Cosmic Diary by Lori Fenton

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Is it an old fossil barchan dune?

May 22, 2017


A Piece of Mars: There are many barchans on Mars, those lovely isolated crescent-shaped dunes. In a few places there are what looks like ancient preserved barchans, now lithified. The mound in the center of this 0.96×0.54 km (0.6×0.33 mi) scene shows what may be an example of a fossil barchan. If so, then this is quite unusual. On Earth, dunes are very rarely preserved in their full form, usually having been at least partially eroded away before being preserved. I love how much geology is visible from orbit on Mars! (HiRISE ESP_049955_1665, NASA/JPL/Univ. of Arizona)

Sand tails

May 08, 2017


A Piece of Mars: Up on the tallest volcanoes, the wind screams downhill at night. This 500x500m (0.31×0.31 mi) scene shows how dust is carried downhill, but only that which is trapped behind boulders and crater rims sticks around. The big hole may be a window into a lava tube. Formation of the window itself is one of the younger events to have formed this landscape, as the screaming dust hasn’t fully filled in the hole (although it has begun the process and formed a tailing wind streak). (HiRISE ESP_050089_1660, NASA/JPL/Univ. of Arizona)

Curiosity, recovering from the Bagnold dunes campaign

May 01, 2017


A Piece of Mars: You’ll probably want to click on this image to see the whole thing, it’s pretty big, and it’s worth seeing. This 850×550 m (0.53×0.34 mi) scene shows the barchanoid dunes of the Bagnold dune field, imperceptibly crawling southwestward (to the lower left). This is the site where the Curiosity rover first encountered an active dune in its trek through Gale crater. This image was taken after the rover’s intensive field campaign of the two dunes in the upper middle of the frame – the rover is in fact in this frame (extra credit if you can find it!), but it’s backed off a bit from the dunes, and it’s sitting on some old sandstone (that we now know was also once a dune field, long ago, much like some of the sandstones we find on Earth). This image was taken in March 2016; the rover has since moved on and across the dune field, and is slowly working its way through the foothills of Mount Sharp.

I chose this image in tribute to a colleague who unexpectedly passed away last week. He worked on both the HiRISE and Curiosity teams, so it’s fitting to show both here, near the dunes that he studied. He’s best known for his work on dune migration and surface erosion on Mars. He also mapped and measured wind-carved stones called ventifacts (we have those on Earth too), and discovered that the ventifacts here in Gale crater were carved (probably long ago) by a wind blowing from the southwest, which is opposite the direction that the dunes are being blown today! There must have been quite a remarkable shift in wind patterns since those stones were carved, and it remains a mystery. Our dear colleague will be greatly missed.

A Piece of Mars: Get out your red and

April 24, 2017


A Piece of Mars: Get out your red and cyan glasses to see an old crater, which fills this 0.775×0.7 km (0.48×0.43 mi) scene. The crater punched through many thin layers when it formed, some of which you can still see in around the rim. The crater is filled with many small dunes called transverse aeolian ridges (TARs), given this laborious and generic name because they aren’t quite like dunes we find on Earth and we don’t yet understand what they are. The TARs are common in this area, but there are even more here, where sand is swept into and then trapped inside this deep bowl. (HiRISE PSP_008735_1700_PSP_007878_1700, NASA/JPL/Univ. of Arizona)

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)