Martian Dust Dynamics Constrained by OMEGA/MARS EXPRESS Orbital Data

Introduction: Dust is omnipresent within Mars’s atmosphere [1] and at its surface [2]. These small, micrometer-sized particles are one of the major features of Mars modern climate [3] and may also represent a key factor controlling some current surface properties such as composition [4] and activity [5]. Multiple orbital and surface observations have already revealed the main features of the dust seasonal cycle (e.g. [1, 3]). However, some characteristics are still imperfectly known, such as the different lifting mechanisms, the preferential locations where dust settles down, and the driving forces of Global Dust Storms (GDS). Recently, it has been suggested that Recurring Slope Lineae (RSL) [6] may be primarily connected to the dust cycle [5, 7]. Our work aims at better understanding where, when and how dust moves on Mars. This may help understanding how the different spatial scales, from global dust storm to local phenomena such as RSL, are connected. We start with the development of a new method to identify local dust storms in Mars Express observations. Data and method: We use the 1 to 2.5 μm “Cchannel” (and “L-channel”: 2.55-5.1 μm for spectral criteria) of OMEGA [8], an imaging spectrometer that has observed the surface with a typical spatial sampling of 1 km over three Martian years (2004-2010). We have developed a new method to detect the presence of dust in the atmosphere in this dataset. This method is based on the 2 μm carbon dioxide (CO2) absorption band. When the atmosphere is “clear” (without dust or cloud), photons travel a given distance in the atmosphere. This distance through CO2 is reduced if an aerosols layer is present. If we are able to predict the expected CO2 path without atmospheric dust, then it should be possible to detect anomalously weak CO2 path length resulting from the presence of dust or clouds in the atmosphere. We first developed a basic physical model to link the CO2 optical depth without dust to pressure, surface albedo and solar incidence angle. Pressure is the main parameter: it characterises the amount of CO2 in the atmosphere as a function of geographical position and season. It is extracted from a climate model [9]. Solar incidence angle controls the geometry of photons’ path as viewing geometry is nadir. Surface albedo modifies the proportion of photons measured by the instrument after surface contact. This model has been calibrated using observations without dust. Observations have been selected inside typical clear atmospheric periods/locations according to previous TES observations [10]. We paid a particular attention on filtering water ice both as clouds and at the surface, using spectral criteria at 1.5 microns and 3.5 microns [11, 12]. Indeed, water ice is a potential aerosols contributor and it has also a large absorption band around 2 μm.