March 31, 2017 | David F. Coppedge

Dry Titan Has Static Cling

Imagine putting a cat in box of packing peanuts. Something like that happens when Titan’s sand dunes grow, some scientists think.

Long gone are the days when Titan was imagined to be an ocean world, buried half a kilometer deep in liquid methane. After Cassini found sand dunes under the haze, that scenario was over. Large dunes wrap around the equatorial regions of this giant moon of Saturn. Experts were so baffled by first radar images of the surface, they at first described them as “cat scratches”.

Scientists at the Georgia Institute of Technology just published a new theory that adds to the bafflement. The grains in the sand appear to be electrified. “The grains that cover Saturn’s moon act like clingy packing peanuts,” the headline reads. One of the researchers has a vivid imagination:

“If you grabbed piles of grains and built a sand castle on Titan, it would perhaps stay together for weeks due to their electrostatic properties,” said Josef Dufek, the Georgia Tech professor who co-led the study. “Any spacecraft that lands in regions of granular material on Titan is going to have a tough time staying clean. Think of putting a cat in a box of packing peanuts.”

The new theory was prompted by a puzzle. The dunes seem to run the wrong way, against the prevailing winds. The scientists built a simulator to model what happens to grains in a dry nitrogen atmosphere. Sure enough, static electricity caused the particles to clump together.

“Titan’s extreme physical environment requires scientists to think differently about what we’ve learned of Earth’s granular dynamics,” said Dufek. “Landforms are influenced by forces that aren’t intuitive to us because those forces aren’t so important on Earth. Titan is a strange, electrostatically sticky world.”

This implies that Titan and its atmosphere are very dry. Except for some large lakes near the poles, the static-clingy dune particles fall out of the atmosphere and clump together. The relatively low average wind speed seems inadequate to build large dunes out of the clumps. New Scientist surmises that “More powerful winds from storms or seasonal changes could blow otherwise-stable sands eastward, forming the dunes that we see today.” And about that dryness, Leah Crane writes,

Grains on Titan can maintain that charge and stick together for much longer than particles on Earth could, because of their low density and the dryness of Titan’s atmosphere.

That might explain the strange opposite orientation of the dunes. But where do the particles come from?

“The atmospheric particles are very small, so they can’t be the things blowing around in those dunes, but this is one way that we could make them grow,” says Jani Radebaugh at Brigham Young University in Utah.

Once enough particles had clumped together, they would fall out of the sky, coating the moon’s surface like electric snow.

But Titan was not supposed to be dry. Its atmosphere was supposed to be drenched with liquid methane and ethane. Yet hydrocarbons must form continuously because of known interactions in the atmospheric methane with the solar wind. It was a major surprise to planetary scientists when the Huygens Probe landed with a thump on a moist sand of organic-coated particles. It had been designed to float on the fabled oceans!

The new hypothesis is published open-access in Nature Geoscience.

None of the articles worry about the time factor. They should. If particles have been falling out of the atmosphere for billions of years, where are they? It’s the same problem as with the missing oceans, only this time with static-cling particles. They don’t seem to want to talk about it.



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