Monitoring atmospheric dust spring activity at high southern latitudes on Mars using OMEGA
Résumé
This paper presents a monitoring of the atmospheric dust in the south polar region during spring of Martian year 27. Our goal is to contribute to identifying the regions where the dust concentration in the atmosphere shows specific temporal patterns, for instance high, variable, and on the rise due to lifting or transport mechanisms. This identification is performed in relation with the seasonal ice regression. Based on a phenomenological examination of the previous results, hypothesis regarding the origin of aerosol activity of the southern polar region is proposed. This is of paramount importance since local dust storms generated in this region sometimes grow to global proportions. The imaging spectrometer OMEGA on board Mars Express has acquired the most comprehensive set of observations to date in the near-infrared (0.93-5.1 μm) of the southern high latitudes of Mars from mid-winter solstice (Ls=110°, December 2004) to the end of the recession at Ls=320° (November 2005). We use two complementary methods in order to retrieve the optical depth of the atmospheric dust at a reference wavelength of 1 μm. The methods are independently operated for pixels showing mineral surfaces on the one hand and the seasonal cap on the other hand. They are applied on a time series of OMEGA images acquired between LS=220° and LS=280° . As a result the aerosol optical depth (AOD) is mapped and binned at a spatial resolution of 1.0° pixel-1 and with a mean period of AOD sampling ranging from less than two sols for latitudes higher than 80°S to approximately six sols at latitudes in the interval 65-75°S. We then generate and interpret time series of orthographic mosaics depicting the spatio-temporal distribution of the seasonal mean values, the variance and the local time dependence of the AOD. In particular we suspect that two mechanisms play a major role for lifting and transporting efficiently mineral particles and create dust events or storms: (i) nighttime katabatic winds at locations where a favourable combination of frozen terrains and topography exists, (ii) large scale (≈10-100 km) daytime thermal circulations at the edge of the cap when the defrosting area is sufficiently narrow. As regards to the source regions around the cap, the sector with the highest AOD values / variability / increase spans longitudes 180-300°E around LS≈250°. Later (LS≈267°) the cryptic sector becomes the most productive while the longitude sector 300-60°E remains moderately dust-generative. Our work calls for new simulations of the martian surface-atmosphere dynamics at mesoscales to reproduce the observations and confirm the interpretations. The atmospheric dust opacity is mapped in the south polar region of Mars during spring. We use a time series of OMEGA hyperspectral images for Martian year 27. We document the dust source regions in the southern polar area. The dust evolution is put in perspective with the seasonal CO2 ice regression. Possible lifting and transport mechanisms are discussed based on previous atmospheric modelling work.