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

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?

[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.

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.