Cosmic Diary by Lori Fenton

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Updated: 5 hours 33 min ago

A change of fluids

April 17, 2017


A Piece of Mars: Water carved this ~800 m (0.5 mi) wide channel billions of years ago. The water dried up, and since then it’s been sand that flows through here (from the right), building up lovely dunes. A single crater on one of the dunes indicates that they’re not very active (dunes of this type on Mars all seem to be inactive, unlike their bigger, darker cousins). Look closely between the dunes and you might see a few little dots – these are boulders that have fallen, weathered out from the channel walls. (HiRISE ESP_022693_1530, NASA/JPL/Univ. of Arizona)

Two directions

April 10, 2017


A Piece of Mars: Sometimes I just want to show the interior of a dune field, because it’s full of waves: ripples and dune crests, slip faces, all of which signs of movement. The dunes in this 0.67×0.47 km (0.41×0.29 mi) view have been made by two winds: one blowing from the top of the frame, and a more-recently-active one blowing from the right. Together, these two winds (and gravity) push this sand between a series of hills and down into Coprates Chasma, one of the longest canyons on Mars. (HiRISE ESP_035278_1655, NASA/JPL/Univ. of Arizona)

Where on Mars is this dune?

April 03, 2017


A Piece of Mars: This 0.48×0.27 km (0.3×0.17 mi) scene shows a rotund barchan dune. Can you tell from looking at it where on Mars it might be? To me the most obvious feature are the bumpy piles at the bottom of the slip face (at the foot of the dune on the right). They’re probably the remains of avalanches that occurred when there was still winter frost on the dunes. This is a summertime image, so the frost is long gone and the wind is reworking the dune, trying to erase signs of the cold season avalanches. This sort of pattern is best seen in dunes near the north pole. (HiRISE ESP_027674_2650, NASA/JPL/Univ. of Arizona)

A big rock in a big air stream

March 27, 2017


A Piece of Mars: Sand pours in from the top of this 1.95×1.95 km (1.21×1.21 mi) scene. The sand piles up and up (here ~115 m or 377 ft high), but ahead (at the bottom) is a mountain poking up. Like water diverting around a rock in a stream, the mountain affects the air flow just upwind of it, causing the sand to move around it. The steep dune slope is a slip face, caused by oversteepened sand avalanching. If you look closely, you’ll see some of those narrow avalanches near the bottom of the slip face (those at the top have been covered by ripples and falling sand). (HiRISE ESP_049045_1760, NASA/JPL/Univ. of Arizona)

More Earth-like views of Mars

March 20, 2017


A Piece of Mars: In a recent post (Dunes in a Colorful Hole), I showed some dunes crawling over layered terrain, with a view that looked a lot like some desert regions of Earth. Here’s another spot on Mars (0.95×1.1 km, 0.59×0.68 mi) showing yet more beautiful layers with dunes filling up the valleys. Part of what makes it seem Earth-like is the lack of craters, although if you go looking you’ll see there are some there. It’s hard to tell from here, but this whole scene is inside an old fluvial channel. The layers are thought to be lake deposits from when the river dammed up, ages ago. Since then the wind has taken over, taking apart the layers one grain at a time, and then building up dunes with some of those grains. (HiRISE PSP_010329_1525, NASA/JPL/Univ. of Arizona)

Windblown or not? Probably…

March 14, 2017


A Piece of Mars: This 0.95×0.95 km (0.59×0.59 mi) scene shows an eroding surface punctured by some old craters. Long, thin lines seem to form in the wake of many brighter knobs. Are those thin lines windblown in origin? They look like erosional features – things that are left behind when other stuff erodes away around it (not like sand dunes, which are things that pile up over time). If so, they don’t look like typical yardangs, which are streamlined bedrock, formed as sand wears down the rock. But this isn’t typical bedrock – it is easily erodible material. The bright knobs and crater rims are what’s left of a once-higher surface. The darker material may be a lag deposit that has built up as that brighter layer eroded down, leaving behind coarser grains that the wind has a harder time transporting (a similar process has occurred in Meridiani Planum, where the Opportunity rover drove through many kilometers of ripples, which now help protect the surface from erosion). If so, these long thin lines are a very unusual sort of yardang. (HiRISE ESP_016843_1590, NASA/JPL/Univ. of Arizona)

Hills made by wind and ice

March 08, 2017


A Piece of Mars: A fluid is something that fills a container it’s put into, and it includes both gas and liquids. This 0.7×0.5 km (0.43×0.31 mi) scene shows hills of sediment left behind by two different fluids (wind and ice). The hill on the left is a rippled sand dune, which has been piled up by the wind as it drops its sandy load. On the right is a layered sinuous hill, leftover from when ice flowed down a slope offscreen to the right. The dune is slowly encroaching on the hill, and will eventually be disrupted by it. (HiRISE ESP_048913_1330, NASA/JPL/Univ. of Arizona)

Dunes in a colorful hole

February 27, 2017


A Piece of Mars: Gray dunes have migrated over reddish rock, moving toward a narrowing cleft surrounded by tall tan cliffs. Bright lines on the dunes are exposed internal layers (bones of the dunes, really) that show you where the lee-side slopes once were (so you can tell they’ve moved to the left). The cliffs are made of layered rocks (extra points if you can find the fault), suggesting these are sedimentary layers, laid down long ago in Mars’ geologic past. The whole HiRISE image is worth a long look, it’s really amazing. (HiRISE ESP_049009_1520, NASA/JPL/Univ. of Arizona)

Who wins in the fight of wind vs. ice?

February 21, 2017


A Piece of Mars: This is the crest of one of the largest dunes on Mars (0.5×0.5 km or 0.31×0.31 mi). The wind mostly blows from the right, slowly pushing sand up the windward slope. But frost accumulates on (and probably in) the sand during winter, and sometimes it gets too heavy and slides down the steepest slope (toward the left), carving out big gullies in the sand. And then the wind blows some more, trying to erase the gullies by 1) making ripples, 2) burying the gullies (the featureless blue patches are grainfall, which is a fancy term for sand that fell as airfall), and 3) forming dust devils that leave faint but wide tracks. Who wins this fight, wind or ice? Neither: gravity wins (it usually does). (HiRISE ESP_020876_1330, NASA/JPL/Univ. of Arizona)

Mars’ yin-yangs

February 13, 2017


A Piece of Mars: Is this 480×270 m (0.3×0.17 mi) scene showing a 150 m (492 ft) wide yin-yang symbol on Mars? Sort of, maybe, if you blur your eyes and lend me artistic license, but it’s not doing so intentionally. One side of the crater is dark and the other is light. Both have their tone because of windblown material blown from the same direction, but the different materials collected where they did for different reasons. The dark material is probably mafic sand (iron and magnesium-rich, like what’s found near many volcanoes), which was bounced along the ground from the lower right, and collected in the lee of the crater rim. The bright material is much finer-grained, dust carried aloft, and it probably settled down on the far side of the crater, and outside as well, as the crater rim poked into the wind and provided enough shelter to let some of the bright material settle out as airfall. (HiRISE ESP_016496_2000, NASA/JPL/Univ. of Arizona)

The two-faced dunes of Mars

February 06, 2017


A Piece of Mars: The focus of this 0.96×0.96 km (0.6×0.6 mi) scene is one of many two-faced dunes on Mars. The bright sunlit slope is one face, formed recently by wind blowing from the upper right. The dark shaded slope is the other face – it’s a little older, formed by wind blowing from the left. Together these two winds alternate, probably in different seasons, forcing the sand into a needle-shaped point that carries sand in a direction that is, give or take, the sum of those two winds. Two-faced dunes like this are rare on Earth, as winds here typically quickly erase older crestlines. (HiRISE ESP_021716_1685, NASA/JPL/Univ. of Arizona)

Dunes + Craters = Mars

January 30, 2017


A Piece of Mars: How do you tell when a planetary landscape shows Mars, instead of Mercury or the Moon or Europa? The easiest way to tell is to look for both craters and dunes, like what’s shown here in this 640×360 m (0.4×0.22 mi) scene. Not all martian landscapes have either feature, and there are some other worlds that do have both (Earth, Titan, maybe Pluto, and probably Venus but we need better data…), but it’s a pretty good bet that if you see both features together, you’re looking at Mars. Anyway, in this lovely view, the dark gray terrain (you’ll see boulders if you look closely enough!) is being eroded away slowly, revealing a much older, brighter surface beneath it. Unfortunately for those who would study ancient terrains on Mars, much of that older, lower surface is covered in dunes. But I like the dunes – they give us information about surface erosion rates and wind patterns. One person’s signal is another person’s noise. (HiRISE ESP_047762_1585, NASA/JPL/Univ. of Arizona)

Mars’ giant sweaters

January 23, 2017


A Piece of Mars: Sometimes in the floors of small craters, the wind blows in from several directions to produce odd polygon-shaped dunes that look like crochet (maybe Mars is making sweaters for its craters – it is, after all, a cold place). This “sweater” segment is 480×270 m (0.3×0.17 mi) in size (the “stitches” are ~20 m, or 66 ft, across). The smaller interior lines are younger windblown features, that are superposed on the larger structures – their alignment is strongly controlled by the topography of the larger polygonal “stitches”. (HiRISE ESP_017833_1975, NASA/JPL/Univ. of Arizona)

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)