Low-density organic granules that cover the surface of Saturn’s moon Titan are ‘electrically charged,’ according to new experiments done by scientists at the Georgia Institute of Technology.
An artist’s rendering of the surface of Saturn’s largest moon, Titan. Image credit: Benjamin de Bivort, debivort.org / CC BY-SA 3.0.
The research, published in the journal Nature Geoscience, was led by Josh Méndez Harper, a geophysics and electrical engineering doctoral student at Georgia Tech.
“When the wind blows hard enough (approximately 15 mph), Titan’s non-silicate granules get kicked up and start to hop in a motion referred to as saltation,” Méndez Harper and co-authors said.
“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.”
The granules maintain that charge for days or even 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,” explained Georgia Tech Professor Josef Dufek, co-author of the study.
“Any spacecraft that lands in regions of granular material on Titan is going to have a tough time staying clean.”
Study results of wind speed needed to affect surface granules on Titan. Image credit: J.S. Mendez Harper et al / Georgia Tech.
To test particle flow under Titan-like conditions, the team built a small experiment in a modified pressure vessel in a lab.
“To recreate granular flows on Titan, we used two organic materials inferred to exist on the surface, naphthalene (C10H8) and biphenyl (C12H10), as well polystyrene as an analogue,” the researchers explained.
“Naphthalene is the simplest fused polycyclic aromatic hydrocarbon and can be used as an end-member proxy for that entire class of molecules. Naphthalene has been identified in Titan’s atmosphere from Cassini/CAPS data.”
“Other work suggests that polyphenyls are likely to be present in the Titan system. The simplest polyphenyl molecule is biphenyl.”
“Polystyrene, an aromatic compound, serves as a good analogue material with similar physical properties.”
The researchers inserted grains of naphthalene and biphenyl into a small cylinder. Then they rotated it for 20 minutes in a dry, pure nitrogen environment. 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% didn’t come out of the tumbler,” Méndez Harper said.
“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.”
Earth sand does pick up electrical charge when it’s moved, but the charges are smaller and dissipate quickly. That’s 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 co-author George McDonald, a graduate student in the School of Earth and Atmospheric Sciences at Georgia Tech.
The team’s results may help explain an odd phenomenon: prevailing winds on Titan blow from east to west across the surface, but sandy dunes nearly 300 feet (90 m) tall seem to form in the opposite direction.
“These electrostatic forces increase frictional thresholds,” Méndez Harper said.
“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.”
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J.S. Méndez Harper et al. Electrification of sand on Titan and its influence on sediment transport. Nature Geoscience, published online March 27, 2017; doi: 10.1038/ngeo2921
This article is based on a press-release from the Georgia Institute of Technology.
