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Wavy dunes and straight dunes

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A Piece of Mars: The dunes here are ~40 m (131 ft) apart and ~200 m (219 yd) long. (They’re not really dunes, but rather a windblown thing nearly unique to Mars that we call TARs.) Look carefully and you’ll see that some have very straight crests, like a sword – this is typical for TARs. But others have wavy crestlines, like huge serrated knives.

Why are some wavy while others are straight? My guess is that the straight-crested TARs formed first. TARs are known for being immobile. The wind forms them, and then they just stop moving, unlike dunes and ripples, which can migrate long distances. At some point after they formed, the wind direction shifted, maybe as the climate in this region changed. The TARs had become somewhat resistant to erosion by that point. They weren’t as hard as rocks, but they’d probably developed a crust that made it hard for this new wind to rework their sediment.

But the wind, like water, is relentless, and it worried away at the TARs. Eventually the crust on some of them gave way, maybe because it was less protected by local topography, or maybe because it just didn’t develop as strongly as those on neighboring TARs, or maybe because the new wind blew more sediment over some TARs and less over others. And so some of the TARs were partly reworked by this new wind, forming tiny little new TARs on the left sides of the older TARs, which led to the wavy crestlines. So today we see the history of two different winds, recorded in the waviness of TAR crestlines. (HiRISE ESP_051995_1525, NASA/JPL/Univ. of Arizona)

The crazy laciness of martian TARs

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A Piece of Mars: Take a look at the windblown stuff in this 0.55×0.625 km (0.34×0.39 mi) scene. Those are intricate patterns of a sort of dune-ripple thing that forms all over on Mars, but not so much on Earth. We call them TARs (transverse aeolian ridges, here are some other examples) because we’re still not sure what they are: dunes or ripples or something else? They’re beautiful, they reflect wind patterns in ways we don’t yet understand, and they might make up a large part of the martian sedimentary rock record. Be glad it’s not your job to try to tease all that out, these things are complex. (HiRISE ESP_051129_1705, NASA/JPL/Univ. of Arizona)

Dust trapped on the lee side

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A Piece of Mars: This 0.95×1 km (.59x.62 mi) scene shows the center of a small dune field. The dunes are shaped by three winds blowing from three different directions: from the west-southwest, east, and south. The north-facing slopes are slip faces made by the south wind, and most of them have bright patches on them that are probably accumulations of airfall dust. Whatever winds brought the dust, none have yet been able to remove it. I’d bet that one of the most recent winds to pick up sand on these dunes blew from the south, because those bright dust patches are still visible on those north-facing slopes, where they’d be protected from southerly winds. (HiRISE ESP_049481_1310, NASA/JPL/Univ. of Arizona).

Leeward and poleward

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A Piece of Mars: The sharp line in this 0.625×0.625 km (0.39×0.39 mi) scene is the crest of a long dune in Mars’ southern hemisphere. The sunlit side is also the lee side: the bright streaks are thin sand avalanches (grainflows) that formed when the wind blew too much sand over the crest from the other side. The dark side is completely different. It’s the side facing toward the south pole, and it’s covered in ripples and erosional gullies that are thought to form when winter ice blocks roll down the darker slopes. (HiRISE ESP_024304_1345, NASA/JPL/Univ. of Arizona).

Endless wind

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A Piece of Mars: This 2.88×1.13 km (1.79×0.70 mi) scene shows quintessential Mars, with a 670 m diameter impact crater heavily modified by wind erosion. Both the crater floor and the surrounding terrain are covered by what is likely loosely-cemented dust. The texture is that of wind-eroded materials, but to make this texture that material must be fine-grained and uniform in cementation (except where punctuated by craters that are, in turn, also wind-eroded). I’ve never seen a texture like that on Earth. Check out the whole HiRISE image to see how extensive that texture is (and note that I’ve only shown it at half-scale here!) – it’s the dominant feature of this landscape for many hundreds of kilometers. This is in Daedalia Planum, high terrain just southwest of the Tharsis Montes, where equatorial easterly winds might be enhanced by nighttime downslope winds coming down Arsia Mons, the southernmost of the three volcanos (HiRISE ESP_017651_1670, NASA/JPL/Univ. of Arizona)

Wind and maybe water too

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A Piece of Mars: Along the right side of this 0.5×0.5 km (0.31×0.31 mi) scene is the rim of a crater – the stripes are layers exposed (and then perhaps draped by falling ejecta) as the crater formed. To the left is the crater’s interior wall, dropping downward. Deep gullies have been eroded into the crater walls, probably by water, carrying sediment downslope. Rivers and landslides are generally great sources of sand-sized sediment, and this place is no exception. The sediment piled up downslope, and then the wind came along and sculpted it into beautiful cross-hatched patterns (click on the image to see full resolution). (HiRISE ESP_015984_1335, NASA/JPL/Univ. of Arizona)

Exhumed dunes!

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A Piece of Mars: The large dunes in the middle of this 375×450 m (0.23×0.28 mi) scene run along a valley (the small dunes at top and bottom are on high ground). What’s amazing about this is that the ends of the large dunes extend into the valley walls. That is, they’re covered by the stuff in the valley walls. Usually dunes sit on top of all the other geologic structures, but not here. These dunes formed a long time ago. And then a lot of sediment piled on top of them – but without destroying them (which is what usually happens on Earth, so we don’t see this sort of thing here). And then those sediments were later eroded to make the 0.5 km wide valley, revealing the buried dunes. Look at all this geology we can do from space! (HiRISE ESP_018347_1660, NASA/JPL/Univ. of Arizona)

Mars’ giant bubble wrap

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A Piece of Mars: This 0.7×0.5 km (0.43x.31 mi) scene shows Mars’ giant yellow bubble wrap, with each “bubble” about 100 m across (seriously, don’t you want to pop them?). These are actually a type of dune called a “dome dune”, and they’re about as small as this type of martian dune can get. Dome dunes form where the wind blows from one main wind direction, but shifts a bit in direction (we call it a “wide unimodal distribution”). These are near the north pole, and at this time of year (early northern spring), they’re still covered in winter frost, with a light powdering of dust to make them yellow. You can see spots where the underlying dark sand is just beginning to show through as the sun sublimates the ice. (HiRISE, ESP_050886_2565, JPL/NASA/Univ. of Arizona).

Cross-strata or not?

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A Piece of Mars: Sand dunes are one of the few sedimentary phenomena that leave behind layers that aren’t horizontal. They tend to have a characteristic lean to them (and we call them cross-strata). So when I see something that looks like tilted layers on Mars, I take notice. This 0.625×0.5 km (0.39×0.31 mi) scene shows a steep slope, the side of a narrow graben system called Sirenum Fossae. The cliff starts at the top where overhanging rocks make shadows, and it ends at the bottom where there are small dunes. Along the slope are many narrow gullies from where sediment has slid downslope. And if you look carefully (click to see the whole image), you’ll see small diagonal lines aligned from upper-right to lower-left.

So are those diagnoal lines the strata produced by ancient dunes? Probably not. I think not, mostly because you can still see those diagonal lines in the gully aprons near the bottom of the slope – and those gullies were made by stuff sliding down this steep graben slope, not dunes. Also, there are a few boulders on the slope that might have wind-tails behind them. If that’s what they are, then these diagonal lines in the graben wall were made by a wind blowing diagonally up the slope, scouring away material as it went.

So, probably not dunes. But still aeolian. And very cool.

(HiRISE ESP_050882_1430, NASA/JPL/Univ of Arizona)

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)