Interpreting Barchan Dunes

This week’s image comes from Mars and is a CTX image of a section of the large dune fields near the northern pole. These circumpolar dunes form a sea of shifting sand, which encircle the northern icecap. Different sections of the dune fields exhibit different types of dune patterns and, as I’ve discussed before the shapes of dunes can tell us a lot about the local wind conditions. 

This area is characterised by "barchan dunes". These crescent like shapes are often found in terrestrial deserts as well. Barchan dunes form when there isn’t enough sand to make a continuous blanket. It shifts from one place to another, forming piles and mounds. the sparse barchans on the left hand side of the image gradually coalesce into more continuous ripples to the right. This suggests that there is less loose sandy material on the one side of the image and more on the other.

Barchan dunes generally occur in places where the wind blows from a consistent direction, and so collects the sand into crescent shaped mounds. The shape of the dune is the result of the flow of air over it, giving them an aerodynamic shape. The points of the crescent point downwind, so the structure of these dunes tells us what the prevailing wind direction is. 

The upwind face of the dune is generally shallower than the downwind “slipface” which can be seen between the two horns of the dune. By measureing these angles we can learn about the movement of granular material. We know that the slip face will almost always be at the “angle of repose”. This is the steepest angle at which a slope made of dry material remains stable. It cannot become any steeper without collapsing, when the forces exerted on the slope exceed the frictional forces keeping the grains of sand together. 

The angle of repose depends on a lot of factors, including the size and structure of the grains and the presence of water. Wet sand sticks together more readily, as the water forms links between the sand grains. This can come in handy when trying to build a sand castle, and allows a slope to form which is steeper than the angle of repose would be for a dry granular material. Thus if we know what the angle of repose of a slope should be at a site, we can tell if a slope is steeper than expected, and thus must consist or wet sand or a different material which is more stable. If it is shallower than expected then we know that this slope is not yet at the angle of repose, and more material can accumulate there before it risks becoming unstable. 

But how do we determine what the expected angle of repose should be? On earth this si quite well constrained for different materials, although it can vary substantially from site to site. For a long time it has been thought that the angle of repose was independent of gravity, and so the same materials should form similarly steep slopes on any planet. However a recent study has cast doubt on this. 

Researchers set up a large number of rotating drums of sand and gravel in an experimental aircraft and filmed them using high speed cameras as they went through several parabolic flights in order to simulate different levels of gravity. The results indicated that there were substantial changes to the angle of repose due to changes in gravity. The paper itself can be found here, and this blog has a good overview of it for anyone who can’t access the paper itself but wants more detail than is included in the abstract. 

If these results are confirmed it could have some very interesting implications for interpreting the geomorphology of other planets. Angle of repose isn’t just significant when looking at sand dunes, but on any loose slope, such as those on alluvial fans and mounds of debris. Figuring out how, and if it changes with gravity could potentially give us a lot more information about these landscapes.

 

Image Credit: NASA/JPL/University of Arizona, via Google Mars