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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."
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.
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?
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