APOD: A Dark Dune Field in Proctor Crater (2010 Nov 22)

[A.A. Verveen]

Stomata in the leaves of corn (maize), see the picture in:
http://weblogs.vpro.nl/uitgekookt/2008/ ... -groenten/

[neufer]

Ah, I hadn't realized we are looking at sand-ripple scale.

I'm not entirely sure that we are..., however:

* Normal ripples or impact ripples occur in the lower part of the lower flow regime sands with grain sizes between 0.3-2.5 mm and normal ripples form wavelengths of 7-14 cm Normal ripples have straight or slightly sinuous crests approximately transverse to the direction of the wind.

* Megaripples occur in the upper part of the lower flow regime where sand with bimodal particle size distribution forms unusually long wavelengths of 1-25 m where the wind is not strong enough to move the larger particles but strong enough to move the smaller grains by saltation.

* Fluid drag ripples or aerodynamic ripples are formed with fine, well-sorted grain particles accompanied by high velocity winds which result in long, flat ripples. The flat ripples are formed by long saltation paths taken by grains in suspension and grains on the ground surface.

Sand Ripples Taller On Mars

<<Mars is kind of like Texas: things are just bigger there. In addition to the biggest canyon and biggest volcano in the solar system, Mars has now been found to have sand ripples twice as tall as they would be on Earth. Initial measurements of some of the Red Planet's dunes and ripples using stereo-images from the Mars Orbiter Camera onboard the Mars Global Surveyor have revealed ripple features reaching almost 20 feet high and dunes towering at 300 feet.

One way to imagine the taller dimension of ripples on Mars is to visualize sand ripples on Earth, then stretch out the vertical dimension to double height, without changing the horizontal dimension. "They do seem higher in relation to ripples on Earth," said Kevin Williams of the Smithsonian National Air and Space Museum. Williams will be presenting this latest insight into the otherworldly scale of Marscapes on Monday, Nov. 3 at the annual meeting of the Geological Society of America in Seattle, WA.

Ripples are common on Mars and usually found in low-lying areas and inside craters, says Williams. On Earth they tend to form in long parallel lines from sand grains being pushed by water or air at right angles to the ripple lines. Dunes, on the other hand, are formed when grains of sand actually get airborne and "saltate" (a word based on the Latin verb "to jump"). That leads to cusp-shaped, star-shaped, and other dune arrangements that allow materials to pile sand much higher.

How exactly Martian dunes and ripples form is still unknown, says Williams, since the images from space give us no clues to the grain sizes or whether they are migrating or moving in any way. Though there are Viking spacecraft images from almost 30 years ago to compare with, the images do not have the resolution to confirm whether ripples have moved much in that time. For now, the dimensions of ripple-forms on Mars are the only indications of whether they are large ripples or small dunes. Williams' results came about from the advantageous combination of image parameters to get the first height measurements of these ripple-like features at the limit of image resolution.

According to Williams, it's likely the doubled heights of Mars ripples relative to their spacing is made possible by the same thing that makes Mars' volcanoes
so tall: lower gravity. With about one-third the gravity of Earth, sand, silt, and dust can theoretically stack up higher before gravity causes a slope failure. However, other differences could play roles in making these large piles of sand as well. "It could also be from different wind speeds, air densities or other factors," said Williams. Mars has a perennially subfreezing, very thin atmosphere in which global dust storms have been known to obscure the surface from view.

The study of Mars dunes and ripples has been underway since Viking spacecraft images of Mars first revealed such features in the late 1970s and early 1980s, says Williams. The primary difficulty of the work continues to be in discerning the close-up details, like the exact heights of features and grain sizes. As with dunes and ripples on Earth, these wind-blown features could reveal a lot about local and regional weather and wind currents if more was known about ripple and dune building under the very un-Earthlike conditions of Mars.>>

But still, we appear to have very dark sand on top of light (rippled) sand.
Is there a material difference, and if so, what process keeps the light and dark materials separate?

[size=135]The size, density & shape of the particles determine whether they move by [color=#FF0000]suspension[/color], [color=#FF00FF]saltation[/color] or [color=#0000FF]creep[/color].[/size]

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[ctidd19]

It appears that all the "dunes" have the same structure - as nearly as one can tell from the picture - and the winds building them blow from the top of the screen to the bottom. Yet, the smaller "dunes" in between the large ones appear to have been formed by a wind blowing from the side of the image and form the usual long dune wave formation. How is this explained? Why don't the larger dunes congregate and instead remain distinct individuals of about the same size? Is there some magnetic quality to the sand of the large dunes?

[rstevenson]

What is seen in the photo is the same stuff Opportunity had been driving through for the past years. The dark depressed areas are full of the "blueberries" The light rocks are broken pieces of the crumbly eroded sedimentary rocks. What we are looking at are large eroded sand dunes that were formed billions of years ago when the climate was different, before the wet era.

Hmmmmm, I think you need to take another look, and follow the links in the description.

Opportunity has not been driving through dunes like this -- far from it. The "dark areas" are not depressed; they're the dunes that we're talking about. They rise high above the lower, smaller light-coloured dunes which are similar to what we see in Opportunity's path. Scale may help you visualize things: the large black dunes are in the range of hundreds of meters long. North is up, and the darker east face is the slip face of the dune. It's unlikely these dunes were formed billions of years ago; they have clean sharp features which would have eroded over such a time span. They "may" (to use the careful wording on the HiRISE site) be active today.

Rob

[kesara]

I thought it kind of looks like stubble under a microscope from some advertisement.

Click to view full size image

[liebencrantz]

I'm struck by the rounded quality of these black hotdog dunes. quite different than the sharper spiky or starlike dunes here on earth or eltsewhere on mars. would this have most to do with the micro structure if this black sand; possibly a particular clumping characteristic? if so I am again amazed at how minute microscopic details can so prominently affect the structure of the macroscopic. makes me think of the molecules of a cristal. or indeed the components of everything we interact with.

[BOREDINHB]

Click to view full size image

[neufer]

Martian dust devils cause twisting dark trails on the Martian surface.

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Golgi-stained neurons in human hippocampal tissue

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Click to view full size image