HiRISE images

A Piece of Mars: These "peas-in-a-pod" are dunes covered in long ripples (the scene is 960x480 m, or 0.6x0.3 mi). They're a bit odd, surrounded by a rippled apron. It reminds me of melted-looking dunes that are common in high southern latitudes, but these are at 21.5ºN. Yet more Mars mysteries to solve... (HiRISE ESP_042697_2055 NASA/JPL/Univ. of Arizona). read more ❯

A Piece of Mars: This scene is 0.96x0.54 km (0.60x0.34 mi) across. There's an old river valley running across it. The walls of the valley have been eroded and there's a washboard pattern with a wavelength of ~6m (20 ft). When I first saw this image I thought it was exposed, tilted layers, but a closer look reveals a much smaller (and younger) set of ripples that are similarly oriented and almost certainly formed by the wind. What do you think? (HiRISE ESP_041982_1535, NASA/JPL/Univ. of Arizona) read more ❯

A Piece of Mars: The dark dunes in this 0.96x0.54 km (0.60x0.34 mi) scene are slowly migrating towards the lower left. Look closer and you'll see brighter ripples between the dunes - the biggest ones are ~8m (26 ft) apart. They form trains in the wake of the dunes. Why? This is where the dunes previously marched on by, obliterating the ripples as they went. Ripples form again after the dunes pass by, growing in size and regularity with time. (HiRISE ESP_01746_2570, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: Not all sand piles up into big dunes. If the grains are the wrong grain size (too fine or too coarse), then it might just form sand sheets with ripples on top, like it does here, slowly migrating from top left to bottom right. There are two different kinds of sand here: the ripples seem more grayish and the underlying sand sheet seems more brownish. Grains of different sizes and densities respond to the wind in varying ways, so that they form different features on the surface. (HiRISE ESP_041977_1515, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: On the left is high ground, covered with dunes (or maybe they're ripples) running from upper left to lower right. On the right is low ground, covered in deeply eroded dunes (ripples?) running almost from left to right. They were probably created at two different times by winds that changed direction in the intervening time. The set on the right is probably much older. (HiRISE ESP_041991_1715, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: These dunes look strangely triangular, a little bit like a flock of stealth bombers. Why? They're two-faced barchans. Each flat face is an avalanche slope that faces downwind, formed by one of two distinct wind patterns that blow in this area (probably seasonally). Dunes like this can form on Earth, but the older slip face tends to be quickly erased as the winds change. (HiRISE ESP_027854_2150, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: There's an egg-shaped plateau here (the whole scene is 480x270 m or 525x295 yd across, the "egg" is ~100 m long). It's partly covered by dunes that have extended across it. Or were the dunes there first and it buried them? Probably the former, but you can try to convince yourself either way. What do you think? (HiRISE ESP_041134_1720, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: So much wind. There are dark swirly tracks of dust devils that have passed by, ripples covering dunes, wind scours around rocks, and of course dunes. Dune crests have a different color than other regions: are they less covered in dust? made of a more grayish sand that is more easily blown up the dune by the wind? or both? (HiRISE ESP_040885_1295, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: In this image (0.96x0.54 km or 0.6x0.33 mi), it's late winter and the sun is barely above the horizon here near the north pole. The dunes are covered in winter frost, most of which is CO2 ice (also known as dry ice). The dark regions are those facing the sun, where the ice has started to sublimate, revealing the dark sand below. (HiRISE ESP_041433_2650, NASA/JPL/Univ. of Arizona) read more ❯

A piece of Mars: These dunes are some of the smallest on Mars. The smallest in this frame is ~150 m long (492 ft). But the smallest Earth dunes are ~20 m across. Why are they so much bigger on Mars? The air is thinner, so the wind has to blow stronger to lift sand grains. So once the sand is moving, it goes fast - and therefore goes farther before it lands. This makes for a bigger dune. (HiRISE ESP_41809_1890, NASA/JPL/Univ. of Arizona) read more ❯