Dunes On Mars: How Sand Shifts Without Wind

It's a scientific mystery: How did dunes form on Mars when the wind doesn't seem strong enough to move the sand? Jasper Kok tells Guy Raz he may have the answer — it has to do with sand "bouncing" and "splashing" in a way that's different than what happens on Earth.

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GUY RAZ, host:

Welcome back to ALL THINGS CONSIDERED from NPR News. I'm Guy Raz.

Picture the surface of the planet Mars. It's a little like a red desert - sand shaped in ripples and dunes. The trouble for scientists is that it takes wind to create those sand dunes. But the Martian atmosphere is so thin that only rare hurricane-strength winds - winds over 100 miles per hour - can move the sand.

So, how then did those familiar Martian dunes form? Well, Jasper Kok is with the National Center for Atmospheric Research in Boulder, Colorado, and he may have the answer to this Martian mystery.

Mr. JASPER KOK (Postdoctoral Fellow, National Center for Atmospheric Research): Actually, most of my work is on Earth, and because here, blowing sand is also very important in creating dunes and especially in creating dust storms. And so I developed a sophisticated computer model to model blown sand on Earth.

But since I had this sophisticated computer model, I thought, well, why not apply to Mars and see what happens. And the result is very surprising.

RAZ: So, if the wind on Mars is generally weak and dunes and ripples are still being formed, I mean, some big, some small, how is that happening?

Mr. KOK: So, you need those hurricane-strength winds to get the sand moving, but once you have the sand moving, it's really easy to keep it moving. Sort of like when you get on a bike. It's really hard to get the bike moving, it costs a lot of exertion, but then once you're going, it's much easier to keep it going.

Now, Mars, in comparison to Earth, the gravity is much lower and the air density is also much lower, so there's much less air resistance. So, when the wind picks up sand on Mars, it's kind of like playing golf on the moon. If you hit a golf on the moon, it's going to go much farther and higher than on Earth.

And it's the same with sand on Mars. So, if the wind picks up sand on Mars, it's going to have these much larger jumps than it does on Earth. And during those much larger jumps, it can pick up a lot of speed from the wind. And then when it comes back down and it slams into the surface it, splashes up other particles, other sand particles, sort of like billiard balls knocking each other around. And it's this process of splashing that keeps wind-blown sand going.

RAZ: Okay. So, a sand particle gets kicked up, it travels far and fast, it hits the ground - I was going to say hits the Earth, but it's on Mars - so, it hits the ground and that creates more motion and more sand particles get kicked up and that keeps going and going and when does it stop?

Mr. KOK: Right. So, that doesn't stop until the wind drops by about a factor of 10. So, before this finding, you would've thought that, you know, you would have sand moving for just a fraction of a second. But now, it can keep going for a much longer time period. Now, exactly how long depends on exactly how the wind varies. But it could be seconds, minutes, maybe even an hour - but much longer than without this effect.

RAZ: Jasper Kok is a fellow at the National Center for Atmospheric Research in Boulder, Colorado.

Jasper Kok, thanks so much for joining us.

Mr. KOK: Thanks so much for having me.

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