Scientists have solved a puzzle about wind-blown dunes on a world that is similar to Earth by using a specially engineered wind tunnel.
Saturn's largest moon, Titan, has a thick atmosphere and lakes filled with methane and ethane, making it the only solar system body with liquid on its surface other than our own.
The Cassini orbiter found wind-driven dunes similar to those seen in the deserts of Earth in its lower latitudes, but hundreds of feet high and hundreds of miles in length, according to SETI.org.
Dunes have also been found on Mars and Venus, but Titan is unlike those worlds.
"The dunes are not made of silicates - sand - as on Earth or Mars," says Devon Burr, a planetary scientist at the University of Tennessee, Knoxville and formerly with the SETI Institute, and lead author of a paper in the journal Nature describing the new results. "They're hydrocarbons, and may possibly include particles of water ice that are coated with these organic materials."
Though the source of this strange sand remains a mystery, the bigger question is the direction of the winds producing the dunes. This can be deduced from the streamline appearance of the dunes when they wrap around high points, like mountains. The streamlines indicate winds that are more west-to-east, which differs from the prevailing easterlies, according to the release.
This problem of reasonable expectation and appearance was solved when the team realized that the normal models for wind transport had to be adjusted for Titan's thicker atmosphere and more viscous sand. The researchers determined that the minimum wind speed needed to transport Titan's hydrocarbon-rich sand was higher than normal for the prevailing winds on that moon.
Burr and her colleagues made this discovery while using a wind tunnel that had been made in the 1980s for modeling aeolian physics on Venus, co-author John Marshall of the SETI Institute confirmed.
"It was a bear to operate, but Dr. Burr's refurbishment of the facility as a Titan simulator has tamed the beast," said Marshall. "It is now an important addition to NASA's arsenal of planetary simulation facilities."
This threshold wind speed helped solve the mystery of the dunes' alignment. The winds on Titan reverse direction occasionally, which increases in intensity due to the changing position of the Sun in its sky.
Since the threshold wind speed is so high, only stronger winds blowing from the west can move the sand and the dunes.
"This work highlights the fact that the winds that blow 95 percent of the time might have no effect on what we see," Burr says. Much like the damage produced by infrequent, but "perfect" storms at sea, it is the relatively rare events that have shaped the dunes of this intriguing moon.
The new research provides new information into wind-borne transport on other bodies, both those with thin atmospheres, like Mars, Pluto and comets, and thick atmospheres, such as Earth-like exoplanets.
Burr believes that these results have down-to-Earth applications, according to the release.
"We see today sediment being wafted over the Sahara desert, across the Atlantic to South America. This wind-blow material accounts for much of the fertility of the Amazon Basin. So understanding this process is essential."
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