Double trouble

A piece of Mars: Bright dunes once marched through the depression created by a double impact. How can you tell the dunes are no longer moving? Because one of them has its own little crater sitting on it. A mobile dune would quickly erase such a distinctive feature. (HiRISE ESP_027018_1925)

Wind erodes rocks into swirls

A piece of Mars: When the wind sandblasts at layered rocks, some pretty swirly patterns emerge. These are flat layers that take on a stacked wedding-cake appearance on an outcrop elongated by the wind. The direction of the stacks indicates the direction of the wind — from upper left to lower right. (ESP_027011_1735)

Patch of blue

A piece of Mars: Most of the image, stretching beyond the edges of this frame, shows a bland gray landscape of lava blanketed in dust. But one small patch of blue shows where sand is still actively moving and piling up. As usual, it’s in the lee of a topographic feature. (HiRISE ESP_027002_1765)

Barely a dune

A piece of Mars: What makes a dune different from a random pile of sand? Usually the requirement is that it has a “slipface”, a steep avalanching slope. The central dune in the image here has a small one, making it a type of dune called a barchan. The other dunes are called “dome dunes”,

An intriguing mess

A piece of Mars: What an intriguing mess this is. These are dunes or ripples, oriented every which way, and mottled with white spots. The different orientations tell us that the winds blow from several different directions, on timescales long enough to seriously influence the dunes. The mottling is a mystery. (HiRISE ESP_025386_1800)

Dunes all gone

A piece of Mars: Way up in the cold northern plains of Mars, some of the dunes are being blown away slowly by the wind. Here are some that are barely there yet — dark patches of sand in the shape of their former glory. (HiRISE ESP_027389_2645)