Particles that cover the surface of Saturn’s largest moon, Titan, are “electrically charged” and can cling together for months, scientists have found for the first time. When the wind blows hard enough, Titan’s non-silicate granules get kicked up and start to hop in a motion referred to as saltation.
As they collide, they become frictionally charged, like a balloon rubbing against your hair, and clump together in a way not observed for sand dune grains on Earth – they become resistant to further motion.
They maintain that charge for days or months at a time and attach to other hydrocarbon substances, much like packing peanuts used in shipping boxes here on Earth. “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, from Georgia Institute of Technology in the US.
“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,” Dufek.
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The electrification findings may help explain an odd phenomenon. Prevailing winds on Titan blow from east to west across the moon’s surface, but sandy dunes nearly 300 feet tall seem to form in the opposite direction.
“These electrostatic forces increase frictional thresholds,” said Josh Mendez Harper, a doctoral student at Georgia Tech. “This makes the grains so sticky and cohesive that only heavy winds can move them. The prevailing winds aren’t strong enough to shape the dunes,” said Mendez Harper.
To test particle flow under Titan-like conditions, the researchers built a small experiment in a modified pressure vessel in their Georgia Tech lab.
They inserted grains of naphthalene and biphenyl – two toxic, carbon and hydrogen bearing compounds believed to exist on Titan’s surface – into a small cylinder.
Then they rotated the tube for 20 minutes in a dry, pure nitrogen environment (Titan’s atmosphere is composed of 98 per cent nitrogen). Afterwards, they measured the electric properties of each grain as it tumbled out of the tube.
“All of the particles charged well, and about 2 to 5 per cent didn’t come out of the tumbler,” said Mendez Harper.
“They clung to the inside and stuck together. When we did the same experiment with sand and volcanic ash using Earth-like conditions, all of it came out. Nothing stuck,” he said.
Earth sand does pick up electrical charge when it’s moved, but the charges are smaller and dissipate quickly. That is one reason why you need water to keep sand together when building a sand castle. Not so with Titan.
“These non-silicate, granular materials can hold their electrostatic charges for days, weeks or months at a time under low-gravity conditions,” said George McDonald, a graduate student at Georgia Tech.
The findings have just been published in the journal Nature Geoscience.