Using a specially engineered wind tunnel, scientists have solved a puzzle about wind-blown dunes on a world that has some striking similarities to our own.
Titan, Saturn’s largest moon, has both a thick atmosphere and lakes filled with methane and ethane, making it the only solar system body other than our own with liquid on its surface. In its lower latitudes, the Cassini orbiter has found wind-driven dunes reminiscent of those seen in the deserts of Earth, but hundreds of feet high and hundreds of miles in length.
Dunes are also known to exist on Venus and Mars, but Titan is unlike those worlds. This raises two questions: (a) what are the dunes made of, and (b) why do they appear to be formed in a direction opposite to that of Titan’s prevailing east-to-west winds?
“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 [http://www.nature.com] describing the new results. “They’re hydrocarbons, and may possibly include particles of water ice that are coated with these organic materials.”
While the source of this otherworldly sand remains a mystery, more puzzling is the direction of the winds producing the dunes. This direction can be deduced from the streamline appearance of the dunes when they wrap around high points, such as craters or mountains. These streamlines indicate winds that are more west-to-east, contrary to the prevailing easterlies.
This conflict of reasonable expectation and appearance was solved when the research team realized that the usual models for wind transport need to be adjusted for Titan’s thicker atmosphere and more viscous sand. The team found that the threshold — or minimum — wind speed needed to transport Titan’s hydrocarbon-rich sand was higher than typical for the prevailing winds on that moon.
Burr and her coauthors made this discovery using a wind tunnel that had been constructed in the 1980s for modeling aeolian physics on Venus, notes co-author John Marshall of the SETI Institute. “It was a bear to operate, but Dr. Burr’s refurbishment of the facility as a Titan simulator has tamed the beast. It is now an important addition to NASA’s arsenal of planetary simulation facilities.”
This greater threshold wind speed solved the mystery of the dunes’ alignment. The winds on Titan occasionally reverse direction and dramatically increase in intensity due to the changing position of the Sun in its sky. Because the threshold wind speed is so high, only these stronger winds blowing from the west can move the sand and streamline 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 important insights into wind-borne transport on other bodies, both those with very thin atmospheres (Mars, Pluto and comets) and thick, such as might be encountered in Earth-like exoplanets.
Burr says that these results also have down-to-Earth applications.
“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.”
Wind transport dynamics are also important to unraveling climate changes in the past, including the ice ages, and the so-called “snowball Earth” episode when the entire planet was encased in ice and snow.
Marshall says that the research “has raised many questions about Titan. There we have low gravity, a dense atmosphere, and light-weight materials — a recipe for unusual aeolian activity. Our work has just begun.”
