Liquid Dunes

I was just explaining to my nephew how air and water can get waves in them and behave like fluids. This is a picture that looks, at first glance to be liquid streaming off a wet surface. It is actually a satellite photo of Mars, showing a series of dunes trailing off a rock formation, like drips of water.

The picture is from the April 21, 2009 Astronomy Picture of the Day. See the whole archives of pictures at http://antwrp.gsfc.nasa.gov/apod/archivepix.html

The photo is titled Flowing Barchan Sand Dunes on Mars. I did not know what Barchan meant, so I ended up pursuing the subject and other sand dune terms through multiple references in Wikipedia. What's cool about it is that the dunes that are made from fine sand and constant winds have this strange horseshoe crab, cylon spaceship shape. Not only is the shape cool, but the movement is, too. Apparently, small dunes can overtake larger ones, and when they do, the small one hits, disappears, and then re-emerges out of the other side of the bigger slower dune. The little dune does not actually move through the big dune, it actually breaks up and reforms on the downwind side, but it looks like it moves through.

Following the links, you can learn about Megabarchans, solitons, and cresentic, star, dome, parabolic, and seif dunes. You probably did not suspect that so much thought went into naming something that we consider to be just sand.