NASA released a series of images from the Mars Reconnaissance Orbiter (MRO) this week showing movement of sand dunes over a 2-4 year period. Yesterday afternoon, the editor here at Wired Science, Betsy Mason, posted a gallery of the animated GIF images, including the one above. These images are a simple and effective way to highlight change over time as documented by repeat photography.
The image above very nicely highlights the process of grain flow on a slip face of the dune. Note how most of the change between static images is in the upper left part of the dune. It’s a bit difficult to see on this dark and low-contrast image so below is a non-animated image that I tweaked in Photoshop and annotated.
Sand dunes are topographic features that interact with and influence the wind patterns that create them. Sand grains are transported up the dune (in this case, from lower right to upper left) very close to the surface of the dune through a combination of rolling and short-hop bounces. When they reach the crest of the dune they will avalanche down the steep leeward side of the dune — also referred to as the slip face. The angle of that slip face corresponds to the steepest angle the sand can pile up before it avalanches down (called the angle of repose). Over time, as more sand is moved to the crest and avalanches down the slip face the entire dune will migrate.
Sand dunes, like many depositional landforms, are fascinating examples of self-organization. Once the dune is started, there is a feedback between the form of the dune and how that form influences the wind flow pattern, which in turn perpetuates the form of the dune. Also note the presence of ripple marks on the annotated image above, which tells us that this interaction between fluid flow and morphology can occur at different scales on one landform.
Bonus: Speaking of sand dunes, check out this fantastic post at the blog Galileo’s Pendulum discussing a recent paper investigating what happens when sand dunes ‘collide’ over a period of several decades. It must be Sand Dune Week (way better than Shark Week), Jennifer Oullette of Cocktail Party Physics has a superb post about the physics of granular materials and ‘singing’ sand dunes.