The concentric ring structure of the Aorounga crater—renamed Aorounga South in the multiple-crater interpretation of SIR data—is clearly visible in this detailed astronaut photograph. The central highland, or peak, of the crater is surrounded by a small sand-filled trough; this in turn is surrounded by a larger circular trough. Linear rock ridges alternating with light orange sand deposits cross the image from upper left to lower right; these are called yardangs by geomorphologists. Yardangs form by wind erosion of exposed rock layers in a unidirectional wind field. The wind blows from the north-east at Aorounga, and sand dunes formed between the yardangs are actively migrating to the south-west.
<ul><li>NASA Expedition 20 crew</li>
<li>Image Science &amp; Analysis Laboratory, NASA Johnson Space Center</li></ul>
2009-08-10
<a rel="nofollow" class="external text" href="http://earthobservatory.nasa.gov/IOTD/view.php?id=39727">NASA Earth Observatory</a>