Granular flows are, not to put too fine a point on it, horrible. In some situations, they flow like a normal fluid, but a minuscule change will cause the flow to jam. In the right situation, the particles will experience enough friction to pick up charge. Then, if the particles are fine enough, the static discharge decorates the landscape with bits of your grain silo.

It's not all bad, though: the movement of sand dunes and their singing in the wind is also part of granular flow. We all love it when inanimate bits of rock serenade us as we die of thirst. And that's just the Earth. Titan also has dunes that seem to be oriented in the wrong direction relative to the prevailing winds, and scientists may have found the corner piece that helps them solve this puzzle: static electricity.

Dunes are not hills of sand; they are more like giant ripples that are oriented in a specific direction. On Earth, that direction is dictated by the prevailing wind. The sand is blown about by the wind until it falls over the ridge on top of the dune and is protected beyond that. Any protrusion that pokes out the wrong way is quickly etched away by the wind.

The dunes on Titan are not made of sand. They are made of tiny frozen mothballs—well, old school mothballs, anyway—pellets of organic chemicals. Titan also has winds, so we might expect the dunes to have the same orientation relative to the prevailing winds as they do on Earth. But the mothball dunes don't lie in the right direction.

So something else is helping to control their flow. One possibility is that static electricity is holding the dunes together and changing their orientation. Why does that work on Titan but not on Earth?

We are all too soggy

Static electrical charge is built up by friction, and as particles bang into each other, electrons get knocked from one particle to another. Over time, a large charge can build up. On Earth, it's very rare for the charge to build up to the point where static electricity dominates over wind; it takes highly unusual conditions, like powder flows in pharmaceutical manufacture, to observe static electricity triumphing over inertia.

That's in part because static charges don't just build up—they are also constantly discharging as well. The air usually has quite a bit of moisture in it, and it's quite happy to carry charge back and forth between particles. This flow of charge limits the total difference in charge. Even if you build up charge faster than the water can transport it, air has a relatively low breakdown voltage, so you get small (and not so small) lightning strikes to reduce the charge differences.

Finally, all particles conduct electricity to some extent. Even though sand particles are good insulators, they conduct electricity well enough to slow the buildup of charge differences. When two oppositely charged particles hit each other, any charge excess slowly moves between them, equalizing the charge difference.

On Titan, however, things are very different. There's almost no water. The atmosphere has a higher pressure, so the breakdown voltage is higher. And hydrocarbons are much worse than sand at conducting electricity. Combine that with the winds of Titan pushing the mothballs around and you have a recipe for high static charges that may influence the orientation of Titan's dunes.

Mini Titan dunes

That's the theory anyway, and any good theory relies on experimental data. Unfortunately, there's no easy way to measure the charge stored in Titan's dunes. But a group of researchers has measured how much charge can typically build up under Titan-like conditions.

They built a drum that was lined with hydrocarbon and partially filled the drum with sand—they used various hydrocarbons, including napthalene (old school mothballs), as sand. The drum was then filled with dry nitrogen to provide a non-conducting atmosphere similar to Titan's. A static charge was built up by rotating the drum for about 20 minutes. The researchers then tipped the drum up, allowing the particles to fall through an electrometer to measure their charge.

At least, that's what the researchers tried to do. To get the particles to fall out, they had to set the drum spinning faster so that the particles could dislodge each other. Even then, some particles and agglomerates of particles remained stuck to the side of the drum because of all the static.

Compared to sand particles under Earth-like conditions, the organic particles collected up to an order of magnitude more charge per unit of surface area. But while the maximum was higher, the charge distributions overlap by quite a bit.

The amount of charge alone doesn't help, because it lacks context. The effect of electrostatic forces is to introduce an additional frictional sticking force that binds a particle to its home dune. The wind applies a mechanical force that must overcome this force. So the relevant number is the ratio between the electrostatic forces and the mechanical forces due to wind. Under Titan-like conditions, the researchers estimate that about one-third of the particles have a large enough charge that their response is dominated by electrostatic forces. For the rest, the wind still dominates.

The researchers go to town on that simple condition, including inertia as well, to come to a bunch of curves that all tell pretty much the same story. On Titan, electrostatic forces should not be ignored.

In comparison, particles tested under Earth-like conditions never collected enough charge to generate electrostatic forces greater than the force of typical wind conditions. The agreement of their Earth-like test with actual conditions that seem to be present on Earth suggests that their Titan experiment isn't too far off the mark. As the researchers point out, though, the set of conditions that they chose to test were very mild. Essentially, the drum speed was set so low that the amount of charge that accumulated on the particles in the drum was about the minimum expected on Titan.

What the paper lacks, unfortunately, is a strong analysis of how increased static electric forces will change the orientation of dunes. Yes, it's pretty clear that the additional static forces will change the rate at which dunes move, and it will change the threshold wind conditions required to move particles. One might even imagine that pockets of material develop that are so strongly charged that, for a short period of time, they remain fixed as the dunes roll by. None of that, however, seems to add up to a change in dune orientation, and the researchers state that wind tunnel tests are required to check their idea.

Nature Geoscience, 2017, DOI: 10.1038/NGEO2921