Martian Dunes Thawing (APOD 03 Mar 2008)

[Arramon]

http://apod.nasa.gov/apod/ap080303.html



So what are those marks that look like impacts? Is that the exploding that happens when the sand thaws?

The process might even involve sandy jets exploding through the thinning ice.

Very interesting image. =)

[neufer]

So what are those marks that look like impacts? Is that the exploding that happens when the sand thaws?

The process might even involve sandy jets exploding through the thinning ice.

Very interesting image. =)

Many of the "sandy jets" seem to occur near the pinnacle of the barchan dunes.

Is it because the surface gravitational pressure is greatest there?
Is it because the surface evaporation is most stymied there?
Is it because the surface thermal blanket is greatest there?

http://en.wikipedia.org/wiki/Barchan

[kovil]

The dark spots or areas, look to be the place where the wind velocity would be highest passing over the dune shape. This might be why it thaws there first, and the most 'snow' is also blown away along that track.

It might be because the wind makes a small tornado as it passes over that cresting point, and that also stirs the snow away.

If any electrical currents are generated from the wind or mini swirling tornado, that would be the place it would happen, and the electrical activity might be what is 'melting' the 'snow'.

Is it possible to get a scale of size of the dunes?
Are they 20 feet tall, 150 feet, 3000 feet?
I am lost as to the real size of the dark spots and the true height of the dune 'cliffs'.

[Qev]

Unless I'm totally misreading the HiRISE site, the APoD picture is approximately 130m across (405 pixels at 32cm/pixel for the non-map-projected original image). Am I wildly off-base?

[neufer]

The dark spots or areas, look to be the place where the wind velocity would be highest passing over the dune shape. This might be why it thaws there first, and the most 'snow' is also blown away along that track.

For a barchan dune to move and still maintain it's characteristic shape there must be maximal sand depletion on the steepest part of the front slope in conjunction with maximal sand deposition on the steepest part of the back slope. Presumably it is these steep regions (rather than the flat top) that represent the points of maximal & minimal wind stress (and to some extent of maximal & minimal surface wind speed) respectively.

It might be because the wind makes a small tornado as it passes over that cresting point, and that also stirs the snow away.

There will be a vortex line in the wake of the ridge of the dune but this vortex line will have a significant vertical (i.e., tornadic) component only on the right & left sides of the dune.

[Delysid23]

Three possibly obvious questions, but just to be sure for understanding the image & the previous posts, particularly neufer's last:

1) Which way is the prevailing wind blowing? I assume it's from the upper left to the lower right of the picture, i.e. the long shallow slope faces the wind, the steep overhung near vertical slope is away from the wind. Am I right?

2) Which way do such dunes move? In the same direction as the wind, or towards the wind? I realize the latter option seems counterintuitive, but so did heliocentrism, so I'm just checking.

3) In the previous post by neufer, when he says "front slope", is that the long shallow slope, or the overhung near vertical slope? This answer may be derivable from the above two, but again, I just want to make sure.

Finally, another hypothesis re the dark crater/jet holes most evident on the crest of several dunes -- could it be that they're jets popping up randomly that do so with little fuss and are quickly and lightly covered over in flat non-dune-covered areas, but that build up pressure and make a big mess when bursting through a thick cover of sand, thickest when the ridge of a dune is over them? In other words, the dunes don't cause the jets but just make them more explosive and visible events.

[Delysid23]

Duh! All 3 of my above questions are succinctly answered in the Wikipedia entry on barchan dunes, linked to from within the text of the March 3/08 Martian dunes APOD.

The answers are:

1) Yes, wind is from upper left to lower right of Martian dunes pic.

2) Dunes move in same direction as wind.

3) If by "front slope" neufer means the slope on advancing side of the movng dune, i.e. the side in the direction the dune is moving, it is the overhung steep slope, facing away from the wind. This would make the "back slope" the long shallow slope facing toward the wind.

Here's the picture from the Wiki page given above:

[neufer]

If by "front slope" neufer means the slope on advancing side of the movng dune, i.e. the side in the direction the dune is moving, it is the overhung steep slope, facing away from the wind. This would make the "back slope" the long shallow slope facing toward the wind.

Sorry, I should have been more specific:

I was thinking of the dune as a automobile convertible windshield
with respect to the wind with
the 'front slope' impinging on the wind and
the 'back slope' in the lee of the wind.

With respect to the dune's own movement
vis-a-vis the ground front & back are reversed.

[neufer]

Finally, another hypothesis re the dark crater/jet holes most evident on the crest of several dunes -- could it be that they're jets popping up randomly that do so with little fuss and are quickly and lightly covered over in flat non-dune-covered areas, but that build up pressure and make a big mess when bursting through a thick cover of sand, thickest when the ridge of a dune is over them? In other words, the dunes don't cause the jets but just make them more explosive and visible events.

There certainly could be random hot spots of vapor release that interact with passing dunes in complex non linear ways. As layers of falling sand on the 34º face cover up these hot spots vapor release (preferentially following micro-cracks between the slip layers?) could induce more slippage and actually nudge the center of the dune towards these hot spots until the "big mess" you refer to occurs.

[Arramon]

In other words, don't play on the dunes once we get there. =b

So I guess sandworms are out. O.o
These couldn't be melange spice fields going up? A good spice blow?
hehehe.... no , i guess not.

d'oh! no arrakis for us.

[neufer]

In other words, don't play on the dunes once we get there. =b

So I guess sandworms are out. O.o
These couldn't be melange spice fields going up? A good spice blow?
hehehe.... no , i guess not.

d'oh! no arrakis for us.

http://en.wikipedia.org/wiki/Arrakis

<<Arrakis, ("the dancer") later Rakis (informally known as Dune) is the third planet orbiting the star Canopus, and it in turn is orbited by two moons. Arrakis is a desert planet with no natural precipitation, and is the only known source of the spice melange, which extends life and makes interstellar travel possible (and is therefore the most essential and valuable commodity in the universe). The planet has no surface water bodies, and giant sandworms (Shai-Hulud) and their immature forms of sandtrout and sandplankton are among the few fauna on the planet. Open canals called qanats are used "for carrying irrigation water under controlled conditions" through the desert. As indicated by large salt flats, Arrakis once had lakes and oceans. "The sandtrout ... was introduced here from some other place. This was a wet planet then. They proliferated beyond the capability of existing ecosystems to deal with them. Sandtrout encysted the available free water, made this a desert planet ... and they did it to survive. In a planet sufficiently dry, they could move to their sandworm phase.">>

http://antwrp.gsfc.nasa.gov/apod/ap070928.html



<<In Dune, the desert of Arrakis is the only known source of the spice melange, the most essential and valuable commodity in the universe. A byproduct of the sandworm life cycle, sandtrout excretions exposed to water become a pre-spice mass, which is then brought to the surface by a buildup of gases and develops into melange through exposure to sun and air. Liet-Kynes describes such a "spice blow" in Dune: "Then he heard the sand rumbling. Every Fremen knew the sound, could distinguish it immediately from the noises of worms or other desert life. Somewhere beneath him, the pre-spice mass had accumulated enough water and organic matter from the little makers, had reached the critical stage of wild growth. A gigantic bubble of carbon dioxide was forming deep in the sand, heaving upward in an enormous "blow" with a dust whirlpool at its center. It would exchange what had been formed deep in the sand for whatever lay on the surface.">>
http://en.wikipedia.org/wiki/Sandworm_(Dune)

[JohnD]

Where does the black/dark blue material come from, where does it go and how is it re-concentrated under the barchans for the next winter?

John

[neufer]

Where does the black/dark blue material come from, where does it go and how is it re-concentrated under the barchans for the next winter? John

One can guess on where it goes from the picture itself:

The black/dark blue material at the top of the dune is blown down the 34º face making that face black/dark blue (especially at the bottom).

The black/dark blue material columns that used to connect to the top of the dune are left behind as dark chevrons on the windward face.

[JohnD]

neufer,
Certainly one can see where bits of dark material (!) have collected at the bottom of the windward slope (bottom left of above pic) but chevrons I don't see. And I don't think that the wind that formed the dunes is blowing now, as blue is spread against that direction.

My question was more concerned with how this intensely dark pigment is collected together again, recirculated, or else what caused it to be so concentrated under the surface, yet rapidly dispersed above it.

JOhn

[geonuc]

As to the dark material that 'spots' the tops of the dunes, a previous APOD seems to suggest the dune sand itself is dark and what we're seeing is the initial points of thawing at the top of the dunes. Or something like that.

http://apod.nasa.gov/apod/ap040831.html

[bystander]

As to the dark material that 'spots' the tops of the dunes, a previous APOD seems to suggest the dune sand itself is dark and what we're seeing is the initial points of thawing at the top of the dunes. Or something like that.

Sand Dunes Thawing on Mars (APOD 2008 March 03) suggests the same:

...Thinner regions of ice typically defrost first revealing sand whose darkness soaks in sunlight and accelerates the thaw...

[Arramon]

As to the dark material that 'spots' the tops of the dunes, a previous APOD seems to suggest the dune sand itself is dark and what we're seeing is the initial points of thawing at the top of the dunes. Or something like that.

http://apod.nasa.gov/apod/ap040831.html

Those images don't appear the same. The newer one shows only dark spots near the crest of the dune, where it falls to the side blocked by the wind. They look like explosions. Just wondering what chemical reactions cause this besides what the explanation was in the APOD summary. Are there processes on earth that create these similar outward bursts on this same type of frozen dune?

[henk21cm]

Are there processes on earth that create these similar outward bursts on this same type of frozen dune?

I lack the experience with frozen dunes, the last time i saw frozen dunes, was in 1963, in my childhood. What i can tell from a more recent experience: an odd five years ago experiments were performed by injecting liquid oxygen (90.2 K) into the soil, i.e. silt (d < 63 μm) and aeolian sands. Since the oxygen was injected a depth of a few meters, no blow out occured. Just cracks in the soil. The effective stress in the soil at a depth of 5 m was too high to allow for a blow out.

Thaw of frozen water does not produce an expansion of volume (the opposite occurs): when ice melts, its volume decreases. So water ice can not produce a blow out. Furthermore the change in volume is quite low, 10-15%. Conversely, when liquid oxygen or any other liquid with a low boiling point (below 100K) evapourates, its volume roughly expands by three orders of magnitude: 1 liter of liquid hydrogen produces 800 liters of hydrogen gas.

A proces similar to boiling, can be expected when frozen carbon dioxide (CO2) sublimates into its gaseous phase. When a bubble of gas is created in the soil, at low depth, the effective stress in the soil is too low to prevent a blow out. Cavity expansion theory gives a mathematical treatment of this phenomenon see e.g. http://www.idswater.com/Common/Paper/Pa ... Risk3.htm

As far as i know only the polar regions of Mars are suffciently cold to allow for freezing of CO2. So these sand dunes with a blow out appearence should only occur at high latitudes, if the CO2 explanation is correct. The professional explanation at the apod image speaks of polar region.

Nevertheless there are some points in this explanation which cause trouble.The thermal conductivity of soil is rather low. So when the sun starts heating up the soil, any frozen CO2 at the surface will sublimate rather quickly. Later, solar heat will seep into slightly deeper layers. For a blow out the surface must remain frozen, while the CO2 deeper in the soil must evapourate. This destroyes the explanation with a loud bang.

When temperature drops, of a mixture of water and CO2 gas, the gas with the highest boiling point or freezing point will freeze first, i.e. water. So i expect a layer of water ice to settle on the soil first. Later, when temperature drops further, CO2 ice will settle on top of the water ice. I can not explain why the CO2 ice will freeze at a higher temperature than water ice and so be burried by water ice. So i feel a bit unhappy when reading the professional explanation.

Just an amateur,
Henk

[JohnD]

henk,
The soil/sand below the water/co2 ice must be dark, very dark if it is the materail thrown upwards by the geysirs. If the ice was transparent, would sunshine penetrate to the dark soil and heat that, beneath the ice? So the ice began to sublime from the bottom up?

JOhn

[henk21cm]

The soil/sand below the water/co2 ice must be dark, very dark if it is the materail thrown upwards by the geysirs. If the ice was transparent,

The ice of carbon dioxide i know is milky white, not as transparent as water ice. Maybe there exists a transparent form of CO2 ice at pressures as low as on Mars. I'm not familiar with that specific transparent form.

would sunshine penetrate to the dark soil and heat that, beneath the ice? So the ice began to sublime from the bottom up?

The equilibrium vapour pressure at roughly 150 K is very low. The lowest value i could find is at 175 K ≅ 2 mPa. IMHO too low for a blow out. Effective pressure (of the soil) at 0.2 m below the Martian surface is 0.2 * 3.75 * 1500 ≅ 1 kPa, 6 orders of magnitude larger than the vapour pressure of water at 175 K.

If you replace water by CO2 in your quote, it might work, if there exists a transparent form of CO2 ice. Maybe an idea for a student at a university to experiment with sand, water, CO2 ice and high-power xenon lamps.

Just an amateur,
Henk

[JohnD]

According to the Wikipedia entry on CO2, there IS a glassy version of solid CO2. But it only forms at enormous pressures that need a diamond anvil to acheive! So probably not relevant.

John

[iamlucky13]

The CO2 ice doesn't melt into a liquid. It sublimates into a gas...requiring several times it's previous volume and building up pressure.

I suspect the darkness is simply due to the relatively smooth surface being disturbed and broken...more shadowed.

[neufer]

http://apod.nasa.gov/apod/ap080303.html

So what are those marks that look like impacts? Is that the exploding that happens when the sand thaws?

The process might even involve sandy jets exploding through the thinning ice.

http://www.youtube.com/watch?v=PNm1kfxl ... re=related


<<The blue-tinted colors associated with the scours and ripple crests are probably due to the presence of basaltic sands mixed with hematite-rich spherules.>>
http://marsrovers.jpl.nasa.gov/gallery/ ... 1123a.html
http://www.aapg.org/explorer/2005/05may ... erries.cfm
http://marsrovers.jpl.nasa.gov/gallery/ ... 4R1_br.jpg
http://www.thunderbolts.info/tpod/2004/ ... 27mars.htm
http://www.space.com/scienceastronomy/u ... 40616.html
http://www.space.com/scienceastronomy/h ... 40319.html

[JohnD]

neufer,
Note that many Mars Rover pictures are displayed in "false-colour", that exaggerates the colour differences markedly. The eyeball appearance would be a near uniform brown. I've not seen anything on the HiRise site to say that their pics are similarly 'falsified', but that intense dark blue could be a slight tinge in reality.

JOhn

[Dr. Skeptic]

For the sake of conversation: What if the dark areas were algae?

(Or, dung from giant sand worms?)