Sand Dunes On Mars In Motion

TheNextCorner writes with news that NASA's Mars Reconnaissance Orbiter has detected ripples and shifts in the sand dunes on Mars, which means the surface of the planet is more dynamic than previously thought. Planetary scientist Nathan Bridges said, "Mars either has more gusts of wind than we knew about before, or the winds are capable of transporting more sand. We used to think of the sand on Mars as relatively immobile, so these new observations are changing our whole perspective." The article explains, "The air on Mars is thin, so stronger gusts of wind are needed to push a grain of sand. Wind-tunnel experiments have shown that a patch of sand would take winds of about 80 mph to move on Mars compared with only 10 mph on Earth. Measurements from the meteorology experiments on NASA's Viking landers in the 1970s and early 1980s, in addition to climate models, showed such winds should be rare on Mars."

Nah, not wind

[bhcompy]

It's just the sandworms. I hope the rover that goes to the sand desert regions has a thumper

Re:Nah, not wind

[bosef1]

"Stilgar, have we wormsign?"

"Usul, we have wormsign the likes of which even God has never seen."

Re:

[madhi19]

"Bless the Maker and all His Water. Bless the coming and going of Him, May His passing cleanse the world. May He keep the world for his people."

If you walk without rhythm

[Anonymous Coward]

you won't attract the worm

Re:

[masternerdguy]

But the worms have turned

Re:

[BlortHorc]

The spice must flow!

Re:

[GregC63]

I need melange.....

Obviously

[Anonymous Coward]

It's obviously aliens. Gogogogo History Channel Documentary!

Local storms...

[JoeMerchant]

Measurements from the meteorology experiments on NASA's Viking landers in the 1970s and early 1980s, in addition to climate models, showed such winds should be rare on Mars.

Yeah, and 80+mph winds were rare in Miami in the 1970s and early 1980s too.

Re:

[phayes]

Nice job missing TFAs point. It's not that there are no storm systems on mars which were judged capable of high winds, it's that the dunes moved without seeing the martian equivalent of hurricanes from orbit.

On the implied subject, the sample size of huriicanes going through Miami (or just hurricanes in general) since the mid 80s is too small to support any conclusions.

Re:

[JoeMerchant]

Nice job missing my point. Miami is a place that experiences relatively frequent hurricanes, but if you landed a probe there and gathered data from January 1, 1970 through January 1, 1990, I don't think, in 20 years of data collection, that you would have ever witnessed a single 80+mph wind event.

Yes, they also used data gathered from orbit and other sources, but, obviously, their methods were flawed in some way - perhaps relying too much on the only two surface probes they had was a part of the problem?

As

Re:

[phayes]

Your point is only valid if you can statistically show that Miami has had more hurricanes than it had previously. The sample size is too small so you cannot, Not everything can be pinned on the coattails of global warming.

Now, young grasshopper try to explain how the sand dunes moved in the absence of any storms visible from orbit. The theory up to now has been. they shouldn't. They do. Do you at last understand why this is news for nerds & not just an occasion for you claim that the higher than normal

Re:

[JoeMerchant]

Wow, a leap to global warming, but from where?

As for the absence of any storms visible from orbit, what's our Martian weather coverage and resolution like? Presumably, not all Martian dust storms are runaway planet-wide events.

There should be some appropriate jab at how conservatives take their own limited view of the cosmos and extrapolate it universally, all the while espousing a knowledge of statistics and sample sizes, but either being ignorant of how they really work, or more insidiously lying (to the

Re:

[phayes]

What, if not global warning, were you referring to in your first post by "Yeah, and 80+mph winds were rare in Miami in the 1970s and early 1980s too.--"?

Re:

[phayes]

much clearer, thanks.

Sands Move Everywhere

[LifesABeach]

When a wind storm on Mars covers a bunch of real estate, it's a easy guess that there's more than enough energy to move sand. Look any where there is sand, the stuff doesn't take much to find the inside of your shoe; go figure. Now JPL has me wondering if their next rover will be able to handle sand traversal; I guess we'll know by controlled experiment.

Dunes eh?

[masternerdguy]

He who controls the spice...no that's too easy.

Re:

[drinkypoo]

He who controls the spice...no that's too easy.

Travel to any Dune on Mars, without moving.

Controlled for all factors?

[Webs 101]

This bugs me: "Wind-tunnel experiments have shown that a patch of sand would take winds of about 80 mph to move on Mars compared with only 10 mph on Earth."

In order to move the sand, the wind must overcome friction. Sealed wind-tunnel experiments with different atmospheres can easily show that winds of low-pressure atmospheres need to have more energy to move sand than winds of higher pressure atmospheres.

But the wording of that statement doesn't mention gravity. In order to move the sand, the wind must overcome the force of friction, and of course friction depends on gravity. Did anyone adjust for Mars gravity being 38% of Earth's?

Re:

[Anonymous Coward]

> Did anyone adjust for Mars gravity being 38% of Earth's?

How about using less dense "sand" for the experiment?

Re:Controlled for all factors?

[Baloroth]

Took me about 30 seconds of computation to figure out: probably. KE=1/2 M*v^2. M(mars atm)=~.01M(earth atm). v(e)=10MPH, v(m)=80MPH. Works out (very roughly) to the same KE needed if you account for the reduced gravity. I'm certainly no fluid dynamicist though.

Re:

[msauve]

You could have probably saved NASA the cost of a million dollar wind tunnel experiment. Too bad they don't have anyone with a BS in physics on staff.

Re:

[Baloroth]

All physics should be checked by experimental methods before being considered valid. Otherwise, all you are doing is mathematics.

Re:

[msauve]

So, back to the original point - how did they duplicate Martian gravity (and sand, for that matter) in a wind tunnel? I suspect they were just "doing mathematics."

Re:Controlled for all factors?

[Baloroth]

Sand is pretty easy, they know the composition from probes. Gravity is a bit harder, but I'm guessing you could adjust the density of the sand to reflect the weight/ size ratio on Mars, which would give you a pretty accurate duplication of Martian conditions. Not exact, but these kinds of physics rarely are.

Re:

[Anonymous Coward]

It kills me how slashdotters upvote crap post like, "Oh, did any of the PhD specialist Mars scientists remember that Mars has lower gravity than Earth??" How the hell is a dead obvious observation like that "Insightful"? Look, I can do it too: I wonder if the scientists remembered that Mars soil probably has a different composition than Earth soil. Dang I'm a fricken genius.

Re:Controlled for all factors?

[WillHirsch]

If anyone's actually interested in the real answer to this, the wind tunnel they used appears to be called MARSWIT [nasa.gov] and to compensate for gravitational differences they use walnut shell dust [nasa.gov] among other particles as their working soil. To fully correct for gravity all you have to do is match the ratio of the air density to the particle density. Since rock is about 5 times denser than wood but Earth air is about 20 times denser than Martian air, they don't seem to be fully compensated - but perhaps at 80 mph equivalent winds the important accelerations are all much larger than 4m/s (g on Mars) and so the difference in gravitational effects isn't that important.

Re:

[llamapater]

that's a really loose approximation O.o the density is off and the friction between the particles is different. they'd get better results doing a test in one of those low gravity flights the cabins are already pressure controlled not much equipment modification to add a wind tunnel. and are they sure it's silicate sand.

The obvious is obvious

[dbIII]

Don't you think they may have noticed that Mars is a different planet? I think it's very safe to assume that if they are going to the trouble of considering an atmosphere so thin that ice sublimes without melting that the lower gravity would also be considered - even a computer game like X-Plane goes that far.

What about gravity?

[msauve]

"Wind-tunnel experiments have shown that a patch of sand would take winds of about 80 mph (nearly 130 kilometers per hour) to move on Mars compared with only 10 mph (about 16 kilometers per hour) on Earth."

I can understand how they could have a low pressure wind tunnel to simulate the lower Martian atmospheric pressure, but how did they reduce the gravity by almost 2/3? There's no mention of Mars' lower gravity anywhere in the article.

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[masternerdguy]

Sssh, you're spoiling the JPL illusion. Keep this up and we might actually have to send humans to Mars, which NASA would find totally unacceptable.

Re:

[LostOne]

Seems to me that if you can work out the air speed needed to move sand with air of a particular density in at 1 g, you should be able to do some math to work out how that would translate at 1/3 g.

Re:

[msauve]

By your logic, you should be able to do the same for the pressure difference, so why then the wind tunnel? What is the relationship between the wind speed needed to move sand around and gravity? Do you have a physical/mathematical formula to share?

Furthermore, it's known that the Mars atmosphere is dusty, so why would they think particulates just sit still? What's the particle size of the sand in those dunes? Density? They obviously weren't using Martian sand in the tests.

Re:

[LostOne]

Theoretically, you can, indeed, do the same for the pressure difference. After all, you can calculate the necessary forces to move any particular grain of sand. You can calculate various forces from the wind. You can even throw in electromagnetic effects. However, the volume of calculation makes that difficult at best. So the wind tunnel is useful, and would be a faster way to see how different air pressure, air composition, etc., affect the result. And what's to say there weren't dozens or more different e

Re:

[msauve]

"However, those are not the particles that form the dunes in the first place. The dunes would be composed of the larger (heavier) particles that need more force to move."

Sure. I'd expect that the winds naturally sort by particle size. Those light enough to be kept aloft, are. Dense/large ones stay in one place. Those in between, which can only be pushed around as dunes, are. Where's the surprise in that? The surprise would be if there were a particle size distribution with a large gap between ones which co

Re:

[dbIII]

By your logic, you should be able to do the same for the pressure difference, so why then the wind tunnel?

Because while gravity is simple and easy to calculate fluid flow isn't. The only time I've ever seen an analogue computer it was sitting next to a very long pipe designed to try to get something closely resembling laminar flow, and apparently even then it was hard to get the computer model and reality to agree. Throw in a rough loose surface and you get turbulent air full of sand and that gets a lot m

Re:

[TheOriginalRevdoc]

Simple solution: simulate the lower gravity with sand made of a material that's 2.6 times denser.

Dunes?

[nege]

Mars. Desert Planet. The only known source of the spice melange.

Martion sand storms are not new news

[AbsoluteXyro]

If Mars has been covered in a global sand storm [spacetoday.org] as recently as 2001, why is it such a shock that there might be winds strong enough to ripple up some sand dunes?

Re:

[phayes]

The dunes moved without us seeing anything like a major sandstorm beeing seen from orbit.

How much energy would be required?

[dutchwhizzman]

It's not about gravity, atmosphere density or any of that, but purely about the amount of energy required to move the sand. If there's enough solar energy to heat up the atmosphere, you'll get wind. If the atmosphere is less dense, it will require less energy to get winds to 80mph. that 80 mph is an arbitrary figure that shouldn't be looked upon the same as it is on earth.

Who says its wind?

[Hognoxious]

Is Mars seismicalogically, er, seismicly, ummm, I'll come in again.

Is there such a thing as marsquakes?
 

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[ChrisMaple]

Caterpillar corporation reports a great upswing in the purchase of earthmoving equipment to little green men.