Earth’s most spectacular dunes can be found in its oldest desert, the 55-million-year-old Namib in southwestern Africa, where windswept piles of sand crest more than 100 meters above the surrounding landscape. The dunes there are memories of ancient winds, twisted like multimillennial weather vanes by the air that flowed across the region tens of thousands of years ago.
The Namib’s dunes may be unmatched in majesty on our planet, but not in the solar system. A billion kilometers away radar imaging by NASA’s Cassini orbiter has uncovered wind-sculpted dunes of similar size and shape. They snake across the equatorial deserts of Titan, Saturn’s largest moon—the only moon in our solar system with a substantial atmosphere. These dunes, however, are curiously askew. They point east, exactly opposite of what would be expected from the moon’s prevailing winds, which atmospheric circulation models suggest must blow westward. Now, some scientists are suggesting a shocking solution to these backward-facing drifts: They may be sculpted not only by wind, but also by electrostatic forces. That is, by static cling. The results appeared March 27 in Nature Geoscience.
“Why do Titan’s dunes look like they’re going the wrong way, against the wind?” asks study co-author George McDonald, a graduate student at Georgia Institute of Technology. “One way to explain it is if the sands are sticky, and don’t move very much most of the time from the wind, which is what you’d expect if the sands were electrified—electrostatically charged and clumped together.” He adds, “It turns out the sands of Titan should be very susceptible to charging in a unique way.” In this scenario only Titan’s fastest winds—stormy gales that might periodically blow in the opposite direction due to large seasonal shifts in the moon’s atmosphere—would sculpt the dunes. Knowing these dynamics, planetary scientists could then better study Titan’s dunes as long-term weather vanes, too, piecing together the moon’s fluctuating climate patterns across millennia.
Sands on Earth can also be sticky—usually when they are wet, something familiar to anyone who has built a sand castle by the seashore. Earth’s sands can also become charged, typically when they are blown aloft in sandstorms or volcanic plumes. Continual midair collisions create friction and build up static charge between the grains, which often discharges as lightning.
On Titan the situation is altogether different. Like Earth, it has a thick nitrogen-suffused atmosphere and rainwashed surfaces carved with rivers, canyons and seas. But Titan only receives about 1 percent as much sunlight as our planet, giving it an average temperature of –179 degrees Celsius and bone-dry air filled with smoglike organic haze. Its “rock” is water ice and its rains are hydrocarbons such as methane and ethane condensed by the cold. The frozen hydrocarbon “sand” of Titan’s dunes, it is thought, is composed of the same low-density volatile vapors that, on Earth, waft from mothballs and plastics. Titan’s gravity is lower, too. All that, it turns out, is a perfect recipe for building up immense charges among windblown, tumbling grains and creating ultrasticky sand dunes—or long-lived sand castles that would resist crumbling for months. “This would be electrostatic charging on steroids,” says co-author Joe Dufek, a professor of Earth and atmospheric sciences at Georgia Tech.
“Imagine opening a box from Amazon on a cold day and seeing it’s filled with those annoying plastic packing peanuts. Imagine your cat getting in the box and the peanuts sticking all over it. That’s kind of what’s happening with hydrocarbon sands on Titan—this force we don’t normally think about on Earth is likely having a big impact on the landscape there in a way that isn’t very intuitive.”
The idea that Titan possesses electrified sand is not exactly new, having appeared for more than a decade in scientific literature and elsewhere. Consider this whimsical snippet from a 2007 Titan-themed poem by study co-author Mike Malaska, a scientist in the Planetary Ices Group at the NASA Jet Propulsion Laboratory: “Methane sky;ethane drizzle.Surface made of organic shizzle.Dunes of plastic;it’s fantastic.Let’s get stickyand electrostatic.” But no one had robustly tested the idea.
Titan in a Tube
Lead author Josh Mendez, another graduate student at George Tech, did just that. Mendez, McDonald and colleagues pumped pressurized nitrogen gas into a small hydrocarbon-coated tube. Then they tumbled and spun tiny particles of polystyrene, biphenyl, naphthalene and other hydrocarbons inside it, watching as the material collided, charged and clumped under conditions simulating Titan’s atmosphere and lower gravity. After each run they dumped the particles out of the tube and through a device that measured the particles’ accumulated individual electrostatic charges.
The results suggested the static-cling sands in Titan’s dunes should be at least an order of magnitude more resistant to winds than sand on Earth, and perhaps much more so. The experiments took place at room temperature, using uniform particle types and sizes, but previous studies have hinted that lower temperatures and varied particle sizes and compositions can magnify electrostatic charging. The group hopes to conduct further experiments at cryogenic temperatures, and plans to perform computer simulations investigating how great numbers of varied particles might clump together to form larger structures. “We don’t really understand this at the macroscale yet,” Dufek says. “How millions or billions of varying grains would interact with this [charge] is tricky. The stronger the charge and the longer they hold it, the bigger the structures you could build, but we don’t know the exact size or morphology.”
The results also suggest a mechanism for how Titan’s sands form in the first place. The dunes are thought to be hydrocarbons based on their dark color in radar images, which means they are probably not produced by erosion and fragmentation of the moon’s water-based bedrock. But electrostatic charging could also occur between microscopic particles of smog in Titan’s air, allowing the particles to glom together, grow and precipitate to its surface. Which means Titan’s electric hydrocarbon “sand” may really more accurately be called snow. “On Earth, it would be like flurries that came down but never melted and formed sticky, persistent drifts,” Dufek says.
Other experts caution, however, that it is too soon to conclude Titan’s sands are electrified based solely on remote images of dunes and Earthbound laboratory experiments. According to Jani Radebaugh, a planetary scientist at Brigham Young University who was not involved in the study, these results are notable because they represent a big step forward in studying the Saturnian moon’s surface. “Appealing to electrostatic charging on grains complicates things,” Radebaugh says. “I suspect this might not be any more complicated than our just not having the models right—that surface winds just blow in the opposite direction than we think. But getting in the lab and working with these exotic materials could well be a stepping-stone to better understanding processes on Titan.”
Ralph Lorenz, a planetary scientist at Johns Hopkins University Applied Physics Laboratory who was also not part of the research, agrees that more work is needed before we understand Titan’s mysteries. “Electrostatic charging could be quite important in ultimately controlling how sand moves on Titan, particularly with regard to relations between dune orientation and [winds],” he says. Even so, he says, there are other explanations for the backward-facing dunes, largely contingent on what exactly they are made of and the speed and directionality of Titan’s winds. “To know what wind you need to make sand move on Titan, you really need to go there,” Lorenz says.
A Sticky Situation
In 2005 the Cassini orbiter dropped a small lander, Huygens, into Titan’s atmosphere, where the tiny spacecraft deployed parachutes and descended to the surface. But Huygens landed in a dry riverbed, not near dunes, and only operated for a short period before succumbing to the cold. NASA is presently awaiting formal proposals from planetary scientists for missions to Titan as part of the agency’s New Frontiers program. Lorenz is one of many experts working on such a proposal, hoping that a more thorough exploration of Titan’s surface is in the not-too-distant future. His is called Dragonfly, a quadcopter-style drone that would hop and fly across Titan’s terrain to investigate the dunes and more. If, however, Titan’s dunes are as electrically charged as this latest work suggests, approaching them may be a bad idea.
“We know dust is a problem on Mars, where it coats solar panels on landers and rovers,” Dufek says. “On Titan, it could be much worse. If you landed in a region with lots of particles, they would stick to any metal surface very efficiently, and they would stay there for a long time. To say this would cause difficulties is an understatement. Instruments that need clear fields of view—cameras, mass spectrometers and so on—could be crippled.”
Lorenz counters that electric sand on Titan wouldn't be a showstopper for surface exploration. "It's a basic principle of spacecraft design that you have to harden electronics against electrostatic discharge," he says. "Now, we would obviously pay attention to the sort of erosion or deposition considerations that blowing sand can introduce, but this is something that is more or less familiar. The Huygens probe landed on the surface of Titan and didn't instantly go dark with stuff sticking to it…. Anytime one goes to an alien environment, one must be cautious, but we’re not concerned."*
Still, this latest work is leading Mendez and McDonald to reconsider what they and many other planetary scientists used to hold certain—that Titan is a close-but-cryogenic cousin of Earth. “The data from Cassini does give the impression of an Earthlike place—Titan has a thick atmosphere, lakes, dunes, mountains and maybe volcanoes,” McDonald says. “But you look deeper and it’s not like Earth at all.”
“What we really gained from this experiment was a sense of how toxic Titan is for humans,” Mendez adds. “We were getting headaches from tumbling naphthalene and biphenyl under fume hoods—and we deliberately picked some of the safest, most benign things that may exist on Titan to work with. This stuff is toxic to touch; it’s toxic to breathe. You wouldn’t want to live there.”
*Editor's Note (4/6/17): A paragraph was added after posting to reflect the ongoing debate over the potential risks of electrostatic sand for surface explorations on Titan.