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The smallest dunes

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A Piece of Mars: There are two small dome-shaped dunes in this frame (0.96×0.54 or 0.6×0.33 mi). If they got any larger, they’d form slip faces. Any smaller and they’d just be random drifts of windblown sand. Dunes form at a particular size (~125 m in this case) related to the distance it takes for sand grains to accelerate to the background wind speed. This distance is bigger on Mars than on Earth, where the smallest dunes are ~20 m across. (HiRISE ESP_044198_1480 NASA/JPL/Univ. of Arizona)

Remnants of erosion

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A Piece of Mars: The gray area in the center of the 480×270 m (0.3×0.17 mi) area is an erosional remnant: once, more of this area was covered by the gray stuff, but some of it has eroded away (most likely by the wind) to reveal the underlying terrain below. The wind probably blew from upper left to lower right, lifting away the finer grains and leaving behind the larger, heavier ones. Some of the larger grains have formed into ripples, that in some places may be the only sign that the overlying layer was ever there. (HiRISE ESP_043136_2020, NASA/JPL/Univ. of Arizona)

Springtime for sand dunes and polar ice

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A Piece of Mars: This 480×270 m (0.3×0.17 mi) scene shows a dark dune peeking out from under its cover of winter frost. In this picture it’s late spring, but still cold up at this latitude – it’s as far north here as the Greenland town of Qaanaaq (pop. 656 as of 2013). The CO2 frost here lingers on shady slopes until summer, preventing the dunes from migrating until it’s gone. (HiRISE ESP_043799_2570 NASA/JPL/Univ. of Arizona)

That which curves and that which is straight

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A Piece of Mars: The long meandering lines snaking across the image (3.2×1.8 km or 2×1.1 mi across) are inverted channels. They are river deposits that once were the lowest part of the landscape (rivers always are), but then the water dried up and wind erosion took over. The river channels were more resistant to erosion and so now they stand above the rest of the terrain. The wind left behind straight, streamlined hills called yardangs. Given enough time, the wind will scrape at the surface until both the yardangs and the river channels are gone, but for now there’s a beautiful landscape. (HiRISE PSP_002424_1765 NASA/JPL/Univ. of Arizona)

Curiosity about sand dunes (part 2/2)

Today is December 21, 2015 (northern winter and southern summer solstice on Earth). On Mars it is Ls = 84º, Mars Year 33 (about 12 sols from northern summer and southern winter solstice on Mars). It is sol 1200 of Curiosity’s mission on Mars, and the rover is working its way around the southern side of Namib Dune. Part 1 of my previous post shows part of the windward (northeastern) side of High Dune. This time I’ll show pictures of the slip face of Namib Dune.

The dunes in this part of the Bagnold Dune Field are slowly marching towards the southwest. Wind blows from the north-northeast and the sand piles up, only to oversteepen on the lee side and form avalanches in what we call a slip face. This happens over and over as the dune moves: saltating sand flies over the dune crest and settles on the upper slip face (what we call grainfall). When enough sand piles up, it oversteepends, and eventually there’s a slope failure: stuff higher up moves down, like a little landslide (what we call grainflow). We see grainflows most commonly when the wind is blowing nearly directly across the crest of the dune.

But when is nature ever so uniform? Sometimes people ask me how strong the wind blows on Mars, as if I could just give them a single value that would apply to all of Mars (its mountains, polar caps, steep crater rims, and flat plains) at all times (its CO2-covered winters, convectively-turbulent summer days, regions prone to seasonal dust storms, and nighttime low-lying flows). Go outside on Earth for a moment and tell me if you can feel the wind moving from different directions, at different speeds, and do it again in 12 hours and again in 6 months. In most places you won’t get the same result, and it is the same on Mars.

You don’t get the same winds blowing here in the Bagnold Dune Field either, even though the dunes are telling us that the strong winds mostly blow from the north-northeast. What can the slip face tell us about that? Let’s have a look:

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What an awesome view! Here’s another view of the whole slip face:

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It’s totally different from the other side of the dune, which is covered in ~2 m sized ripples that are themselves covered in smaller ripples. Instead, this looks like a giant wall of sand, textured with features that all look like they’ve moved downhill. The smoother surfaces are probably the newest: these are fresh grainflows. But some of the slip face is covered in small ripples. Here’s what I mean:

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What you’re seeing is the interplay of at least two different winds. There’s the main wind that blows over the crest, which forms grainflows, and then a secondary wind that blows in a different direction, forming ripples out of sand from the grainflows. Based on the orientations of other ripples, that secondary wind probably blows from the northwest, which is roughly along the slope of the slip face. That northwesterly wind is responsible for making the large ripples at the base of the slip face – when that wind blows, those large ripples would march towards the camera.

It’s pretty typical for two to three wind directions to dominate all other winds in a region, at least in terms of sand transport. On Earth this usually comes down to seasonal changes in weather patterns: winter storms vs. steady summer winds. Perhaps where you live, most of the weather arrives from one direction, but occasional storms may blow in from another direction. Those winds that blow strongest are most likely to move the most sand. This appears to be the case on Mars as well.

It looks to me like the most recent wind activity has formed grainflows, suggesting that the NNE wind has been more recently active than the ripple-forming wind. However, most of the slip face is covered in small ripples, suggesting that this NW wind was, until recently, the prevailing wind. The ripples weren’t able to fully rewrite the topography of the slip face, as you can see they cover slightly larger undulations that were probably older grain flows – this supports the idea that they are formed by a secondary wind that cannot move enough sand to rewrite the entire dune (if it did then the slipface would point towards the southeast, instead of towards the southwest as it does now). We’re probably seeing a seasonal tradeoff between the NNE and NW winds. I might even cautiously suggest that the grainflow-forming NNE wind is active in the current season (local autumn) and that the ripple-forming NW wind blew in a previous season (perhaps local spring or summer). I’d love to get a shout-out from the REMS folks, if they can pull out any new wind data from their partially-broken anemometer.

Happy solstice everybody, and I hope you have a good holiday.

Curiosity about sand dunes (part 1/2)

Sorry for the pun in the title there, but NASA asked for it by naming their rover like that. And you’ve seen it done a hundred times, so let’s grit our teeth, smile, and carry on.

Anyway.

So I’m more excited now about a space mission than I have been in a long time. A Mars rover is finally visiting sand dunes, after so many years of peering at them from orbit and seeing them in rover images in the far distance. They took their time getting there, but now it’s there. Taking images of the dunes, and presumably other data as well. Here’s what it looks like from above:

Map credit: NASA/JPL/CalTech/MRO/UofA/HiRISE/Processed by Phil Stooke
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Today, December 14, 2015, is Sol 1193 for Curiosity. On Sol 1176, Curiosity took a bunch of color images looking up at “High Dune”. Here’s what they look like all mosaicked together:

Image credit: NASA/JPL-Caltech/MSSS
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Looking from the bottom up, you are panning up the windward side of the dune, looking at the lovely ripples that cover the entire dune. You can see the biggest ripples in the first image from space. Using those images, my colleague Simone Silvestro and others (including me, yay) measured that they move downwind (towards the left in the above image) at a rate of 0.66 meters (26 inches) every Earth year. Here’s a 3-frame movie from that paper, showing how the dunes and ripples move over the course of 5 years (which is ~2.5 Mars years) in 2006, 2008, and again in 2011. Note that this isn’t the same dune that Curiosity imaged above.

Image credit: Silvestro, S., D. A. Vaz, R. C. Ewing, A. P. Rossi, L. K. Fenton, T. I. Michaels, J. Flahaut, and P. E. Geissler (2013), Pervasive aeolian activity along rover Curiosity’s traverse in Gale Crater, Mars, Geology, 41(4), 483–486, doi:10.1130/G34162.1.
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So yes, these dunes are moving! We might even see changes as Curiosity takes more images. We might see sand grains that have bounced onto the deck of the rover. We might even be able to learn something about how the wind moves sand on Mars, how that process differs from that on Earth, and how strong the wind blows. Cool, eh? Check back next week for more pictures of these dunes. I’ll be showing some closeups of the sand and speculating on what it’s made of and why it’s different from windblown sand on Earth.

Too steep for ripples

ESP_043085_1670_1.0x A piece of Mars: This 480×270 m (0.3×0.17 mi) area is a steep slope that plunges down to the upper left. A pile of dark sand, covered by brighter tan dust, clings to the hillside. Usually the martian wind blows sand into ripples, and you can see where it’s tried to do that here. But the steep slope triggers thin dark avalanches of dark sand that compete with the wind in shaping the sandy surface. (HiRISE ESP_043085_1670 NASA/JPL/Univ. of Arizona)

Windy windows

ESP_043086_1715_1.0x A Piece of Mars: This 0.96×0.54 km (0.6×0.33 mi) area shows ripples forming on a layer of dark gray material. In a few spots, the gray layer has been eroded away (probably by wind scour), revealing the lighter, tan-colored terrain below. Geologists call these exposures windows, because you can see through one layer to another that’s underneath. (HiRISE ESP_043086_1715 JPL/NASA/Univ. of Arizona)

Crochet ripples

ESP_042360_1755_1.0x A Piece of Mars: This 480×270 m (1575×886 ft) area shows a seemingly endless field of ripples. They’re big, about 50 m (164 ft) from crest to crest, and probably about 5 m (16 ft) high. Is there a knit or crochet pattern out there that looks like this? You could market it to some Mars aeolian scientists… (HiRISE ESP_042360_1755, NASA/JPL/Univ. of Arizona)

Very long ripples

ESP_043098_1650_1.0x A Piece of Mars: Most of the scene (0.96×0.54 km or 0.6×0.34 mi) is one long slope of a dune. The crest is the line in the top right; the ground below is in the bottom left. If you ever walk along a dune or beach, you’ll see small ripples that can reach up to about a meter in length. But the ripples on this dune extend from the crest to the ground – they’re more than half a kilometer (more than a third of a mile) long! (HiRISE ESP_043098_1650 NASA/JPL/Univ. of Arizona)