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Westward moving

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A Piece of Mars: No great scientific insights today, just a really lovely view of bright TARs and some very dark sand in this 0.875×0.5 km (0.54×0.31 mi) scene. Only one major wind acts in this region, moving sediment toward the west. Jezero crater, a prime landing site candidate for the Mars 2020 rover, lies 50 km to the west, so some of the sand blown into that crater passed through this area at some point in the past. (HiRISE, ESP_050899_1985, NASA/JPL/Univ. of Arizona)

Experimenting with 3D views

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A Piece of Mars: I often use JMARS to visualize Mars data sets, especially images. They’ve recently updated their 3D layer, allowing folks to make lovely vistas by overlaying DTMs with images. I’m new at this, but I’ll experiment and see what I can do to make nice views. Here’s a series of barchan dunes marching away from a tall stack of layers in Becquerel crater, with no vertical exaggeration. (HiRISE, DTEEC_045140_2015_044784_2015, NASA/JPL/Univ. of Arizona)

Reversing slip faces

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A Piece of Mars: This 523×750 m (0.32×0.47 mi) scene shows a large dune. It’s quite colorful for some reason, although it’s partially false-color. What caught my eye is that the slip face on this dune has reversed direction, which is somewhat rare on Mars (but common on Earth). The main sand-moving wind blows from the right, forming a long avalanching slope (you can see long bright lines of grain fall slips at the lower center). But at some point a wind blew from the left, forming a small slip face in the opposite direction. Although many other wind directions have also help to build this dune, those two are the main winds apparent here. (HiRISE ESP_050887_2225, NASA/JPL/Univ. of Arizona)

The real tetrahedrons of Mars

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A Piece of Mars: The real tetrahedrons of Mars are dunes, built by winds blowing sand from more than one direction. This 0.5×0.5 km (0.31×0.31 mi) area shows a dune formed from two winds that are about 90 degrees apart: one blowing from the bottom and one blowing from the right. This makes the dune have two slip faces, which is a rare occurrence on Earth dunes. (Earth dunes are complicated by superposed secondary dunes that interfere with and obscure this pattern. Or if they’re small enough to not have those secondary dunes, then they are changing fast enough that one slip face will quickly erase the other. I’ve only ever seen two slip faces at once for very short periods in Earth dunes – they don’t last.) Here, the two winds have worked together to form a little spit of sand off to the upper left. The result is a 3-sided “pyramid”, with no advanced civilization required for its construction. (HiRISE ESP_050479_1360, NASA/JPL/Univ. of Arizona).

The thinnest landslides

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A Piece of Mars: In the dustiest regions of Mars, steep slopes occasionally produce very thin avalanches of dust, revealing a darker surface under the top layer of dust. This shows one that is 610 m (0.38 mi) long, running from its tiny point of initiation near the top of the slope down to the bottom of the slope where accumulated landslides have slowly buried old windblown dunes (or TARs). These landslides occur every spring, and may be triggered by sublimation of small accumulations of winter ice, or perhaps by the wind. This one formed some time between May 7, 2012 and May 22, 2013, as it appeared between two successive images of this spot. It’s still there today, most recently imaged on May 5, 2017, slowly accumulating dust until it fades into the background with the rest of the slope. (HiRISE ESP_035307_2115, NASA/JPL/Univ. of Arizona)

Dunes fighting for survival

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A Piece of Mars: Having a bad day? You’re in good company with these dunes in this 0.96×0.48 km (0.6×0.3 mi) scene. The gray barchanoid dunes are covered in ripples, as the wind valiantly tries to push the sand to the dune crests. But they are besieged by other processes at work. Dark scribbles show how dust devils have swept by, removing dust and probably scattering a little bit of the sand. The steep slip faces are not covered in dry avalanches typical of active dunes, but rather they appear eroded, as if some force locked the dune in place and started eroding the surface wherever ripples couldn’t rescue it. Splotches on the tan ground between the dunes, and narrow furrows attest to seasonal ice reworking the surface. And in this great battle, what I wonder is: can those dunes have formed like this amidst such turmoil, or are they relics of an older, windier, perhaps less icy age? (HiRISE ESP_050488_1150, NASA/JPL/Univ. of Arizona)

Weird and wild southern polar dunes

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A Piece of Mars: This 0.96×0.48 km (0.6×0.3 mi) scene shows a bit of a south polar dune field. The more recently-active dark sand is rippled, but there are bright splotches where something else has happened. Presumably it’s ground ice that sublimated away explosively, as happens at many high latitude locations on Mars, only here the dunes are stabilized enough that those spots aren’t eroded away by wind activity every summer. Because the dunes aren’t active, their crests have diminished to subtle bumps on the landscape (would you even know they were dunes if I hadn’t told you? Look at the whole HiRISE image to be sure!) (HiRISE ESP_013224_1080, NASA/JPL/Univ. of Arizona)

Is it an old fossil barchan dune?

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A Piece of Mars: There are many barchans on Mars, those lovely isolated crescent-shaped dunes. In a few places there are what looks like ancient preserved barchans, now lithified. The mound in the center of this 0.96×0.54 km (0.6×0.33 mi) scene shows what may be an example of a fossil barchan. If so, then this is quite unusual. On Earth, dunes are very rarely preserved in their full form, usually having been at least partially eroded away before being preserved. I love how much geology is visible from orbit on Mars! (HiRISE ESP_049955_1665, NASA/JPL/Univ. of Arizona)

Sand tails

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A Piece of Mars: Up on the tallest volcanoes, the wind screams downhill at night. This 500x500m (0.31×0.31 mi) scene shows how dust is carried downhill, but only that which is trapped behind boulders and crater rims sticks around. The big hole may be a window into a lava tube. Formation of the window itself is one of the younger events to have formed this landscape, as the screaming dust hasn’t fully filled in the hole (although it has begun the process and formed a tailing wind streak). (HiRISE ESP_050089_1660, NASA/JPL/Univ. of Arizona)

Curiosity, recovering from the Bagnold dunes campaign

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A Piece of Mars: You’ll probably want to click on this image to see the whole thing, it’s pretty big, and it’s worth seeing. This 850×550 m (0.53×0.34 mi) scene shows the barchanoid dunes of the Bagnold dune field, imperceptibly crawling southwestward (to the lower left). This is the site where the Curiosity rover first encountered an active dune in its trek through Gale crater. This image was taken after the rover’s intensive field campaign of the two dunes in the upper middle of the frame – the rover is in fact in this frame (extra credit if you can find it!), but it’s backed off a bit from the dunes, and it’s sitting on some old sandstone (that we now know was also once a dune field, long ago, much like some of the sandstones we find on Earth). This image was taken in March 2016; the rover has since moved on and across the dune field, and is slowly working its way through the foothills of Mount Sharp.

I chose this image in tribute to a colleague who unexpectedly passed away last week. He worked on both the HiRISE and Curiosity teams, so it’s fitting to show both here, near the dunes that he studied. He’s best known for his work on dune migration and surface erosion on Mars. He also mapped and measured wind-carved stones called ventifacts (we have those on Earth too), and discovered that the ventifacts here in Gale crater were carved (probably long ago) by a wind blowing from the southwest, which is opposite the direction that the dunes are being blown today! There must have been quite a remarkable shift in wind patterns since those stones were carved, and it remains a mystery. Our dear colleague will be greatly missed.

(HiRISE ESP_045293_1755, NASA/JPL/Univ. of Arizona)