HiRISE images

Buried by ejecta
Published 12/26/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: To see this one well you'll have to click on the image. At the lower right, a 240 m (787 ft) diameter crater formed when a bolide hit the surface, throwing out ejecta on the surrounding terrain. Zooming in, you can see that the ejecta has a distinctive rough surface. Farther from the crater there are smooth patches where ejecta didn't fall. What I like about this is the many small bedforms (ripples), some of which are covered by ejecta and some of which aren't. Closer to the crater, you don't see so many of these... read more ❯

The mysterious bright streaks
Published 12/18/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: Some things just go unexplained (so far, anyway). Here's a mysterious bright streak (scene is 1.2x1.8 km, 0.75x1.12 mi) concentrated between two sets of ripple-like bedforms. It looks sort of like a river, but it's on flat terrain and it's not water. It's part of a larger set of bright streaks that you can see throughout the top of this broader CTX image (the bright streak shown in detail here is visible as a distinct white stripe on the floor of a crater). My guess is that at some point, probably at least several million years... read more ❯

Dunes in a row
Published 12/11/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: Look at the alignment of the ~100 m dunes in this 713x750 m (0.44x0.47 mi) scene. How do dunes form in such straight lines? And why don't they always do that? It's likely that these dunes were once long ridges stretching from the lower right to upper left. The shape of the slip faces suggests they're formed from two winds that blow from similar directions, both of which push sand toward the upper left. To stay stable, this sort of dune needs a constant influx of sand from upwind (from the lower right), but if that... read more ❯

Ripples of rock
Published 12/4/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: To the upper right of this 0.85x0.6 km (0.53x0.37 mi) scene is a flat-lying plain strewn with large ripples. To the lower left is a rugged hill with gray rock laced with white veins (this might be part of an impact megabreccia identified nearby in Holden crater). Notice that some of the ripples on the rugged hill are also veined - this is evidence that they are actually eroded into the bedrock, rather than fine-grained deposits like their counterparts on the plain. It's not yet clear how these "Periodic Bedrock Ridges" form, and they may be... read more ❯

The corpse of a dune
Published 11/27/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: The rippled darker patch in this 600x600 m (0.37x0.37 mi) scene is the former site of a sand dune. This is one of a few "dune corpses" found just upwind of a dune field in Holden crater. The dunes are migrating to the south and east - you can see that the arc of this former dune opens to the south, the way a barchan slip face would. This dune is what's left behind after most of its sand has migrated downwind. (HiRISE ESP_052367_1540, NASA/JPL/Univ. of Arizona) read more ❯

Is it windblown or not (#2)?
Published 11/20/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: This 0.93x1.25 km (0.57x0.78 mi) scene shows what I'm starting to think are windblown features. I posted something similar to this once before, from a location not that far from here. In this one region of Mars there are parallel lines cut into the tops of hills. A geologist would first presume they were exposed, tilted layers. But the regularity of their spacing (especially when you zoom in) is a bit unusual, and suggests some sort of self-organization (like windblown ripples). And then the questions begin: why just in this spot on Mars? what's unusual about... read more ❯

Overhang
Published 11/15/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: There's a fabric of erosion in this 1x1 km (0.62x0.62 mi) scene, with the main wind blowing from lower right to upper left (and if you look carefully you'll see there's a second, subtler fabric a bit clockwise from that one). The result is a landscape strewn with streamlined rock called yardangs. The darkest areas are shadows from rock faces scoured by the wind so deeply that they've been undermined until there's overhang. Normally this would lead to collapse features, like rock piles, but you don't see those here. That's an indication that the rock here... read more ❯

Island in the stream
Published 11/8/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: In the floor of what might have been an old fluvial channel there are a bunch of really neat dunes (or maybe ripples, they're TARs and we don't know yet what they are). One spire pokes up here, ~200 m (656 ft) across and ~90 m (295 ft) tall. The TARs reveal the wind direction here, as wind flowed from top to bottom around the spire, converging on the lee side. (HiRISE ESP_026557_1525, NASA/JPL/Univ. of Arizona) read more ❯

Black and tan
Published 11/6/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: Dunes in the top row in this 0.73x0.47 km (0.46x0.29 mi) scene are dark but those in the lower row are brighter. Why? They're all probably made out of the same kind of sand, which is dark. And they all probably got covered by fine-grained airfall dust, which is bright. At some point after that, a wind blew, probably from top to bottom of the view, and moved enough sand to kick off the fine bright dust. But the relief from those top dunes took energy from the wind, so that by the time it reached... read more ❯

Mars' corduory
Published 10/30/2017 in Lori Fenton's Blog Author lfenton
A Piece of Mars: The wind on Mars likes to make textiles (unfortunately the term geotextiles is already taken for other purposes). This 1x0.6 km (0.62x0.37 mi) scene shows two different sets of ripples. The larger set has straight to wavy crests, and they're ~18 m (~59 ft) apart, which is pretty big for ripples (really they're TARs). Inbetween those (click on the picture so you can see them) are small ~2 m (~6.5 ft) ripples that make Mars look like it's made of kahki corduroy (which is a thing but it's not on trend, so Mars could stand to... read more ❯