A Piece of Mars: The focus of this 0.96×0.96 km (0.6×0.6 mi) scene is one of many two-faced dunes on Mars. The bright sunlit slope is one face, formed recently by wind blowing from the upper right. The dark shaded slope is the other face – it’s a little older, formed by wind blowing from the left. Together these two winds alternate, probably in different seasons, forcing the sand into a needle-shaped point that carries sand in a direction that is, give or take, the sum of those two winds. Two-faced dunes like this are rare on Earth, as winds here typically quickly erase older crestlines. (HiRISE ESP_021716_1685, NASA/JPL/Univ. of Arizona)

Comments (4)

  1. I’ve seen snow drifts curve around objects, but in this pic there is no apparent obstruction to cause such a curve in the dune. Fascinating, and beautiful.

    1. The slip face (avalanching slope) of a dune tends to curve, especially when the dune is made by a wind blowing from one single direction (like here: It’s then called a “barchan” or sometimes, a “crescentic” dune. The top of the dune has plentiful sand, so the dynamics make the crest try to align perpendicular to the wind. But lower down, there’s less sand, and there the wind forces the sand to align downwind. So you get these lovely curves.

      1. Sorry no question regarding the wind dynamics but, what makes the sand look green especially in the upper center of the image?

      2. Moss!


        It’s some combination of 1) this is not a standard Red-Green-Blue image, but rather an Infrared-Red-Blue image that has been mapped to Red-Green-Blue (this is common in astronomy – lots of galaxies and quasars are really imaged in odd light, like x-rays, and then remapped to wavelengths our eyes can see). I’ve done my best to adjust it to make it seem more like something the human eye would see, but sometimes it doesn’t quite map from IRB to RGB that well (usually you see it as bluer dark sand than normal, but for whatever reason this one went a different way), and 2) my tenuously thin artistic/Photoshop skillz.

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