Cosmic Diary by Lori Fenton

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Where we have been

July 18, 2016


A Piece of Mars: This 0.98×0.54 km (0.61×0.34 mi) scene shows ancient windblown bedforms (maybe dunes) that have been partially eroded by the wind. The wind has left behind ghostly stripes: these are remnants of where these things once were, back when they were still actively migrating. Some of the bedforms have been almost entirely erased, except for those remaining bits. Much smaller (2-3 m wavelength) ripples have since formed between some of them, probably made from material scavenged from the larger bedforms. (HiRISE ESP_037700_1710, NASA/JPL/Univ. of Arizona)

Landslides unlike any on Earth

July 05, 2016


A Piece of Mars: Click on this 0.96×0.54 km (0.6×0.33 mi) scene to see it in detail. Many thin, narrow landslides have formed on these dust-coated hills. As far as I’m aware, there’s nothing like this on Earth. Inside the landslide scars, there are small dusty ripples about 1.75 m (~6 ft) in wavelength, smaller than the ripples found on dark sandy dunes. These landslides are visible in images at least as far back as 2007, although they clearly formed after the small crater on the slope (which is slowly being filled with the dusty debris). (HiRISE ESP_045605_1715, NASA/JPL/Univ. of Arizona)

How to hide geology on Mars

June 27, 2016


A Piece of Mars: Three things are trying to hide in this 0.96×0.48 km (0.6×0.3 mi) scene. 1) Craters are slowly being both scoured and buried by migrating sand, 2) the sand itself is hiding in the lee of crater rims and other topographic obstructions to the wind, and 3) small patches of ice (blue in this stretch) are hiding on shady slopes (north is to the right in this southern hemisphere image, taken during southern winter). (HiRISE ESP_045792_1395 NASA/JPL/Univ. of Arizona)

New craters and wind

June 13, 2016


A Piece of Mars: The two small dark craters (2.25-2.4 m, or 7.4-7.8 ft across) are brand new, having appeared in CTX images sometime between May 2007 and April 2008. They punched through a layer of bright dust and threw up some darker material, which the wind carried downwind (near-surface winds blowing from the southwest, and higher winds blowing from the southeast). Application of an atmospheric model could further constrain the season and time of day when the impact happened, based on the prevailing wind direction. This picture from May 2016 shows the wind streaks are still there, having faded only a little in the 8-9 years since they formed. (HiRISE ESP_045798_1965, NASA/JPL/Univ. of Arizona)

Itty bitty changes: places where the wind barely moves sand

June 06, 2016


A Piece of Mars: Not all dunes on Mars are moving at a measurable pace. This 0.96×0.45 km (0.6×0.28 mi) scene looks a lot like one I posted 3 years ago called Martian Sports. This image shows the same dunes 9.5 years apart (that’s 5 Mars Years). There are a few places where patches of sand have appeared or been removed, but it would take some detailed work to figure out whether the bulk of the dunes has shifted much. In the first post I guessed that the upper dune would crash into its topographic hurdle in 20 years, but after nearly 10 years of relative inactivity, I’ll have to revise that estimate upwards to perhaps 100 years. (HiRISE <a href="http://www.uahirise.org/ESP_045785_1995"ESP_045785_1995 NASA/JPL/Univ. of Arizona)

Neverending dust

May 31, 2016


A Piece of Mars: Some parts of Mars, like this one, are very dusty. This 1.92×1.1 km (1.2×0.67 mi) area has built up a thick deposit of dust that slowly buries the impact craters until they’re mere ghosts of the deep bowls they once were. If you knew the dust fallout rate, you could date the age of the craters. Or if you knew the age of the craters, you could estimate the mean dust fallout rate. (HiRISE ESP_044884_2050, NASA/JPL/Univ. of Arizona)

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.