Saturn's moon Titan is one of the most fascinating worlds in our solar system. It has a thick atmosphere packed with aerosols, a rich system of hydrocarbon rivers and lakes, and islands that disappear and reappear like something out of Lost.
But Titan is also home to another mysterious feature: sand dunes. These formations have puzzled scientists since they were first observed by the Cassini-Huygens mission in 2004. The dunes appear to be sculpted by east-to-west winds, when in reality the moon's winds predominantly blow in the exact opposite direction, from west to east. What gives, Titan?
A study published today in Nature tackled that question, using a wind tunnel to recreate conditions on the distant moon. "This work has been a long time in the making," lead author and planetary scientist Devon Burr told me over Skype, after giving me a great tour of her current digs in Antarctica (she is there to hunt meteorites as a part of the ANSMET project).
"About ten years ago we got the first images from the surface of Titan," she continued, "and to everyone's surprise, these images showed these really extensive, very long, linear dunes that covered most of the tropics."
Burr told me that the scientific community was initially perplexed by the presence of sand full-stop, let alone their accumulation into counterintuitive dune shapes.
"On Earth, we think, 'well, sand is everywhere,'" she said. "But it's actually a specific grain size, and it's not readily made everywhere. On Earth, sand is made out of breakdown of the crust but it appears that on Titan, that sand is made maybe from materials from the atmosphere."
According to spectral analysis—in which the light emitted or absorbed by an object is used to unlock its composition—Titan's sands do indeed have more in common with its atmosphere than its crust. But the shape of the moon's dunes proved trickier to explain.
In a 2010 paper, planetary scientist Tetsuya Tokano suggested that Titan's wind direction might reverse for very short periods, when the moon's summer crosses hemispheres. Though these westerly winds would be very brief, they could be much stronger than the prevailing easterlies, allowing them more influence over the shape of the dunes.
That's where Burr's work with an antiquated wind tunnel came in handy. Burr and her colleagues spent six years refurbishing the long defunct equipment, which was originally designed in the 1980s to simulate conditions on Venus. In the summers of 2012 and 2013, her team was at last able to run tests in the wind tunnel, located in the Planetary Aeolian Laboratory at NASA Ames Research Center.
"Essentially, what we did was put 23 different unique combinations of sediment density and size in the wind tunnel," she told me, "and then blew the wind, inspecting very closely to see at what wind speeds sand was saltating"—in other words, how the particles were moved and distributed under different conditions.
The team discovered that in order to shape Titan's dunes, the moon's westerly winds must be about 50 percent stronger than previously predicted. Though these westerlies only prevail about two percent of the time on Titan, they are the driving forces shaping the moon's dunes. "That's what does all the geomorphic work," Burr confirmed.
The findings are further proof that Titan is a world of extremes, in which brief periods of seasonally-driven unrest can have more influence than the moon's "normal" weather during the rest of the Saturnian year. It also demonstrates how a discarded, antiquated piece of equipment can be reinvented to resolve modern questions.
Along those lines, Burr plans to use Ames wind tunnel to investigate Titan's past. "We just had some new work funded, and we get to go back now and experiment with different paleoclimates on Titan," she told me. "There's the thought that Titan has gone through some very significant climatic shifts over the age of the solar system, and the atmosphere we see there now may be unusual."
Given that the moon supports such a variety of bizarre features, it wouldn't be surprising to find out that it's an atypical place not just by the solar system's standards, but by its own as well. If that's true, then we are just lucky enough to catch it during its more dynamic episodes, when rivers are flowing, winds are blowing, and sand is formed in its skies.