Cosmic Diary by Lori Fenton

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Updated: 5 hours 16 min ago

Dunes carving up rock (3D)

January 16, 2017

A Piece of Mars: Get out your 3D blue/red glasses (or look here for a 2D version if you can’t find them). This is a 3.2×1.8 km (2×1.13 mi) scene showing dark dunes carving lanes 50-70 m (165-230 ft) deep into a stack of brighter sedimentary layers. Over time, the sand wears down the rock into yardangs, the elongated remnants of rock the sand didn’t manage to reach. Here we see the process ongoing; perhaps in a few million years there will be nothing left but a few streamlined peaks. Those murdering basterds [sic]. (HiRISE ESP_034419_2015, NASA/JPL/Univ. of Arizona)

Tortoise and hare

January 12, 2017

A Piece of Mars: There’s a lot of evidence for both fast and slow movement in this 480×270 m (0.3×0.17 mi) scene.

The tortoise: The rippled surface at the top is high ground: the top of a dune. Wind pushes the ripples toward a steep sunlit slope, creating long thin, dark avalanches that slowly inch the slipface forward. At the bottom of the slope, which is shielded from winds blowing from the top, ripples have been formed by wind blowing from the left.

The hare: Oblivious to both the slow progression of ripples and dunes, 5-25 m wide dust devils have blazed on by, leaving behind erratic trails.

(HiRISE ESP_048592_2070, NASA/JPL/Univ. of Arizona)

Crater ejecta on old ripples

January 03, 2017

A Piece of Mars: Mars rarely does anything without drama. Long ago in this 0.96×0.54 km (0.6×0.34 mi) scene, large ripples formed and then, presumably, lithified (turned into rock). Some time after that, an impact formed the crater in the center, throwing debris into an ejecta blanket that covered the lithified ripples. That ejecta blanket sat around long enough to acquire some smaller impact craters of its own. Since then, most of that ejecta blanket has eroded away, exposing the ripples to view once again. (HiRISE ESP_011699_1910, NASA/JPL/Univ. of Arizona)

Steno’s principles, or “how to make sense of pretty landscapes”

December 26, 2016

A Piece of Mars: Nicholas Steno was a 19th century geologist, who came up with some principles that are still used today to guide interpretation of exposed sedimentary rocks. The principles seem a bit obvious, but then some of the most profound principles can be like that. Emily Lakdawalla of the Planetary Society describes them in more detail here, with really good examples. You can use these principles to do forensics on a landscape, to see what happened and when.

You can see all three principles at work in this image.

#1: Stuff makes horizontal layers. (This isn’t always true, e.g., dunes and deltas make tilted layers, but most sediments pile up into flat, horizontal layers.) You can see that at work here: A thick layer of dark gray stuff once piled up on a flat surface of brighter stuff. Some of the dark gray stuff has since eroded away, but you can see that both the gray and the brighter stuff originally piled up in flat-lying layers.

#2: Older stuff is at the bottom. (Because newer stuff buries the older stuff, like the papers on my desk and the veggies in my fridge.) In this image, the brighter stuff must be older than the darker gray stuff, because the bright stuff is on the bottom.

#3: You can’t see the layers until they’re exposed by erosion or tectonics. (Because they’re buried. So if you see layers, you know something has happened so you can see them.) You can see the edges of the dark gray stuff, so you know it’s been partially eroded away – otherwise you’d never know the underlying bright stuff was ever there. Some of the material from the dark gray layer has been reformed into dark bedforms on the brighter layer, and those bedforms are probably the youngest features in this scene.

What I like most about this image has to do with yet another principle of layered stuff: Things that cut across other things are younger. Things that have been cut across are older (Like if you chop down a tree, then the axe cuts on the tree trunk must have been made after the tree itself grew. Duh, right?). You can see that in this image: on top of the dark gray layer are some old bedforms. They must be quite old, even cemented or lithified (turned into rock that the wind can’t easily move), because they’ve been cut by erosion at the edge of the gray layer. So not only was the gray layer once more extensive, but it had ripples on it, and those ripples formed and became immobile before that erosion ever happened.

(HiRISE ESP_030460_1525, NASA/JPL/Univ. of Arizona)

The trail of a dune

December 19, 2016

A Piece of Mars: A low, broad dune occupies the center of this 800×450 m (0.5×0.28 mi) scene, blown by a dominant wind towards the lower left. The slip face on the lee side has several small avalanches, formed as the slope oversteepens (this is how dunes crawl along the surface). Upwind, among other fainter lines, is a prominent bright line: it is a former slip face of this dune, possibly formed from a thick accumulation of bright dust (maybe there was a big dust storm that year). Farther upwind, another dune slowly approaches. (HiRISE ESP_033955_2065, NASA/JPL/Univ. of Arizona)

Them that make ripples and them that don’t

December 12, 2016

A Piece of Mars: Higher ground is to the left. You’re seeing a tan layer sandwiched between two gray layers in this 0.96×0.54 km (0.6×0.34 mi) scene. Large ripples have accumulated in the lowest area to the right, which is the floor of an old river channel. Ripples have also formed on the gray upper layer. But not the middle tan layer – maybe it’s too fine-grained to erode into sand grains, or maybe it erodes too slowly to allow any eroded sand grains to pile into ripples before they’re blown away. (HiRISE ESP_048196_1995, NASA/JPL/Univ. of Arizona)

Martian spiders

December 05, 2016

A Piece of Mars: Martian spiders, or araneiforms, are geological structures found at high latitudes on Mars. The dark splotch with branching arms in this 0.48×0.27 km (0.3×0.17 mi) scene is a good example. They form in the springtime, when bright frost still covers a darker sandy soil, but some sunlight filters through the frost to warm the underlying surface. Sublimation of gas (under the frost but just above the soil) creates enough pressure that little explosions occur like dry geysers, punching through the frost and blowing up sand that then falls back to the surface as a dark splotch. If the wind is blowing when this happens, then the dark splotch is carried a ways downwind, but that hasn’t happened in this case. (HiRISE ESP_048189_0985, NASA/JPL/Univ. of Arizona)


November 28, 2016

A Piece of Mars: The dunes climbing over a rocky surface in this 0.96×0.54 km (0.6×0.34 mi) scene are mostly yellow because they’re covered (and therefore kept immobile) by dust. The crest of one dune, though, shows recent activity: dark sand has been pushed by the wind up the lower right side, and then shot (cannonball-style) over the brink, where it slowly piles up on the upper left side. This pileup is called grainfall, because that’s what the sand grains have done here (rather than sliding downhill, avalanche-style, which is called grainflow). There’s a dune on the left side of the image that hasn’t experienced this activity, maybe because it’s a little more sheltered from the wind. (HiRISE ESP_047779_1655, NASA/JPL/Univ. of Arizona)

Bedforms on crater rims

November 21, 2016

A Piece of Mars: Dunes and ripples most commonly form in topographic lows. But not in this 0.96×0.54 km (0.6×0.34 mi) scene. Here, and in other places on Mars, these bedforms (called TARs) form on plains, and sometimes appear to cling to the rims of craters – which are topographic highs, not lows. It’s not clear how this happens: Does the topography of the crater rim provide a wind shadow that allows windblown sediment to accumulate there? Or was there simply more loose material on the crater rims to begin with, allowing these things to form in place? I’m open to suggestions. (HiRISE ESP_047787_1910 NASA/JPL/Univ. of Arizona)

Shadows behind boulders

November 14, 2016

A Piece of Mars: Bright material (either dust or sand) has accumulated in the lee of wagon- to car-sized boulders in this 0.96×0.54 km (0.6×0.34 mi) scene. It’s perhaps something like the Rocknest sand shadow that Curiosity visited a few years back. The wind blows from lower right to upper left, carrying along sediment that occasionally gets trapped in the protected areas behind the boulders. These sand shadows aren’t very thick, as the underlying texture (polygonal terrain!) is visible through them. (HiRISE ESP_047798_1150, NASA/JPL/Univ. of Arizona)

Dunes and rock hurdles in Gale crater (3D)

November 07, 2016

A Piece of Mars: Wind from the upper left is blowing dark dunes toward the lower right in this 1.92×1.08 km (1.19×0.67 mi) anaglyph (if you don’t have your red/blue 3D glasses handy, you can also check out the black and white 2D version). The dunes are crossing through hurdles aligned to make their progress as difficult as possible, but the dunes nevertheless are slowly making their way through. Ironically, the bright “hurdles” are themselves lithified dunes that are perhaps billions of years old. (HiRISE ESP_020555_1755/ESP_047139_1755 NASA/JPL/Univ. of Arizona)

On Mars the wind carves stream channels

October 31, 2016

A Piece of Mars: This 1.6×2 km (1×1.24 mi) scene mostly shows what wind will do to fine-grained, weakly-consolidated surfaces. It has created topography that further strengthens wind scour in the hollows, which even leave kilometers-long grooves reminiscent of water-carved streams. If this were Earth I’d guess they had been carved by water first. But this is Mars, where the wind is in charge. (HiRISE ESP_046504_1785, NASA/JPL/Univ. of Arizona)

The spire in Eberswalde crater

October 10, 2016

A Piece of Mars: OK, you have to bring out the red/blue glasses for this one. (Or click here if you’re missing your glasses and want the black and white version.) Eberswalde crater has some lovely layered deposits, long ago laid down by running water, and since eroded steadily by the wind. The wind leaves behind the most resistant parts (mainly fluvial channels that were more cemented). The center of this image shows a tall spire: the tallest of the flat layers (top of the “wedding cake”) is 290 m (950 ft) across and casts a shadow indicating it’s 200 m (656 ft) above the next layer down. That central spike is another 70 m (230 ft) taller yet, by itself nearly rivaling the “Totem Pole” in Monument Valley. Check out the rest of the red/blue anaglyph, it’s stunning. (HiRISE, ESP_047185_1560/ESP_047119_1560, NASA/JPL/Univ. of Arizona)

Windblown: ancient and recent

October 03, 2016

A Piece of Mars: HiRISE is celebrating 10 years of success by showcasing its first high resolution image, taken back in 2006. Here is a portion of it, shown at 1/4 the full resolution (the scene is 2.5×2.5 km across). I highly recommend downloading the HiRISE image viewer and looking at the whole thing, it’s an amazing landscape. The portion shown here has many different ripple-like features, formed by a wind blowing from left to right. Notice that those in the middle and middle-left are a bit fainter: these are ripple-like features that were carved into the bedrock by the wind, and they may be much older than the sharper-edged ones nearby. (HiRISE TRA_000823_1720, NASA/JPL/Univ. of Arizona)