust events may be related to local RSL activity occurring in the Chryse-Acidalia area. We will present further comparisons between atmospheric dust and RSL activities at the conference.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgments:&amp;#160;&lt;/strong&gt;The OMEGA/MEx data are freely available on the ESA PSA at https://archives.esac.esa.int/psa/#!Table%20View/OMEGA=instrument.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References:&amp;#160;&lt;/strong&gt;[1] Smith M. D. (2004) &lt;em&gt;Icarus, 167,&lt;/em&gt; 148-165. [2] Lemmon M. T. et al. (2015) &lt;em&gt;Icarus, 251,&lt;/em&gt; 96-111. [3] Montabone L. et al. (2015) &lt;em&gt;Icarus&lt;/em&gt;, &lt;em&gt;251&lt;/em&gt;, 65-95. [4] Szwast M. A. et al. (2006) &lt;em&gt;JGR&lt;/em&gt;, &lt;em&gt;111&lt;/em&gt;, E11008. [5] Vincendon M. et al. (2019) &lt;em&gt;Icarus, 325,&lt;/em&gt; 115-127. [6] Bibring J-P. et al. (2004) &lt;em&gt;ESA Publication Division&lt;/em&gt;, 1240, 37-49. [7] Leseigneur Y. et al. (2021), &lt;em&gt;52nd LPSC&lt;/em&gt;, LPI contribution No. 2548, id. 1741. [8] Wang H. et al. (2015) &lt;em&gt;Icarus, 251,&lt;/em&gt; 112-127. [9] Smith M. D. (2004) &lt;em&gt;Icarus, 167,&lt;/em&gt; 148-165. [10] Battalio M. (2021) &lt;em&gt;Icarus, 354,&lt;/em&gt; 114059. [11] Stillman D. E. et al. (2016) &lt;em&gt;Icarus, 265,&lt;/em&gt; 125-138. [12] Vincendon M. et al. (2019) &lt;em&gt;Icarus, 325,&lt;/em&gt; 115-127.&lt;/p&gt;