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The bright barchan

ESP_039568_1120_0.794xA piece of Mars: Most dunes on Mars are dark, like these and these. So why is this one bright? It’s adjacent to a more typical, dark dune. It’s possible that there are two populations of sand here that are different enough in size or density, and so they respond to different winds – thus producing remarkably different dunes in the same location. (HiRISE ESP_039568_1120, NASA/JPL/Univ. of Arizona)

Sometimes I just have no idea

ESP_039595_1230_0.631xA piece of Mars: The smooth areas are eroded dunes, separated by fields of boulders (the scene is 1.51×1.14 km or 0.93×0.71 mi). The largest boulder near the center is 7.5 m (25 ft) across, the size of a small RV. The interesting wave patterns on the lower sides of the smooth dunes… well, I don’t know. My best guess is it’s another type of bedform created from the sand of the smooth dunes. Do you know? (HiRISE ESP_039595_1230, NASA/JPL/Univ. of Arizona)

Dunes ignoring small hills

ESP_039524_1445_1.0xA piece of Mars: What happens to dunes as they move over rough terrain? This is what a barchan looks like on a relatively flat surface. If the hills are smaller than the dune, then it does its best to pretend they don’t exist, like the one in this image. It’s 175m (574ft) wide and 190m (623ft) long, with a slipface indicating overall migration to the northeast. (HiRISE ESP_039524_1445, NASA/JPL/Univ. of Arizona)

On Mars, the wind wins

ESP_039057_1485_1.0x A piece of Mars: This scene (600×450 m or 1969×1476 ft) is covered in small craters, formed by the splash of a larger crater nearby. They cover everything, even the bright ripples visible on the right. So the ripples were there before the impact that formed all these little craters. And yet… there are itsy little gray ripples on the upper right, merging with the crater rims – these are new ripples, younger than the craters. On Mars, it’s the wind that wins in the end. (HiRISE ESP_039057_1485, NASA/JPL/Univ. of Arizona)

Wind eroded mantle

ESP_039195_1755_0.398xA piece of Mars: The curving ridge of a mountain has signs of many small landslides. Mantled on top of these is an older set of landslides that has been partially eroded away. The rippled edge of this older deposit suggests that it is wind that has done the erosion. So the history here goes: mountains, then landslides, then wind erosion, then new smaller landslides. (HiRISE ESP_039195_1755 NASA/JPL/Univ. of Arizona)

Bearded craters and dunes

ESP_0038826_1700_1.0x
A piece of Mars: This 600×450 m (1969×1476 ft) scene has a complex sedimentary history. How are bearded craters and dunes formed? They weren’t always bearded. At some point, a deposit of bright material accumulated on this surface, and was then eroded so that all that remains of it is what is protected by topography (anything that pokes up like dunes or crater rims). Can you find the boulder that has tumbled downslope (it too has a beard!). (HiRISE ESP_038826_1700, NASA/JPL/Univ. of Arizona)

The long, low dune

ESP_038615_1665_1.0xA piece of Mars: A long, low dune covered in long, linear ripples stretches across the scene (600×450 m; 1969×1476 ft). Dark gray areas on the dune show where sand has most recently moved. A small slip face has formed on the southeast side of the dune, but ripples have formed on it, so there haven’t been any recent avalanches here. (ESP_038615_1665, NASA/JPL/Univ. of Arizona)

Wind, wind, impact(!), and then more wind…

ESP_038918_1650_0.437xA piece of Mars: Some time ago, something hit the ground on Mars and made this impact crater, right into a field of ripples. Stuff thrown up during the impact fell back down, burying the ripples with the gray ejecta rays that radiate from the crater. But the wind kept blowing, and in some places you can see where new ripples have formed on top of the ejecta. That’s Mars for you: wind, wind, wind, impact(!), more wind… (HiRISE ESP_038918_1650, NASA/JPL/Univ. of Arizona)

Aeolian shoreline

ESP_038799_1590_1.0xA piece of Mars: On the left is a steep slope leading to a hill. On the right are waves – but not waves of water or any other kind of liquid. These are dunes or very large ripples, blown by the wind into intricate patterns. Sharp eyes might spy boulders that have rolled downslope into this “sea” – there’s even a dotted track that one boulder made as it went. Can you find the boulder? (HiRISE ESP_038799_1590, NASA/JPL/Univ. of Arizona)

Inverted crater

ESP_038309_1870_0.501xA piece of Mars: This circular hill is 200 m (~656 ft) across and ~48 m (~160 ft) high. It stands alone on a relatively flat plain. Why is it there? The surface here used to be ~48 m higher than it is now – on that old surface, a crater formed. The crater was filled in by sediment. And then the surrounding terrain was eroded away by the wind (that’s a whole lot of stuff to be removed over time!). What’s left is the old crater fill, but one day it too will be blown away. (HiRISE ESP_038309_1870, NASA/JPL/Univ. of Arizona)