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."

Comments

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."

If you walk without rhythm

[Anonymous Coward]

you won't attract the worm

Re:If you walk without rhythm

[BlortHorc]

The spice must flow!

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.

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.

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: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:Controlled for all factors?

[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 and to compensate for gravitational differences they use walnut shell dust 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: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.

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.

Re:What about gravity?

[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:What about gravity?

[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 experiments calibrated to measure different aspects of the problem?

And no, I don't have a specific formula to share. But I also have not conducted a detailed study (and I never claimed I did) so I do not have a detailed understanding of the dynamics in play. Still, I would expect formulas used in fluid dynamics and, get this, the formulas related to gravity, would likely apply. And given there is probably an electrostatic effect of some kind with the small particles, formulas related to electricity and magnetism might be involved.

In any event, my assertion was that it *should* be possible. I don't have any special knowledge of the problem.

Also, sure, there are suspended particulates in the Martian atmosphere. 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. And before you ask, I'm basing that statement on having observed drifting sand.

Finally, let me direct to you my signature, "If it works in theory, try something else in practice." I have no delusions that my "theory" is any more correct than any other.

Re:What about gravity?

[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 more complicated. To solve it on a computer you've first got to work out what the computer has to do - that's the hard bit and probably only possible after collecting wind tunnel data. There is no "physical/mathematical formula to share" that's going to cover all situations and it's not trivial to work out where to start.

Who says its wind?

[Hognoxious]

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

Is there such a thing as marsquakes?