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Mystery of Titan's
dunes solved

Posted: 30 July 2010

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Seasonal changes that temporarily reverse the wind patterns on Saturn's moon Titan explain the orientation of the moon's dune fields, a new report says.

The dunes exist within 30 degrees of the equator, are about a kilometre wide and tens to hundreds of kilometres long, and in some cases stand over 100 metres tall. Data collected by ESA's Huygens probe as it descended through Titan's thick atmosphere in 2005, combined with basic principles of atmospheric circulation, suggested that surface winds generally flow east-to-west around the moon's equator. However, this assumption contradicted the first images of Titan's dunes, the orientation of which implying that the dominate winds blow from west-to-east.

Cassini radar sees sand dunes on Saturn's giant moon Titan (top) that are sculpted like Namibian sand dunes on Earth (bottom). The bright features in the upper radar photo are not clouds but topographic features among the dunes. Image: NASA/JPL (upper photo); NASA/JSC (lower photo).

The answer to this paradox appears to lie in seasonal changes. Tetsuya Tokano of the University of Cologne in Germany reports in a new paper featuring in a forthcoming edition of the journal Aeolian Research, that gusts of winds, persisting for no longer than about two years, sweep across the moon in a west-to-east direction, with much more strength than the 'normal' wind direction.

"It was hard to believe that there would be permanent west-to-east winds, as suggested by the dune appearance," says Tokano. "The dramatic, monsoon-type wind reversal around equinox turns out to be the key."

Tokano arrived at this conclusion by re-analysing a computer simulation model for Titan's global atmospheric circulation and adding in additional data on the moon's topography based on recent Cassini data. Rather than focusing on averages of wind patterns he studied the variations in the wind patterns at specific points in time. Equinoxes, which occur twice a Titan year (29 Earth years), seemed to provide the missing link. During this period, the Sun shines directly over the equator, heating and mixing the atmosphere such that the winds reverse and accelerate. A similar effect is seen over the Indian Ocean on Earth, during the transitional seasons between monsoons.

On Titan, the reversed winds race around the moon at speeds of 1-1.8 metres per second, much faster than the typical east-to-west wind speeds. "This is a subtle discovery – only by delving into the statistics of the winds in the model could this rather distressing paradox be resolved," says Ralph Lorenz, a Cassini radar scientist based at the Johns Hopkins University Applied Physics Laboratory. "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."