Just as with some dunes here on Earth, the wind moves the dunes on the floor of the Herschel crater downwind until their shape gets elongated to the point of merging with dunes beside them. To put the icing on this geological cake, nature decorates the tops of these dunes with delicate ripples and scallop shapes.
A vast vertical rock face divides the area between the layered northern polar deposits and the more southerly lands where the dunes reside. You can see their ripple effect at the bottom of the image, where they’re covered in frozen carbon dioxide even though it’s the Martian spring.
You’d be forgiven for thinking this was a close-up image of reptilian skin, or even the tissue of some gruesomely infected body part. You might, then, be surprised to find out that it is, in fact, an image of Martian polar sand dunes. The white is frozen carbon dioxide (dry ice) and the darker flecks are composed of basaltic sand. During Martian spring the ice sublimates (meaning it turns directly to gas without ever melting into liquid), which brings some of this dark sand up to lie on the surface of the ice. Scientists believe that these deposits formed at the same time, due to the fact that they’re all in similar shapes and directions.
This stunning image of the Victoria Crater shows that beauty can even be created by a devastating cosmic event. The edge patterns are from debris falling from the sides into the crater and the neat pool of ripples in the center are sand dunes covering the floor.
Namibia has some of the highest and largest sand dunes in the world. This image from ASTER (the Advanced Spaceborne Thermal Emission and Reflection Radiometer) shows Northern Namibian dunes, which are often colored red, running in eerily-ordered patterns across the entire expanse.
The Thar Desert is a huge expanse in Northwest India and Eastern Pakistan. The sand dunes are prone to shifting due to high-velocity winds, which has caused problems when the sand dunes have shifted and blocked roadways. Some effort has been made to stop this by planting trees and shrubs, which give the dunes more structural stability with their roots.
The carbonate sand dunes in Tarpum Bay, Bahamas look like beautiful blue satin in shallow waters. The erosion of limestone coral reefs supply the sand that is then carved into the soft patterns by the ocean current.
The Richat structure in Mauritania’s Sahara Desert is known for its resemblance to an eye, but it’s also well known for its large fields of sand dunes. In this image you can see that the dunes stop before the cliff on the right hand side as winds from the northeast clear sand from the base of the cliff. It’s incredible how this image looks so otherworldly but is, in fact, on Earth. It certainly wouldn’t be out of place on the cover of a sci-fi novel.
This may look like an impressionist’s study of a lake but in actuality, it is an image of barchan sand dunes in Kandahar, Afghanistan. Barchan dunes are arc-faced and have two horns on the downwind side. You can tell the winds are from the west in this section from the direction of the dunes.
The Bodele sand dunes in Chad, Africa are used by scientists to calculate the speed at which sand dunes migrate. This information is often used to understand the impact wind has on dry landscapes. Images are taken in increments of anything between one month and 6.5 years and have proved that these dunes are among the fastest moving on Earth!
This is another image of Mars that shows dark basalt sand dunes in a crater. As the ice-covered dunes defrost, the underlying basalt is exposed. The defrosting is simply frozen carbon dioxide changing to gas as the Martian spring warms the surface.
Both light-toned bedrock and darker material containing sand dunes appear in this image, which looks astonishingly like the wall of a cave. It is the Ganges Mensa on Mars, and scientists use images like this to determine the strengths and age of surface material. Impact craters are more apparent in the veneer of sand dunes than the bedrock below, which is smoother — an apparent paradox, as the bedrock has to be older than the sand layer. The answer lies in the structure of the bedrock: It crumbles more easily, and impact craters are smoothed over faster than in the sand.