Like the Oprah of gorgeous space imagery, America’s venerable space agency just can’t stop handing out stunning images of the deep and lovely dark above our heads, and all the planets, moons, and stars that reside in it. Next up – under the groaningly pun-tastic title of “Once in a Blue Dune” – is a pair of shots of the surface of Mars, one of which depicts what appears to be a turquoise-hued pile of gorgeous-looking sand.
As with plenty of space imagery, what you’re seeing in the photograph isn’t actually what it looks like. Depending on the filter used, or depending on what needs to be highlighted, the colors of the real deal are frequently altered. In this case, although the dunes are different from the surrounding geology, they’re certainly not quite as vibrantly blue as the Mars Reconnaissance Orbiter’s masters would have you briefly believe.
What you’re seeing here, in the region of the sand-trapping Lyot Crater, is a field of still awesome-looking “classic barchan dunes” with a more complicated, larger dune just south of the field. Here's the zoomed-out image in full (rotated):
Now, sand dunes on other worlds – from the methane-comprised marvels of Pluto to the electrified eccentric examples over on Saturn’s moon of Titan – can be both similar and strikingly different from those on Earth. This time around, however, the dunes on our planetary neighbor are extremely familiar.
Barchan dunes are typical, crescent-shaped sand dunes found in desert regions all over the planet, with the archetype found in Turkestan, a vast region stretching from the Caspian Sea to the Gobi Desert. Unidirectional wind causes sand to build up on the so-called stoss side of the dune, and little avalanches of sand fall down on the steeper lee side of the dune.
The wind moving around the side of the dune leads to smoothed-out pointed corners, and voila – you’ve got a barchan dune, one of many types.
That’s what you can see in the above image: plenty of barchan dunes, looking all swish, like sandy manta rays or something.
The main, eye-catching image, the one virtually painted blue to indicate its extremely fine sandy composition, is a large complex dune, showing barchan-like features.
Like those on Earth, they’ll be slowly moving across the land as sand progressively tumbles up the stoss side and down the lee side. Although common, Mars is somewhat lucky to have these dunes.
Dune formation depends, in part, on the density of the air. The Red Planet’s barely-there atmosphere is around 1 percent as dense as Earth’s, which means that wind speeds there are comparatively low – but still just strong enough to push sand and form those lovely-looking dunes.