Blowin’ in the (Martian) wind.

Mars is frequently considered to be a ‘lifeless’ planet where nothing of geomorphological note has occurred for billions of years. However, in many cases the dynamic processes taking place in the aeolian sand seas rival those in dryland regions on Earth.

Barchan Dunes, Paracas National Park, Peru (George Steinmetz)
Barchan Dunes, Paracas National Park, Peru (George Steinmetz)

When studying the climate and geomorphology of extra-terrestrial bodies we see that similar physical processes act on their landscape as on our own. For example the ‘runaway’ greenhouse effect on Venus is an exaggerated version of the greenhouse effect experienced by Earth and yields temperatures of at least 462°C on the surface of the planet.

By studying the Martian landscape it has become clear that many of the regions on Mars are similar in geomorphology to desert regions of Earth. Barchan dunes are an example of a landform which has developed on both Earth and Mars. These are a form of sand dune which is crescent in shape with two horns facing downwind (figure 1).

On Earth barchan dunes occur where there is high aridity and a narrowly unimodal wind regime. Examples of barchan dunefields are as far flung as Peru, Namibia, California and Antarctica.

On Mars, dunefields are found in the North Polar Region and in craters in the southern hemisphere. The dunes themselves are generally of a scale that would dwarf most terrestrial barchans. The shape, size and migration patterns of these dunes is determined, in part, by the winds blowing over them.

There has been significant progress in understanding the controls on barchans planform shape and change to that shape. There are increasingly refined numerical models (e.g. Parteli et al, 2013) that show a relationship between barchan dunes and their formative wind conditions. The data suggest that barchan form can be used as a proxy for wind conditions in regions where detailed meteorological records are often scarce.

Asymmetric barchan dunes of Arkhangelsky Crater in the Southern hemisphere of Mars.                                  (HiRISE ESP_019559_1390).
Asymmetric barchan dunes of Arkhangelsky Crater in the Southern hemisphere of Mars. (HiRISE ESP_019559_1390).

What has been missing from this approach is a test of the modeled relationships with the instrumented record. For my undergraduate research project I showed the potential importance of dune asymmetry (that is the difference in length between a barchan’s two horns) as an indicator of secondary winds that act upon a dune. By tracking the asymmetry of dunes in Peru and Namibia alongside the region’s wind records I was able to show, for the first time, the morphological response of the dune to changes in the wind regime. The dune response to bimodal winds was to cause a change in barchan planform from symmetric to asymmetric. These data add strength to attempts to infer regional wind regimes using dune two dimensional dune shape on other bodies in our solar system.

It is compelling as a researcher to know that observing these forms on Earth can greatly increase our knowledge of the environment which exists or once existed on a planet 56 million kilometers away from our own.

Reference:

Parteli, E.J.R., Duran, O., Bourke, M.C., Tsoar, H., Poschel, T., Herrmann, H.J., 2014. Origins of barchan dune asymmetry: Insights from numerical simulations. Aeolian Research, 12, 121-133.

Written by Diarmuid Dwyer, Undergraduate, Geography Department, Trinity College, Dublin.

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