Cassini Helps with Dune Mystery

A recent paper published by Japanese astronomer Tetsuya Tokano suggests a solution for one of the mysteries found on Saturn’s moon, Titan. Using data retrieved from the Cassini spacecraft, he designed a model that solves one of the moon’s paradoxes.
 Cassini radar sees sand dunes on Saturn's giant moon Titan (upper photo) that are sculpted like Namibian sand dunes on Earth (lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes. (Source: NASA/JPL (upper photo); NASA/JSC (lower photo)
Cassini radar sees sand dunes on Saturn’s giant moon Titan (upper photo) that are sculpted like Namibian sand dunes on Earth (lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes. (Source: NASA/JPL (upper photo); NASA/JSC (lower photo)

NASA’s Cassini spacecraft obtained the first images of Titan’s dunes in 2005. In these images, it was evident that the winds and sands move from west to east. However, the basic principles of planetary rotation and their atmosphere led astronomers to think that Titan’s orientation was east to west. Up until now, this contradiction has led scientists in the European Space Agency (ESA) to claim this is a paradox, a puzzle without explanation.

Now, Tetsuya Tokano suggests a new approach in a recently published paper appearing in the journal Aeolian Research. According to his study, seasonal changes appear to reverse wind patterns on Titan for a short period. These gusts, which occur intermittently for perhaps two years, sweep west to east and are so strong they do a better job of transporting sand than the usual east to west surface winds. Those east to west winds do not appear to gather enough strength to move significant amounts of sand.

"It was hard to believe that there would be permanent west-to-east winds, as suggested by the dune appearance," said Tokano, of the University of Cologne, Germany. "The dramatic, monsoon-type wind reversal around equinox turns out to be the key." He is not the only scientist to hold this opinion; Ralph Lorenz, the lead author on a 2009 paper mapping the dunes, has recently published an article claiming similar causes made Titan to have this paradoxical state.

Similarly to Earth’s desert, the dunes on Titan are only in latitudes within 30 degrees of the equator. They are about a kilometer (half a mile) wide and tens to hundreds of kilometers long. Furthermore, they can rise more than 100 meters (300 feet) high. The sands that make up the dunes appear to be made of organic, hydrocarbon particles, and the ridges generally run west-to-east, as wind here generally sheds sand along lines parallel to the equator.

Most scientists, until this study, predicted winds in the low latitudes around Titan’s equator would blow east-to-west because at higher latitudes the average wind blows west-to-east. The wind forces should balance out based on basic principles of rotating atmospheres. In his work, Tokano re-analyzed a computer-based global circulation model for Titan he put together in 2008. That model, like others for Titan, was adapted from ones developed for Earth and Mars. The new input Tokano used was topographic data retrieved from Cassini. In his new analysis, he also looked more closely at variations in the wind at different points in time rather than the averages. Equinox periods jumped out.

The “years” in Titan are 29 times longer than the Earthly year. In this time period, equinoxes (equations between the length of day and night) occur twice – just like on Earth (March 21 and September 21). During an equinox, the sun shines directly over the equator, and heat from the sun creates atmospheric upwelling. The turbulent mixing causes the winds to reverse and accelerate. This rare kind of wind reversal happens on Earth, too: during equinoxes, it is observed over the Indian Ocean in transitional seasons between monsoons.

According to theory, the threshold of sand movement is about 1 meter per second (2 mph), a speed that the typical east-to-west winds never appear to surpass. Measures show that the episodic reverse winds on Titan blow around 1 to 1.8 meters per second (2 to 4 mph). Dune patterns sculpted by strong, short episodes of wind can be found on Earth in the northern Namib sand seas in Namibia, Africa.

Ralph Lorenz, a Cassini radar scientist based at the Johns Hopkins University Applied Physics Laboratory, said: "This is a subtle discovery – only by delving into the statistics of the winds in the model could this rather distressing paradox be resolved. This work is also reassuring for preparations for proposed future missions to Titan, in that we can become more confident in predicting the winds which can affect the delivery accuracy of landers, or the drift of balloons."

TFOT has covered other stories regarding Saturn’s moons, such as the origins of tiger stripes on Enceladus, revealed by researchers from the University of Melbourne, and Titan’s mysterious landscape, which is similar to Earth’s landscape.

For more information about the recent discovery regarding Titan’s sands, see NASA’s press release.