Cosmic Diary by Lori Fenton

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Updated: 2 days 22 hours ago

Dust trapped on the lee side

September 17, 2017


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

September 05, 2017


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

August 28, 2017


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

August 14, 2017


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!

August 07, 2017


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

July 31, 2017


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?

July 25, 2017


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

July 17, 2017


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)

Experimenting with 3D views

July 13, 2017


A Piece of Mars: I often use JMARS to visualize Mars data sets, especially images. They’ve recently updated their 3D layer, allowing folks to make lovely vistas by overlaying DTMs with images. I’m new at this, but I’ll experiment and see what I can do to make nice views. Here’s a series of barchan dunes marching away from a tall stack of layers in Becquerel crater, with no vertical exaggeration. (HiRISE, DTEEC_045140_2015_044784_2015, NASA/JPL/Univ. of Arizona)

Reversing slip faces

July 10, 2017


A Piece of Mars: This 523×750 m (0.32×0.47 mi) scene shows a large dune. It’s quite colorful for some reason, although it’s partially false-color. What caught my eye is that the slip face on this dune has reversed direction, which is somewhat rare on Mars (but common on Earth). The main sand-moving wind blows from the right, forming a long avalanching slope (you can see long bright lines of grain fall slips at the lower center). But at some point a wind blew from the left, forming a small slip face in the opposite direction. Although many other wind directions have also help to build this dune, those two are the main winds apparent here. (HiRISE ESP_050887_2225, NASA/JPL/Univ. of Arizona)

The real tetrahedrons of Mars

June 19, 2017


A Piece of Mars: The real tetrahedrons of Mars are dunes, built by winds blowing sand from more than one direction. This 0.5×0.5 km (0.31×0.31 mi) area shows a dune formed from two winds that are about 90 degrees apart: one blowing from the bottom and one blowing from the right. This makes the dune have two slip faces, which is a rare occurrence on Earth dunes. (Earth dunes are complicated by superposed secondary dunes that interfere with and obscure this pattern. Or if they’re small enough to not have those secondary dunes, then they are changing fast enough that one slip face will quickly erase the other. I’ve only ever seen two slip faces at once for very short periods in Earth dunes – they don’t last.) Here, the two winds have worked together to form a little spit of sand off to the upper left. The result is a 3-sided “pyramid”, with no advanced civilization required for its construction. (HiRISE ESP_050479_1360, NASA/JPL/Univ. of Arizona).

The thinnest landslides

June 15, 2017


A Piece of Mars: In the dustiest regions of Mars, steep slopes occasionally produce very thin avalanches of dust, revealing a darker surface under the top layer of dust. This shows one that is 610 m (0.38 mi) long, running from its tiny point of initiation near the top of the slope down to the bottom of the slope where accumulated landslides have slowly buried old windblown dunes (or TARs). These landslides occur every spring, and may be triggered by sublimation of small accumulations of winter ice, or perhaps by the wind. This one formed some time between May 7, 2012 and May 22, 2013, as it appeared between two successive images of this spot. It’s still there today, most recently imaged on May 5, 2017, slowly accumulating dust until it fades into the background with the rest of the slope. (HiRISE ESP_035307_2115, NASA/JPL/Univ. of Arizona)

Dunes fighting for survival

June 05, 2017


A Piece of Mars: Having a bad day? You’re in good company with these dunes in this 0.96×0.48 km (0.6×0.3 mi) scene. The gray barchanoid dunes are covered in ripples, as the wind valiantly tries push the sand to the dune crests. But they are besieged by other processes at work. Dark scribbles show how dust devils have swept by, removing dust and probably scattering a little bit of the sand. The steep slip faces are not covered in dry avalanches typical of active dunes, but rather they appear eroded, as if some force locked the dune in place and started eroding the surface wherever ripples couldn’t rescue it. Splotches on the tan ground between the dunes, and narrow furrows attest to seasonal ice reworking the surface. And in this great battle, what I wonder is: can those dunes have formed like this amidst such turmoil, or are they relics of an older, windier, perhaps less icy age? (HiRISE ESP_050488_1150, NASA/JPL/Univ. of Arizona)