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SYFY WIRE Bad Astronomy

Federation Scum Is Attacking Mars!

By Phil Plait
Barchan dunes on Mars. Credit: NASA/JPL-Caltech/Univ. of Arizona

What’s the last thing a Romulan commander sees on their tactical screen before a volley of phasers turns them into space vapor?

I’m guessing this:

Barchan dunes on Mars. Credit: NASA/JPL-Caltech/Univ. of Arizona

That’s not a bunch of Federation symbols flying across some alien world. Well, it is an alien world, but it’s not in the distant reaches of the Alpha Quadrant: That’s Mars, and those are actually sand dunes, seen by the HiRISE camera on board the Mars Reconnaissance Orbiter.

Technically, they’re called barchan dunes. They can form when the wind blows predominantly from one direction. If there’s an obstacle, like a big rock or small hill, the wind will blow around the obstacle, the same way water flows around a rock. Sand will pile up on the leading edge and also be swept around to the backside. Eddies in the wind create circular currents on the downwind side, building up walls of sand on the sides and creating that horseshoe crab-like appearance.

Eventually, you get a long, shallow slope leading to a crest, a sharp edge, then a steeper slope downwind. The wind supports the sand from rolling back down the upwind side, but downwind the sand is free to roll down, creating a steeper slope. The long arms leading downwind are due to the eddies in the wind behind the obstacle. Once formed, these dunes can actually move as a piece; the sand rolls up the shallow slope and then down the steep one, keeping the overall shape of the dune even though the individual sand grains change. In that way, it’s very much like a traffic jam on a highway, which persists and can move even as individual cars enter and leave it.

Barchan dunes are common on Earth and on Mars. They can form long, narrow chains called seifs, too—those make for very dramatic and beautiful formations.

I have to laugh: The first thing I thought when I saw the picture was how these dunes looked like Star Trek Federation symbols. And I’m not the only person who thought that …

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