July 30, 2010

Study Explains Titan’s Mysterious Dune Patterns

A new study finds that the mysterious equatorial dune patterns on Saturn's largest moon, Titan, are the result of gusty winds blowing in the opposite direction of prevailing weather.

Titan's vast dunes of tiny organic, hydrocarbon particles cover more than one-fifth of its surface, but one band in particular has puzzled scientists for years.

The band's shape suggested it was formed by winds flowing from west to east, despite climate models predicting that surface winds nearly always blow in the opposite direction.

The climate models were based upon basic principles of planetary atmospheric rotation and data from the European Space Agency's Huygens probe, which suggested surface winds generally flow from east to west around Titan's equator.

However, the first images of Titan's dunes, obtained from NASA's Cassini spacecraft in 2005, suggested the sands and winds were blowing in the opposite direction.

A new paper by Tetsuya Tokano of the University of Cologne in Germany clarifies the contradiction, explaining that seasonal changes seem to reverse Titan's wind patterns for relatively short periods of time.

These intermittent gusts occur for perhaps two years, and are so strong they have a greater effect on the sand than the conventional east to west surface winds, which do not appear strong enough to move substantial amounts of sand, Tokano said. 

"It was hard to believe that there would be permanent west-to-east winds, as suggested by the dune appearance," said Tokano, whose paper was published in the journal Aeolian Research.

"The dramatic, monsoon-type wind reversal around equinox turns out to be the key."

The dunes, which are half a mile wide and hundreds of miles long, are spread across Titan's vast sand seas at latitudes within 30 degrees of the equator, and reach heights of 300 feet in some places.

The ridges typically track west-to-east, as Titan sheds sand along lines parallel to the equator.

Scientists had suspected winds in Titan's lower latitudes would blow east to west since they generally blow in the opposite direction at higher latitudes.  Indeed, these wind forces should balance out according to basic principles of atmospheric rotation.

But Tokano re-analyzed a global circulation model for Titan he had created in 2008 adapted from others developed for Earth and Mars.  He also incorporated new information on Titan's shape based on Cassini radar and gravity data, and more closely examined variations in Titan's wind at different points in time, rather than just considering averages.

The new analysis revealed something interesting during equinox periods, which occur twice per Titan year (one Titan year is equivalent to about 29 years on Earth).

During equinox, the heat from the sun shining directly over the equator creates upwelling in the atmosphere. This chaotic mixing causes the winds to reverse and accelerate.

The same rare wind reversal happens here on Earth over the Indian Ocean during transitional seasons between monsoons.

But on Titan, the episodic reverse winds seem to blow around 2 to 4 mph, exceeding the 2 mph wind speed necessary to move the sand.   Conventional east to west winds do not seem to surpass this threshold.

"This is a subtle discovery -- only by delving into the statistics of the winds in the model could this rather distressing paradox be resolved," said Cassini radar scientist Ralph Lorenz, lead author of a 2009 paper mapping Titan's dunes and author of a related article about Tokano's work published in this week's journal Science.

"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," he said.


Image 1: 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. Image credit: NASA/JPL (upper photo); NASA/JSC (lower photo)

Image 2: Scientists have used data from the Cassini radar mapper to map the global wind pattern on Saturn's moon Titan using data collected over a four-year period, as depicted in this image. Image credit: NASA/JPL/Space Science Institute


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