HiRISE images

A piece of Mars: Barchan, or crescent dunes, are generally thought to form from winds blowing from a single direction. Reality isn't usually that nice. Here are two barchans with crescent-shaped slip faces on their eastern (right) sides, indicating that the main dune-building winds blow from the west (left). However, ripples, an elongated dune arm, and steeper northern slopes hint at a secondary wind from the SE (lower right). (ESP_033272_1400, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: This is what the relentless work of the wind can do. That vaguely circular structure in the center of the image is probably the remains of a ~150 m diameter impact crater. Since it formed, it's been at least partially buried in dust. That dust was probably scoured from elsewhere on the planet during many successive dust storms, and slowly accumulated on the surface here. That dust lithified and is now once again being eroded away by, you guessed it, the wind. (PSP_010345_1635, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: These sharp-tipped hills are dunes near the north pole of Mars. At the height of summer they're lovely dark dunes, but because it's just barely spring here they're still covered in white frost (mostly CO2 ice, but a little water ice). Like penguins who huddle in the dark of the polar night, these dunes are waiting for the sun to return so they can wake up and start moving with the wind. (ESP_032895_2615, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: All that sand on Mars -- where does it come from? It's often hard to figure that out. Here is a rare case where we can see material shedding from a stack of bright layered rocks, moving downslope (toward the top of the frame). (PSP_003684_1730, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: In the center are two dark dunes, racing one another to jump over hurdles formed of older, now inactive dunes (or maybe those bright things are ripples, we still don't know for sure). The dune in the lead has been slowed by this barrier, but it is starting to crawl over it, one sand grain at a time. The dune in second place is still well-formed but will likely struggle once it runs into its own hurdle. Perhaps 20 years from now. (PSP_001756_1995, NASA/JPL/University of Arizona) read more ❯

A piece of Mars: These C and S-shaped things were once dunes that marched across the scene (from upper left to lower right), formed from sand deposited by the wind. Then that sand somehow became cemented, locking the dunes in place. But the wind didn't stop -- it continued to blow from the same direction, eroding grooves and flutes into the cemented dunes. (ESP_017741_1745, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: Abstract modern art? No, these are north polar dunes almost completely covered in winter frost (mostly CO2 ice). It's early spring in this image, and the dark sand underneath the ice is only just beginning to peak out from under the ice cover. The ice looks a little yellow because some dust from the atmosphere has settled on the ice. Soon the ice will sublimate away and reveal the spectacular dark dunes. (ESP_024876_2590, NASA/JPL/University of Arizona) read more ❯

A piece of Mars: The blue structures are rippled dunes found not far from the south polar cap of Mars. They are probably partially active, enough to create the fine ripples found on and between the dunes. But the steeper bluish slopes on the right are pitted and rough, which is not characteristic of actively migrating dunes. Ice in the dunes probably keeps them from being mobilized by the wind. (ESP_031749_1080, NASA/JPL/University of Arizona). read more ❯

A piece of Mars: Topography makes for great landscapes. It also funnels winds, forcing dunes and ripples along particular pathways. Here a cliff (high ground is yellow) has forced low-lying, dark ripples to migrate up a slope at the end of a box canyon. (ESP_032069_1525, NASA/JPL/University of Arizona) read more ❯

A piece of Mars: Yep, these are dunes shaped into polygons. Each "cell" is about 25 m (~80 ft) across. The crests of these things are outlined in blue, as if somebody traced them with a pen (well it's stretched to look blue, but it's really more of a grayish color). These don't look like typical dunes -- they were formed in a place where the wind blows from many different directions, so it's tough to tell what winds formed them. (ESP_031138_1380, NASA/JPL/University of Arizona) read more ❯