A little while back, I posted one of the greatest examples of the concave-convex illusion I’ve ever seen, where sand dunes in the Sahara look like little craters when flipped upside-down.
At the time, I was more interested in the illusion than the dunes themselves, but that all changed when the NASA Earth Observatory posted a stunningly lovely satellite picture of the same kind of dunes, called star dunes:
Seriously, right? Dunes are formed when winds pick up and move sand grains. The shapes of the dunes depend mostly on the size of the grains, and the direction of the wind. A steady wind, for example, can generate familiar barchan dunes, shaped like horseshoe crabs. But if the wind changes direction, swirling, the dunes can take on the twisted ribbon and star shapes seen above.
The photo is from the Terra Earth-observing satellite, and shows these dunes over a vast expanse of Saharan Algeria called the Grand Erg Oriental. Most of the dunes you see are about a kilometer (0.6 miles) across, so they’re decent-sized hills. The full resolution picture is astonishing, with hundreds of dunes dotting the landscape, each different in shape.
One of the most basic principles of astronomy is that the forces and rules that govern the Universe are the same everywhere. A corollary of that is that given the same circumstances—say, for example, wind, grains of material, and time—you’ll get similar features. Mars, Titan, and our home world are very, very different, but it’s somewhat comforting to know that in some ways, at least, they act the same.