Properties of a Martian local dust storm in Atlantis Chaos from OMEGA/MEX data

We retrieve maps of effective grain radius (r_eff), optical depth at 9.3 μm (τ_9.3) and top altitude (ta) of the dust layer. Our results show that large particles (r_eff = 1.6 μm) are gathered in the centre of the storm (lat = 33.5° S; lon = 183.5° W), where the optical depth is maximum (τ_9.3 > 7.0) and the top altitude exceeds 18 km. Outside the storm, we obtain τ_9.3<0.2, in agreement with the estimates derived from global climate models (GCM).

We speculate that a low thermal inertia region at the western border of Atlantis Chaos is a possible source of the dust storm. Moreover, we find evidence that topography plays a role in confining the local storm in Atlantis Chaos. The vertical wind component from the GCM does not provide any hint for the triggering of dust lifting. On the other hand, the combination of the horizontal and vertical wind profiles suggests that the dust, once lifted, is pushed eastward and then downward and gets confined within the north-east ridge of Atlantis Chaos.

From our results, the thickness of the dust layer collapsed on the surface ranges from about 1 μm at the storm boundaries up to more than 100 μm at its centre. We verify that a layer of dust thicker than 1 μm, deposited on the surface, can prevent the detection of mafic absorption features. However, such features are still present in OMEGA data of Atlantis Chaos registered after the storm. Hence, we deduce that, once the storm is over, the dust deposited on an area larger than the one where it has been observed.