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… and gone with the wind

The two-kilometre-high central peak in the crater is surrounded by dark, sandy material. Winds have piled it up into groups of individual dunes and also into contiguous dune fields. The grey-black dunes (they only appear bluish in these contrast-enhanced colour images) consist of volcanic sand and fine ash with a basalt composition. This is mainly comprised of the minerals olivine and pyroxene, which cause the dark colouring of basaltic materials. Both minerals are silicates and have a high content of magnesium and iron.

The dune fields in Moreux Crater show slight colour variations, possibly caused by differences in the composition of the dunes. The OMEGA spectrometer on Mars Express can be used to investigate the mineralogical composition of surface materials. The large contiguous dune field to the north of the central peak was found to have a significantly higher olivine content than the rest of the dunes, which are predominantly composed of pyroxene.

In addition to the glacial formations, the crater also has aeolian formations that are visible in the different dune shapes. The most common type of dune is crescent-shaped (referred to as a barchan) which, when they grow together and merge, form barchanoidal ridges. The dune fields in Moreux Crater are made up of these ridges. Barchans are generally associated with limited sediment availability and a unimodal wind regime – this is, they are formed by wind that always blows from the same direction.

However, orientation of the dunes varies in different regions of the crater, which indicates a complex system of prevailing wind directions. This is due to the specific topography of the crater and its central peak. The large barchanoidal dune field to the north of the central peak is primarily formed by winds from the northeast. At its southern end, winds from the northwest meet the dunes and create star dunes, which are typically formed by winds from different directions.

If one follows the dune shapes counterclockwise from there, around the central peak, and uses them to discern the wind directions, the winds follow a semicircle. First, they come from the northwest, then the west, and later from the southwest until one reaches the eastern side of the dune ring. Here, downdrafts from the west blowing from the slopes of the central peak meet winds from the east coming from the crater rim. The barchanoidal ridges change into transverse dunes at this point. Moreux Crater is thus a prime example of how local topography can influence wind flows and thereby have an indirect influence on morphological features.