Martian dune dynamics Options

[Arizona Dave]

It an ok video, but I must point out at saying that Mars Atmosphere is Less that 1% as "Thick" as Earth's" is both incorrect and misleading, its supposedly less than 1% as Dense...im sure this will rub a few ppl the wrong way, but in science terminology is very important. (there is debate and precedence for dust blocking tubes in all pressure sensors used to date, and the Arm that was designed to measure wind speeds broken upon impact on Curi - and dust devils based on rotation havnt presented speeds over about 50-55 mph...less than half the minimum AIMS needed to replicate them in their near-vacuum experiment -- another discussion I know)

Thick vs Dense - Not only does they way one describe something give other's important information about that something, using wrong terms can drastically change the meaning of something, leading others to design things incorrectly, and/or perpetuate incorrect information. Articulate and Precise communication is important, and especially so when presenting things for the public sphere - whether or not the average person knows thick vs dense, many do, and someone unfamiliar with previous data sets and facts could very well take the "thick" to mean distance extending up from the surface, or even some other way...in other words, regardless of who the audience is..clarity and correctness should prevail, of for no other reason than to be able to always say "We said X, and it meant X, not Y"

The fact that these dunes have large and small grains at the surface that migrate, is empirical proof that some force is moving them. We know the sizes of the grains, we know the composition of much of the minerals, and thus the density and weight of the grains, we know Mar's gravity, and using all those data sets, we can then calculate what force it takes to move grains...that amount of force then tells us what force of wind it takes to move them...which all leads back to atmospheric density, not how thick the atmosphere is.

The distance these dunes (and others) move per MOC and HiRise images contradicts known wind speeds vs *observed* Atm pressures...so something is off, or some other forces assist (static elect, CO2 out gassing, Water Vapor suspension?), and Im hoping they can really study these things to figure out why and how things are observed moving in atmospheric pressures that would suggest only the finest of particles could be moved.

Im also very intrigued by the colors seen in these latest images and all the different mineral types being seen in close-ups.

Mars keeps throwing curve balls.

[Deimos]

As I wandered out of the thick forest, the air filled with a thick fog, and I looked forward to getting home for a hearty meal of thick soup, I also pondered whether "Mars' Atmosphere is Less that 1% as "Thick" as Earth's" is correct. Since I use that phrasing, and it seems natural and accurately descriptive to me, I've just checked several hard-copy and online dictionaries and they all had dense, abundant, close together, or something similar among the top 3 definitions. 'Dense", in atmospheric science, is a precise word, and that level of precision is not always what one wishes to communicate. Science communication needs an expansive palette, thickly filled with options.

[My usages above came straight out of Oxford, but are expressions I've used conversationally.]

[mcaplinger]

The distance these dunes (and others) move per MOC and HiRise images contradicts known wind speeds vs *observed* Atm pressures...so something is off...

Uh, no. See http://www.mars.asu.edu/christensen/docs/e...unes_JGR_91.pdf

[Arizona Dave]

As I wandered out of the thick forest, the air filled with a thick fog, and I looked forward to getting home for a hearty meal of thick soup, I also pondered whether "Mars' Atmosphere is Less that 1% as "Thick" as Earth's" is correct. Since I use that phrasing, and it seems natural and accurately descriptive to me, I've just checked several hard-copy and online dictionaries and they all had dense, abundant, close together, or something similar among the top 3 definitions. 'Dense", in atmospheric science, is a precise word, and that level of precision is not always what one wishes to communicate. Science communication needs an expansive palette, thickly filled with options.

[My usages above came straight out of Oxford, but are expressions I've used conversationally.]

Thick fog is a great example...you can see it, taste it, feel it (as in its wetness) - and the "thicker" it is, the less one can see through it...just like "thick" soup - its not as "runny"...And...to better bring that point home, Weather reports will often list fog banks as "thick & dense or just "Dense" or "Thick" - and there is a difference, thick fog banks extend up off the ground a high distance, and they use Dense to describe how hard it is to see through it, thus "Thick Dense Fog engulfed LAX airport today causing numerous flight delays"

Now, how about that unique and odd egg-shaped vein structure with what appears to be a dark patch of soil in and around it here on SOL 1191...wish we had some close-ups:

http://mars.jpl.nasa.gov/msl/multimedia/ra...DXXX&s=1191

How often have you said "My car vents are blowing pretty "thick" today :-) - never.

[Arizona Dave]

Uh, no. See http://www.mars.asu.edu/christensen/docs/e...unes_JGR_91.pdf

Interesting that the report is from 1991...before we had close-up images of actual dunes and thus particle sizes, and before we had more wind data.

Looking at the latest Curi Images, one might be prudent to reconsider that these dunes are of the same particle size, porosity, and mineralogical make-up that this 1991 report assumed...

I also think its funny that they consider images taken from orbit from 1981 as "Very High Resolution".

In any event, here is this: http://www.nasa.gov/centers/ames/research/...cumchamber.html

AMES had to use air speeds of "230 feet per second" (thats 156.8mph), and "Dust on Mars is extremely fine, being about the size of flour used in baking bread," Greeley said. For simulated Mars dust, researchers often use 'Carbondale red clay,' because it is less than 0.0000787 of an inch (two microns, or 2 millionths of a meter) in size." in order to get the dust devils to form.

They further went on to explain that "There is a difference between Mars wind blowing dust, and 'dust devils' lifting dust. Like a vacuum cleaner, a dust devil has lower pressure in its core, which means that it is easier for dust devils to put dust particles in motion than for wind to do so."

Which is saying "Dust Devils can move the fine particles easier than just blowing wind"...

Yet wind speeds have only been measured up to around 58 mph...or about 1/4th the AMES speed it took to lift particles.

I have worked with vacuum chambers before, once your down to the 10mb range, the effect of the remaining "air" on objects is virtually non existent...I even did the feather vs a quarter test in a 24" tall chamber with viewing glass...they fell at virtually the same speed...In order to get that remaining air to move anything in that chamber (we used it specifically to blow extremely fine dust off very high-tech ceramic circuit boards) we had to use extremely high pressure air, the reason was there just wasnt enough force at 10mb to move much of anything.

When I see particle sizes in the 1/4 to 1+mm range moving with the sand dunes, and the sand dunes themselves, I cant help but wonder exactly what force at 6.5mb and 7 to 70 ft/sec move those particles that are thousands of time larger than the 2 micron particles that AMES moved with 230 ft/sec / 156 mph winds.

I take what I see and try to make better sense of it, without being stubborn and assuming someone else "got it right".

The math wont lie.

[elakdawalla]

ADMIN NOTE: I split a few posts out from the main thread to keep it from being derailed. Please feel free to continue discussing how dunes work on Mars, but in the past conversations on semantics have gone in boring circles without adding anything of value to the forum; further posts about semantics will be removed.

[mcaplinger]

I take what I see and try to make better sense of it...

Moving dust is utterly unlike moving sand. Google for "martian saltation kok" for example.

J.F. Kok "Difference in the wind speeds required for initiation versus continuation of sand transport on Mars: Implications for dunes and dust storms"

We show here that saltation can be maintained on Mars by wind speeds an order of magnitude less than
required to initiate it. We further show that the resulting hysteresis effect causes saltation
to occur for much lower wind speeds than previously thought.

[HSchirmer]

Does grain movement on dunes differ from grain movement on bare rock?

That mention of morning fog raises an interesting point - phase changes.
The void space inside a well sorted sand dune should be huge.
Rather like activated charcoal, it should have a huge surface area for adsorption of gasses.
Throw in some perchlorates and you might have adsorption of liquids, which also dissolve gasses.

Perhaps the dunes "breath" a bit as they warm, releasing adsorbed gasses to form a transient denser atmosphere.
Heck, perhaps they go "mentos & diet coke" and release gasses dissolved in some aqueous phase.

[serpens]

It will be interesting to find out the composition of these active dunes and hopefully identify the source. Reworked local sediment, eroded impact glass from deposits on the rim and ejecta field, volcaniclastic material etc.