Cosmic Diary by Lori Fenton

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Dunes with comet tails

January 08, 2018


A Piece of Mars: The north polar dunes in this 575×325 m (0.36×0.2 mi) scene are made of dark sand covered by bright winter frost (which will soon sublimate away, as this image was taken in late spring). To the right of the dunes extend pale yellow bumpy hills, making the dunes look like they have little “comet tails”. What’s going on here? These dunes are migrating towards the left, so the tails are what they leave behind. The dunes are located very far north, where the ground is always frozen. Ice freezes the lowest parts of the dunes, so that as the upper sections can be pushed downwind, the lower sections remain locked in place behind. This can happen on Earth too, but here it’s usually the water table stabilizing the lower portions of the dunes (and many of the thick continental sandstones, like the Navajo sandstone, were built this way millions of years ago). (HiRISE ESP_052773_2650, NASA/JPL/Univ. of Arizona)

Fuzzy dunes

January 02, 2018


A Piece of Mars: The dunes (or maybe they’re ripples) in this valley appear to be fuzzy (the view is 625×775 m, 0.39×0.48 mi). They’re not really fuzzy, but it’s not actually clear what’s going on. They seem to have smaller ripples superposed on them, and maybe bright dust has settled into the troughs between ripple peaks, so that they take on a striped, feathered look. It’s unlike anything I’ve seen on Earth. (HiRISE ESP_052776_1785, NASA/JPL/Univ. of Arizona)

Buried by ejecta

December 26, 2017


A Piece of Mars: To see this one well you’ll have to click on the image. At the lower right, a 240 m (787 ft) diameter crater formed when a bolide hit the surface, throwing out ejecta on the surrounding terrain. Zooming in, you can see that the ejecta has a distinctive rough surface. Farther from the crater there are smooth patches where ejecta didn’t fall. What I like about this is the many small bedforms (ripples), some of which are covered by ejecta and some of which aren’t. Closer to the crater, you don’t see so many of these bedforms because the ejecta is thick enough to have buried them. The crater itself is fairly “young” for a geologic feature on Mars, but enough time has passed to allow small bedforms to accumulate inside it (ripples like these don’t form overnight). (HiRISE ESP_052794_1545, NASA/JPL/Univ. of Arizona)

The mysterious bright streaks

December 18, 2017


A Piece of Mars: Some things just go unexplained (so far, anyway). Here’s a mysterious bright streak (scene is 1.2×1.8 km, 0.75×1.12 mi) concentrated between two sets of ripple-like bedforms. It looks sort of like a river, but it’s on flat terrain and it’s not water. It’s part of a larger set of bright streaks that you can see throughout the top of this broader CTX image (the bright streak shown in detail here is visible as a distinct white stripe on the floor of a crater). My guess is that at some point, probably at least several million years ago, a bunch of fine bright silt was blown towards the north-northwest, settling in a few places. I’m guessing it’s silt (finer-grained than sand) because it didn’t form dunes, and it’s not as fine as the clay-sized dust that blows into dust storms because this stuff wasn’t completely blown away (dust is easily mobilized by saltating sand). (HiRISE ESP_052786_1740, NASA/JPL/Univ. of Arizona).

Dunes in a row

December 11, 2017


A Piece of Mars: Look at the alignment of the ~100 m dunes in this 713×750 m (0.44×0.47 mi) scene. How do dunes form in such straight lines? And why don’t they always do that? It’s likely that these dunes were once long ridges stretching from the lower right to upper left. The shape of the slip faces suggests they’re formed from two winds that blow from similar directions, both of which push sand toward the upper left. To stay stable, this sort of dune needs a constant influx of sand from upwind (from the lower right), but if that flow of incoming sand lessened, then a long dune would be forced to break up into a series of smaller dunes. That may be what’s happened here. Check out the whole image, and you’ll see other long ridges that haven’t yet broke up into smaller dunes. (HiRISE ESP_052798_2565, NASA/JPL/Univ. of Arizona)

Ripples of rock

December 04, 2017


A Piece of Mars: To the upper right of this 0.85×0.6 km (0.53×0.37 mi) scene is a flat-lying plain strewn with large ripples. To the lower left is a rugged hill with gray rock laced with white veins (this might be part of an impact megabreccia identified nearby in Holden crater). Notice that some of the ripples on the rugged hill are also veined – this is evidence that they are actually eroded into the bedrock, rather than fine-grained deposits like their counterparts on the plain. It’s not yet clear how these “Periodic Bedrock Ridges” form, and they may be unique to Mars. (HiRISE ESP_052789_1520, NASA/JPL/Univ. of Arizona)

The corpse of a dune

November 27, 2017


A Piece of Mars: The rippled darker patch in this 600×600 m (0.37×0.37 mi) scene is the former site of a sand dune. This is one of a few “dune corpses” found just upwind of a dune field in Holden crater. The dunes are migrating to the south and east – you can see that the arc of this former dune opens to the south, the way a barchan slip face would. This dune is what’s left behind after most of its sand has migrated downwind. (HiRISE ESP_052367_1540, NASA/JPL/Univ. of Arizona)

Is it windblown or not (#2)?

November 20, 2017


A Piece of Mars: This 0.93×1.25 km (0.57×0.78 mi) scene shows what I’m starting to think are windblown features. I posted something similar to this once before, from a location not that far from here. In this one region of Mars there are parallel lines cut into the tops of hills. A geologist would first presume they were exposed, tilted layers. But the regularity of their spacing (especially when you zoom in) is a bit unusual, and suggests some sort of self-organization (like windblown ripples). And then the questions begin: why just in this spot on Mars? what’s unusual about the rocks (or the wind) here? I still have no good answers. (HiRISE ESP_052386_1565 NASA/JPL/Univ. of Arizona)

Overhang

November 15, 2017


A Piece of Mars: There’s a fabric of erosion in this 1×1 km (0.62×0.62 mi) scene, with the main wind blowing from lower right to upper left (and if you look carefully you’ll see there’s a second, subtler fabric a bit clockwise from that one). The result is a landscape strewn with streamlined rock called yardangs. The darkest areas are shadows from rock faces scoured by the wind so deeply that they’ve been undermined until there’s overhang. Normally this would lead to collapse features, like rock piles, but you don’t see those here. That’s an indication that the rock here is easily eroded and fine-grained, so that as it’s eroded, it’s simply carried off by the wind. (HiRISE ESP_052384_1800, NASA/JPL/Univ. of Arizona)

Island in the stream

November 08, 2017


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)

Black and tan

November 06, 2017


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)

Mars’ corduory

October 30, 2017


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)

Just do it

October 23, 2017


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)

Wavy dunes and straight dunes

October 09, 2017


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)