A Piece of Mars: In the floor of what might have been an old fluvial channel there are a bunch of really neat dunes (or maybe ripples, they’re TARs and we don’t know yet what they are). One spire pokes up here, ~200 m (656 ft) across and ~90 m (295 ft) tall. The TARs reveal the wind direction here, as wind flowed from top to bottom around the spire, converging on the lee side. (HiRISE ESP_026557_1525, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Dunes in the top row in this 0.73×0.47 km (0.46×0.29 mi) scene are dark but those in the lower row are brighter. Why? They’re all probably made out of the same kind of sand, which is dark. And they all probably got covered by fine-grained airfall dust, which is bright. At some point after that, a wind blew, probably from top to bottom of the view, and moved enough sand to kick off the fine bright dust. But the relief from those top dunes took energy from the wind, so that by the time it reached the lower row, it wasn’t strong enough to move sand anymore. So until the next windstorm, we see two different colors of dunes. (HiRISE ESP_052399_1885, NASA/JPL/Univ. of Arizona)
A Piece of Mars: The wind on Mars likes to make textiles (unfortunately the term geotextiles is already taken for other purposes). This 1×0.6 km (0.62×0.37 mi) scene shows two different sets of ripples. The larger set has straight to wavy crests, and they’re ~18 m (~59 ft) apart, which is pretty big for ripples (really they’re TARs). Inbetween those (click on the picture so you can see them) are small ~2 m (~6.5 ft) ripples that make Mars look like it’s made of kahki corduroy (which is a thing but it’s not on trend, so Mars could stand to catch up a little). What does this all add up to? There are at least two different sets of wind directions, and each probably formed on its own timescale. If we learn how to decipher these, then we could better understand weather patterns on Mars, because ripples like these are pretty common there. (HiRISE ESP_051244_1315, NASA/JPL/Univ. of Arizona)
A Piece of Mars: It’s all about wind scour here in this 0.75×0.75 km (0.47×0.47 mi) view. The big “swoop” is an erosional channel dug into the surface by winds (blowing from the lower left) trying to erode the hills in the center. But notice that the hills are all aligned to the upper left/lower right, like a school of fish swimming the same way). That alignment tells us there’s a second wind that came along later, blowing (I think) from the lower right. That wind also left behind some ripples (TARs, really) that swirled around the older big “swoop” channel. (HiRISE ESP_016372_1975, NASA/JPL, Univ. of Arizona)
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)
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)
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).
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).
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)
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)