Sen—Scientists have used a wind tunnel to study the dunes on Saturn's largest moon, to find out what they are made of and why they appear to be formed in a direction opposite to that of Titan's prevailing winds.
Titan has a thick atmosphere and lakes filled with methane and ethane, making it the only Solar System body other than ours with liquid on its surface. The Cassini orbiter found wind-driven dunes in Titan's lower latitudes like those in the deserts of Earth, but hundreds of feet high and hundreds of miles long.
"The dunes are not made of silicates, sand, as on Earth or Mars," says Devon Burr, a planetary scientist at the University of Tennessee and formerly with the SETI Institute, and lead author of the paper published in the journal Nature. "They're hydrocarbons, and may possibly include particles of water ice that are coated with these organic materials." The source of this sand remains a mystery.
The direction of the winds producing the dunes is equally puzzling. The 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 west-to-east, contrary to the prevailing easterlies.
The usual models for wind transport need to be adjusted for Titan's thicker atmosphere and more viscous sand. Using a wind tunnel constructed for modelling what happens on Venus the team found the minimum wind speed needed to transport Titan's hydrocarbon-rich sand was higher than typical for the prevailing winds on that moon.
Burr explained to Sen: "The Titan Wind Tunnel is fairly basic, actually, because it's the refurbished Venus Wind Tunnel from the 1980s. To enhance repeatability, we used two observers, one at each observation port (upwind and downwind), and recorded multiple stages of movement. Threshold was considered 50 per cent of the bed in motion."
The winds on Titan occasionally reverse direction and dramatically increase due to the changing position of the Sun in its sky. 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 per cent of the time might have no effect on what we see," Burr told us. It is these relatively rare events that have shaped the dunes.
The new research provides important insights into wind-borne transport on other bodies, both those with very thin atmospheres (Mars, Pluto and comets like Rosetta's 67P/Churyumov–Gerasimenko) and thick, such as might be encountered in Earth-like exoplanets.
Burr added: "In jets on comets, like the comet 67P, and on Pluto, sediments are transported by very thin atmospheres. Our results for Titan's thick atmosphere substantiates the use of the old threshold wind speed models for modelling sediment transport by those thin atmospheres."
Wind transport dynamics also have down-to-Earth applications. They can be used to unravel climate changes in the past, including the ice ages, and "snowball Earth" when the entire planet was encased in ice and snow.
Co-author John Marshall of the SETI Institute notes 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."
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