Cosmic Diary by Lori Fenton

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Dune cannibals II

May 23, 2016


A Piece of Mars: This 0.96×0.54 km (0.60×0.34 mi) scene shows two sets of bedforms (dunes), each aligned in different directions. The more closely-spaced set has sharper crests, and it’s superposed on top of (and it is therefore younger than) the more widely-spaced set. Like a previous post I wrote, the younger set has cannibalized sediment from the older set (although in aeolian geology we say it has “reworked” the sediment). If you click on the image, you might be able to convince yourself that some internal bedding from the older set is being exposed by erosion, but it’s hard to tell for sure at this resolution (maybe we could tell if we had a full resolution HiRISE image to work with here – hmm, maybe I’ll go request one). (HiRISE ESP_045299_1545 NASA/JPL/Univ. of Arizona)

Wind shadow

May 16, 2016


A Piece of Mars: There’s a dune field migrating past a 230 m (755 ft) diameter crater, creating a 1.6 km (1 mi) long “shadow” that’s empty of dunes. Why? The rim of the crater pokes up just enough to affect the wind, like pebbles in a stream. Either the sand is diverted around the crater, or the rim produces turbulence that increases erosion (or possibly both at different times). I like the dunes that are disrupted as they migrate into the crater. (HiRISE ESP_037948_1645, NASA/JPL/Univ. of Arizona)

Craters and wind

May 09, 2016


A Piece of Mars: This 90 m (295 ft) crater impacted into a windy, cratered plain. It’s now partly filled with dark sand, but where did that sand come from? Looking closely you’ll see that many of the boulders that were flung out during the impact have little “tails”. These tails show that wind from the upper right blows sediment toward the lower left: some of it gets trapped behind the boulders (and other topographic projections), and some of it is the dark sand that got trapped inside the crater. (HiRISE ESP_045397_1885, NASA/JPL/Univ. of Arizona)

Giant “combs” on Mars

May 02, 2016


A Piece of Mars: This 480×270 m (0.3×0.17 mi) scene shows a herd of 100-300 m fine-toothed combs grazing on the surface of Mars. Wait, what? No, it’s not really combs. This is actually a landscape covered by two sets of windblown bedforms. The larger ones (the “comb” shafts) are very old, now inactive windblown features. The smaller ones (the “comb” teeth) are ~2 m apart, and they extend downwind (eastward) from the older bedforms, which effectively serve as filters that block winds from the west (left to right), allowing only the northerly or southerly components of most winds to shape the ripples on their lee sides. Beyond the influence of the larger bedforms, the small ripples merge with those on the surrounding sand sheet, which show the influence of several different winds (HiRISE ESP_045166_1690, NASA/JPL/Univ. of Arizona)

Old ripples

April 25, 2016


A Piece of Mars: In this 480×270 m scene (0.3×0.17 mi), there are a bunch of “ripples” spaced by 5-20 m (the quotes are because we don’t know yet if these are ripples, dunes, or some other new kind of bedform). They’re old: they’re eroded by winds blowing from the bottom to the top of the frame (exposing layers on the upwind side), and if you look carefully you’ll see some craters superposed on them. The craters don’t have any obvious ejecta blankets, which suggests they’re not that young either, so there’s been enough time for the ejecta to erode away. (HiRISE ESP_017766_1535, NASA/JPL/Univ. of Arizona)

Stripes by wind and gravity

April 19, 2016


A Piece of Mars: This scene (800×450 m or 0.5×0.28 mi) is a steep slope, with high rocky outcrops on the upper right and both gullies and ripples heading downslope to the lower left. The wider, brighter stripes are gullies that were carved by stuff eroding from the outcrops and falling downhill, just like on Earth. Beneath that are some finer stripes: this time the straight lines are made by a combination of wind blowing sand into ripples (from upper left to lower right) and gravity elongating the ripples downslope (stretching them from upper right to lower left). (HiRISE ESP_044997_1755 NASA/JPL/Univ. of Arizona)

Windy windows

April 11, 2016


A Piece of Mars: Here’s a tiny bit (0.69×0.39 km or 0.43×0.24 mi) of Jezero crater, one of the candidate landing sites for the Mars 2020 rover. On the bottom and left is high-standing volcanic terrain, former lava that flowed out on the crater floor. On the upper right is a much older deposit of stuff that piled up at the bottom of the lake that once, more than 3.5 billion years ago, filled the crater. Those lake deposits are so easy to erode that they’ve been worn down by the wind (see those bedforms there?) to the point that they’re now lower than the volcanic stuff. I wonder if they’ll eventually be completely covered by those ripples. (HiRISE ESP_037330_1990, NASA/JPL/Univ. of Arizona)

Reference:
Schon, S. C., J. W. Head, and C. I. Fassett (2012), An overfilled lacustrine system and progradational delta in Jezero crater, Mars: Implications for Noachian climate, Planet. Space Sci., 67(1), 28–45, doi:10.1016/j.pss.2012.02.003.

The wind paints

April 04, 2016


A Piece of Mars: For the last few billion years, the wind has (by far) moved more sediment around on Mars than any other geological process. Not tectonics, volcanism, fluvial activity, or impact cratering (although a case has been made for glacial activity). Here’s yet one more swipe at the ground, scouring off bright dust to reveal darker terrain underneath. (HiRISE ESP_044511_2005, NASA/JPL/Univ. of Arizona)

Three types of windblown piles of stuff

March 28, 2016


A Piece of Mars: The wind blows different sorts of sediment in different ways. Ultimately they pile up because some oddity in nature makes one spot accumulate more sediment than other spots, allowing that windblown pile of stuff to grow. Sometimes it’s because of the wind interacting with the shape of the pile, and sometimes it’s because of the trajectories of moving grains as the wind blows them along the ground. Here’s an example of three types adjacent to each other: 1) a big dune on the left (migrating towards the right), which is covered in 2) smaller ripples, and downwind of the big dune are 3) brighter intermediate-scale piles (that are surrounded by larger and, presumably, better-developed versions of the “smaller ripples”). (HiRISE ESP_044515_1620, NASA/JPL/Univ. of Arizona)

Dune shadows

March 16, 2016


A Piece of Mars: Normally I post in color, but sometimes you need to back out to the grayscale images to see the big awesome things. This scene is 4.6×2.6 km (2.8×1.6 mi); the conical hill is 1.4 km (0.89 mi) wide. Sand-laden wind from the right is blowing streamers of dunes around the hill, which leaves a wake that stretches downwind. Some of the luckier hills on Mars have lovely dunes scarves like this, slowly shifting over the centuries as the wind brings in more sand. (HiRISE ESP_044258_1715 NASA/JPL/Univ. of Arizona)

How far does the wind blow stuff?

March 07, 2016


A Piece of Mars: Hargrave crater has an amazing array of colorful surfaces, each of which reflects a different type of rock (this scene is 480×270 m or 0.3×0.17 mi). I like the ripples sitting on top of it all; I’ve long thought that much of the material in those ripples hasn’t moved very far from where it originated. Here’s a good example of why. The ripples on the greenish surface have incorporated some local greenish material. The same is true of the tan ripples in the lower left. I’d bet most of this stuff has only moved as far as it took to make the ripple it’s in. (HiRISE ESP_044161_2005 NASA/JPL/Univ. of Arizona)

Sizes of worlds

February 25, 2016


A Piece of the Solar System: This isn’t my usual sort of post. But lately my 6 year old kid has been into planets, and thanks to the many informative videos on YouTube, has been reciting various names and numbers about the many worlds in the Solar System. I decided to show him just how much smaller than the Earth some of those worlds are. Here’s what I made for him. It’s not exhaustive, but it gives a good idea of just how small Pluto is relative to, say, the Moon. The scale is ~2 km/pixel. (Images attributable to NASA or ESA).

It’s a rock-eat-rock world

February 20, 2016


A Piece of Mars: This 738 x 415 m (0.46 x 0.26 mi) scene shows dark sand flowing down a channel bisected by a ~60 m (~200 ft) tall, thin “island”. That island, and many others around it (see the whole image), is what remains after windblown sand slowly carved away the rest of the rock, the same way rivers slowly cut through rock on Earth. The presence of the dark sand shows that the process is still active today. (HiRISE ESP_04400_1750, NASA/JPL/Univ. of Arizona)

The smallest dunes

February 08, 2016


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)