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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)

The bowl of windstuff

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A Piece of Mars: Get out your red and cyan glasses to see an old crater, which fills this 0.775×0.7 km (0.48×0.43 mi) scene. The crater punched through many thin layers when it formed, some of which you can still see in around the rim. The crater is filled with many small dunes called transverse aeolian ridges (TARs), given this laborious and generic name because they aren’t quite like dunes we find on Earth and we don’t yet understand what they are. The TARs are common in this area, but there are even more here, where sand is swept into and then trapped inside this deep bowl. (HiRISE PSP_008735_1700_PSP_007878_1700, NASA/JPL/Univ. of Arizona)

A change of fluids

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A Piece of Mars: Water carved this ~800 m (0.5 mi) wide channel billions of years ago. The water dried up, and since then it’s been sand that flows through here (from the right), building up lovely dunes. A single crater on one of the dunes indicates that they’re not very active (dunes of this type on Mars all seem to be inactive, unlike their bigger, darker cousins). Look closely between the dunes and you might see a few little dots – these are boulders that have fallen, weathered out from the channel walls. (HiRISE ESP_022693_1530, NASA/JPL/Univ. of Arizona)

Two directions

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A Piece of Mars: Sometimes I just want to show the interior of a dune field, because it’s full of waves: ripples and dune crests, slip faces, all of which signs of movement. The dunes in this 0.67×0.47 km (0.41×0.29 mi) view have been made by two winds: one blowing from the top of the frame, and a more-recently-active one blowing from the right. Together, these two winds (and gravity) push this sand between a series of hills and down into Coprates Chasma, one of the longest canyons on Mars. (HiRISE ESP_035278_1655, NASA/JPL/Univ. of Arizona)

Where on Mars is this dune?

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A Piece of Mars: This 0.48×0.27 km (0.3×0.17 mi) scene shows a rotund barchan dune. Can you tell from looking at it where on Mars it might be? To me the most obvious feature are the bumpy piles at the bottom of the slip face (at the foot of the dune on the right). They’re probably the remains of avalanches that occurred when there was still winter frost on the dunes. This is a summertime image, so the frost is long gone and the wind is reworking the dune, trying to erase signs of the cold season avalanches. This sort of pattern is best seen in dunes near the north pole. (HiRISE ESP_027674_2650, NASA/JPL/Univ. of Arizona)

A big rock in a big air stream

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A Piece of Mars: Sand pours in from the top of this 1.95×1.95 km (1.21×1.21 mi) scene. The sand piles up and up (here ~115 m or 377 ft high), but ahead (at the bottom) is a mountain poking up. Like water diverting around a rock in a stream, the mountain affects the air flow just upwind of it, causing the sand to move around it. The steep dune slope is a slip face, caused by oversteepened sand avalanching. If you look closely, you’ll see some of those narrow avalanches near the bottom of the slip face (those at the top have been covered by ripples and falling sand). (HiRISE ESP_049045_1760, NASA/JPL/Univ. of Arizona)

More Earth-like views of Mars

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A Piece of Mars: In a recent post (Dunes in a Colorful Hole), I showed some dunes crawling over layered terrain, with a view that looked a lot like some desert regions of Earth. Here’s another spot on Mars (0.95×1.1 km, 0.59×0.68 mi) showing yet more beautiful layers with dunes filling up the valleys. Part of what makes it seem Earth-like is the lack of craters, although if you go looking you’ll see there are some there. It’s hard to tell from here, but this whole scene is inside an old fluvial channel. The layers are thought to be lake deposits from when the river dammed up, ages ago. Since then the wind has taken over, taking apart the layers one grain at a time, and then building up dunes with some of those grains. (HiRISE PSP_010329_1525, NASA/JPL/Univ. of Arizona)

Windblown or not? Probably…

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A Piece of Mars: This 0.95×0.95 km (0.59×0.59 mi) scene shows an eroding surface punctured by some old craters. Long, thin lines seem to form in the wake of many brighter knobs. Are those thin lines windblown in origin? They look like erosional features – things that are left behind when other stuff erodes away around it (not like sand dunes, which are things that pile up over time). If so, they don’t look like typical yardangs, which are streamlined bedrock, formed as sand wears down the rock. But this isn’t typical bedrock – it is easily erodible material. The bright knobs and crater rims are what’s left of a once-higher surface. The darker material may be a lag deposit that has built up as that brighter layer eroded down, leaving behind coarser grains that the wind has a harder time transporting (a similar process has occurred in Meridiani Planum, where the Opportunity rover drove through many kilometers of ripples, which now help protect the surface from erosion). If so, these long thin lines are a very unusual sort of yardang. (HiRISE ESP_016843_1590, NASA/JPL/Univ. of Arizona)