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JohnD
2008-Aug-08, 08:22 PM
All,

http://hirise.lpl.arizona.edu/PSP_008982_1965

HiRise pic of a crater floor, with enormous barchan dunes marching across it.
What's more they are BLACK.

The caption is hopeless - it rabbits on about layered deposits. Fascinating to an areologist with an obsession neurosis and tunnel vision, but what about the dunes?

How big are they - they look enormous, far bigger than any Earthly dune?
Why are they black?
How fast are they moving? Martian dust devils move fast, but what is the velocity of a steady Martian wind, necessary for barchans?
Why are some of them leaving a trail? Barchans are blown before the wind, their concave surface going first, and the sand on the back is blown forwards, so it should be blown away.
Where do they come from? No black sand to be seen in the wide view.

So much to know!

John

01101001
2008-Aug-08, 11:52 PM
What's more they are BLACK.
They look black in the image. Is that true color? Stretched? (From the same page: About color products (http://hirise.lpl.arizona.edu/pdf/color-products.pdf) (PDF))

A similar "black" dune field explored by the Spirit rover on the lee of Husband Hill was olivine-rich sand.

Wikipedia: List of extraterrestrial dune fields (http://en.wikipedia.org/wiki/List_of_extraterrestrial_dune_fields):


[...] a small dune field unofficially named El Dorado on the south side of Husband Hill in Gusev crater was investigated by the Spirit rover from sols 706 to 710. Analysis of El Dorado showed that it consists of black wind-blown sand which is "well-sorted, well-rounded and olivine rich.

How big are they - they look enormous, far bigger than any Earthly dune?

I measured one to be about 850 pixels across. At 25 cm/pixel that would be 212.5 m.


How fast are they moving?

That would take multiple images over time to determine. I've read of others similar said to be moving slowly if at all.

Here's an abstract that says some dunes just sit there, at the rate we can observe: First Evidence of Dune Movement on Mars (http://adsabs.harvard.edu/abs/2006AGUFM.P31B0128B)


Many sand dunes on Mars have pristine morphology, fresh grain flows on avalanche faces and sand streamers extending from barchan horns. This suggests that the saltation threshold for sand is exceeded and dune migration is possible under the current Martian climate. However, sand dune movement has not been observed and there is evidence that many of the dunes may be stabilized or indurated. We report the first evidence for the movement of bodies of windblown sand under current climate conditions on Mars. Repeat images of three sand dunes using the Mars Orbiter Camera were acquired between March 1999 and December 2004. We detected the complete removal of sediment from two small dome dunes in a barchan dune field in the North Polar Sand Sea during this time. The third and largest dome dune (77 m wide) in the sample suite had a slight reduction in size, but dune form remained intact. On Earth, dome dunes are circular to oval low mounds of loose, well-sorted, very fine to medium sand. Slip faces are absent or ephemeral and stand only a meter or so high. That these dunes did not migrate, but were eroded, suggests that they were not in equilibrium. Dome dune morphology is not always as effective as e.g., barchan morphology, for trapping sediment, particularly in locations of high velocity winds. In these situations, the removal of sand downwind can lead to the depletion of the dune. Our data confirms that first; the threshold wind speed for saltation is exceeded under present Martian climate conditions. Second, not all dunes on Mars are stabilized or indurated. Third, dune migration is possible under current Martian conditions; however it is likely to be limited to the smallest barchan and dome dunes (i.e. < 20 m wide).

Popular account: Marching sand dunes stir up Mars mystery (http://www.msnbc.msn.com/id/24368968/)


Sand dunes puzzled astronomers because Mars has almost no atmosphere

But scientists were unsure if the dunes were simply relics of Mars' past, when its atmosphere was denser and capable of generating stronger winds than it is now. The planet's current atmosphere is less dense than Earth's, and wind speeds 10 times faster than those on Earth would be required to pick up Martian sand particles.
[...]
Dune migration, also caused by saltation, has only been noticed on the red planet in a few before-and-after images in the last couple years, which seemed to indicate that saltation could still be going on, at least in some areas of the Martian surface.
[...]
Though Parteli and his team's results show that saltation can occur if the winds are strong enough, the winds on Mars rarely reach above the proper threshold, which makes saltation "a very seldom event on Mars," Parteli said.

JohnD
2008-Aug-09, 11:35 AM
Thanks, Mugs,

Searching the HiRise site finds a much more dramatic pic of barchans. This indicates a definite prevailing wind and the source of the sand, an eroding mesa to the east.
http://hirise.lpl.arizona.edu/PSP_007676_1385

John

JESMKS
2008-Aug-09, 07:12 PM
The Arizona paper has me confused. It states "a dune type that on Earth has the horns in the downwind direction. Also, the steepest faces of the dunes are on the dunes eastern side, indicating easterly winds" The steepest side of these dunes are on the sides with the horns and if they are on the eastern side, it should indicate westerly winds (winds from the west blowing east)

01101001
2008-Aug-09, 07:29 PM
The steepest side of these dunes are on the sides with the horns and if they are on the eastern side, it should indicate westerly winds (winds from the west blowing east)

The IRB color (map projected) (http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/PSP/ORB_007600_007699/PSP_007676_1385/PSP_007676_1385_COLOR.abrowse.jpg) has north at the top, according to the observation toolbox, and the horns point to the left, west.

The excerpt image on the Eroding Mesas page (http://hirise.lpl.arizona.edu/PSP_007676_1385) is non-map projected -- and presumably has north on the bottom.

The "steepest sides" (and horns) are on the western side when north is up. The caption doesn't seem right.

JohnD
2008-Aug-10, 09:56 AM
"The caption doesn't seem right"

Absolutely! The magnified image at the top of the "Eroding mesas" page is inverted compared to the much larger image you get if you click on it.

Who writes the captions? They are much less interesting than the Mars Explorer ones. Did the latter employ someone with journalism skills awhile the HiRise rely on academics?

John

JonClarke
2008-Aug-10, 10:33 AM
"The caption doesn't seem right"

Absolutely! The magnified image at the top of the "Eroding mesas" page is inverted compared to the much larger image you get if you click on it.

Who writes the captions? They are much less interesting than the Mars Explorer ones. Did the latter employ someone with journalism skills awhile the HiRise rely on academics?


Mars explorer? What mission was that?

I suspect it is the HiRISE captions thaat are written by a jounralist. An academic would be much more informative.

JohnD
2008-Aug-10, 11:56 PM
An academic more interesting?
Weeeeeeeeell........
Some academics, maybe, but we've all met the boring onanistic one-track acedemics or the teacher that knows their subject but cannot get it across. HiRIse has got one too.

Have you read the caption to the pic I first posted about? It reads, "This image shows the floor of an unnamed impact crater in Arabia Terra that has layered deposits. There are many craters in this region where layering is observed. These layers are often exposed along the sides of large isolated mounds, small knobs and mesas, and other positive relief features. In some cases, the layering is expressed as narrow sinuous ridge-like structures along crater floors. The presence of layered deposits is of particular interest because these materials are not likely to be related to the impact event, but rather post-impact infill of the crater. Modification of the deposits has now revealed layers of material."

Nothing, but nothing, about the feature that any non-specialist would pick out as being of interest - the dunes!

John

Ps Sorry! "Mars ExplorATION ROVERS" mission. Pedant. (big, big smiley)

JonClarke
2008-Aug-11, 10:44 AM
An academic more interesting?
Weeeeeeeeell........
Some academics, maybe, but we've all met the boring onanistic one-track acedemics or the teacher that knows their subject but cannot get it across. HiRIse has got one too.

Seems like you have a thing against academics. Well, to me it reads more like a journalist who quotes big word but does not know how to make them intelligible. A good academic, and HiRISE has plenty, knows how to communicate.


Have you read the caption to the pic I first posted about? It reads, "This image shows the floor of an unnamed impact crater in Arabia Terra that has layered deposits. There are many craters in this region where layering is observed. These layers are often exposed along the sides of large isolated mounds, small knobs and mesas, and other positive relief features. In some cases, the layering is expressed as narrow sinuous ridge-like structures along crater floors. The presence of layered deposits is of particular interest because these materials are not likely to be related to the impact event, but rather post-impact infill of the crater. Modification of the deposits has now revealed layers of material."

Nothing, but nothing, about the feature that any non-specialist would pick out as being of interest - the dunes!

Why are the dunes more interesting to the non specialist? Besides, the caption relates to the purpose the image was takem, in this case layered materials. It also excplains why the layers are important.

There are plenty of other images of dunes for people interested in them, it takes only seconds to find them http://hirise.lpl.arizona.edu/science_themes/aeolian_processes_1.php .



Ps Sorry! "Mars ExplorATION ROVERS" mission. Pedant. (big, big smiley)

My impression is that most of those are written by the scientists, i.e. academics.

Jon

JohnD
2008-Aug-12, 07:46 PM
Written by scientists -who can write.

And dunes - dunes are more interesting because you, I, everyone here can see a dune with our own eyes, and understand to how they form and how they move. And - AND! - they move! What other geological feature moves in 'real' time? Except for tides and river driven phenomena.

'Layering' may be fascinating geologically, but to me, the non-specialist (and I can claim to be an eminent non-specialist!) a dune wins! I'm not running down academics. Of course the descriptions and discussions of this pictures should be written by the academics and scientists whose work they are (and aren't we priveliged to be in their seminar?) but I can remember, and I'm sure you can, teachers from primary to post-graduate level, who couild bring thier subject to life, as it were.

John

JonClarke
2008-Aug-12, 10:10 PM
Written by scientists -who can write.

They are good enough.


And dunes - dunes are more interesting because you, I, everyone here can see a dune with our own eyes, and understand to how they form and how they move. And - AND! - they move! What other geological feature moves in 'real' time? Except for tides and river driven phenomena.

A number of geological phenomena move in real time, even on Mars. Boulders move down hill, craters form, gullies form, dust devil and slope streaks come and go. There is nothing special about dunes in this regard.



'Layering' may be fascinating geologically, but to me, the non-specialist (and I can claim to be an eminent non-specialist!) a dune wins!

It is very easy for non-specialists to appreciate layers. Terrestrial rocks are layered, layers record the history of a planet. Layered rocks can be just as aesthetically pleasing as dunes. And the significance of layered rocks has been explained repeatedly, through many missions.


I'm not running down academics.

Actually you have been.

QUOTE]Of course the descriptions and discussions of this pictures should be written by the academics and scientists whose work they are (and aren't we priveliged to be in their seminar?) but I can remember, and I'm sure you can, teachers from primary to post-graduate level, who couild bring thier subject to life, as it were.[/QUOTE]

Science communicators and professional educators are a very different beast from the journalists you mentioned earlier!

Jon

JohnD
2008-Aug-12, 10:38 PM
Have I offended you personally?
Because you are taking this very seriously.

See my poll
John

JonClarke
2008-Aug-13, 11:20 PM
Have I offended you personally?
Because you are taking this very seriously.

Not really, but I do think you are being unfair to researchers.

Jon