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SYFY WIRE Bad Astronomy

Spectacular Martian Dune

By Phil Plait
hirise_barchandA big pile of sand on Mars. Credit: NASA/JPL/University of Arizonaune_color_hero

When I wrote my review of the movie The Martian, I mentioned how realistic the landscapes were. Much of the literally other-worldly scenery was based on images taken by the HiRISE camera on the Mars Reconnaissance Orbiter, which can resolve objects on the surface of the planet down to about a half meter.

I may have missed it, but in the movie I don’t think they showed any dunes. There was a lot of sand, lots of texturing in it like ripples, but I don’t remember seeing any large-scale dunes.

Too bad. Because Martian dunes are jaw-droppingly beautiful. If you don’t believe me, then take a look at the photo here, and also check it out in superduper high-res, because wow.

A big pile of sand on Mars. Credit: NASA/JPL/University of Arizona

That is what’s called a barchan (pronounced BAR-can) dune—a horseshoe-shaped sandpile. In this HiRISE shot, the wind is coming from the (very slightly upper) right, blowing particles to the left. If something blocks the wind a little bit, like a rock or other outcropping, the sand piles up. The wind then splits, forming a bow wave pattern around the obstacle, and the sand follows suit.

When it gets tall enough, the sand will flow down the back of the slope, creating a sharp peak more-or-less perpendicular to the direction of the wind, so it too follows the bow shape. Irregularities in the dune surface force the wind to flow up and down in waves, creating the ripples in the dune. The inside curve of the dune is in the lee of the wind, so the sand sliding down there creates a much smoother dune surface.

The color is gorgeous, but I’ll note it’s not what your eye would see if you were flying over Mars. The actual image is the combination of a red image (shown as red) and one taken with a blue-green filter (shown in green, since that filter lets through mostly green light). To make the red-green-blue image, the blue color is approximated as a mathematical combination of the green and red image.

However, this image is telling us there’s a lot of red in the dune, most likely due to very fine grain dust. Roughly speaking there are two kinds of grains on Mars: sand, which is millimeter-scale gray volcanic basalt, and dust, which is much finer (like talcum powder) and has a lot of iron oxide in it. Iron oxide is more commonly known as rust, and that’s why Mars has so much red to it.

This dune is part of a much larger dune field, and as the image release points out, the surface under the dune field is just as interesting as the dunes themselves. It looks fractured and has resisted wind erosion, so it must be made of tougher stuff. It’s likely very old bedrock. The red region on the right must be sprinkled with dust, while on the left it’s grayer, meaning less dust. In the larger scale pictures (warning: 200 Mb file!) you can see that this dune sits at the edge of the dune field, upwind (to the right) of most of the dunes. Since that’s also where the gray/red line is in the surface, I’m guessing there’s some surface sloping going on here.

When I see pictures like this, I have to chuckle. Years ago, I used to think Mars wasn’t terribly interesting or pretty. In my defense, all the photos we had were low resolution, so it looked rather dull. But I chalk this up more to a failure of my imagination. Now that we have better tech spying on the planet, it’s obvious just how gorgeous it is. I hope I’ve learned my lesson; now when I look at a photo of some object that doesn’t have a lot of features, I wonder what amazing thing lurks there that we just barely can’t see.

The Universe isn’t actively hiding anything from us. It just is what it is. We have to be motivated to dig deeper, see better, to uncover its truth.

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