Sand dunes are not unique to planet Earth: Astronomers have observed them on Mars and Venus. Now Titan, the largest moon of Saturn, may be included in this list, as new radar images of its surface reveal 100-kilometer-long dark stripes that resemble the rows of tall dunes seen in southwest Africa and Saudi Arabia. But what exactly makes up Titan sand, no one is entirely sure.
Sand dunes can reveal a lot about a region's climate and geology, as they require both erosion to break rocks into sand and wind to blow the sand into piles. Not too long ago, planetary scientists thought Titan lacked both of these ingredients. But as the European probe Huygens descended onto Titan's surface in January 2005 (ScienceNOW, 21 January 2005), its instruments detected winds that appear to be generated not by sunlight as here on Earth but by the strong gravitational pull from nearby Saturn. Huygens' cameras also caught glimpses of channels carved into the ice surface presumably by liquid methane, just as rock is eroded by water on Earth.
The potential for dunes on Titan was confirmed by more recent flybys by NASA's Cassini spacecraft. It recorded large stretches of east-west lines that appear to be seas of longitudinal dunes. Planetary scientist Ralph Lorenz of the University of Arizona in Tucson and his colleagues report in the 5 May issue of Science that these waves of sand are roughly 100 to 150 meters high and spaced 1 to 2 kilometers apart--similar to what is found in the Namib desert of Namibia. To form Titan's dunes, Lorenz's team estimates that surface winds vary around an average eastward direction. Wind speeds of about 2 kph--weaker than Earthly winds--are sufficient to move sand on Titan, says Lorenz.
The researchers calculate that typical Titan sand grains are roughly 3 times bigger than those on Earth are. In addition, they can't be made of silica, because no rock is exposed on Titan's surface. Lorenz and colleagues offer two alternate candidates: ice or organic solids such as complex hydrocarbons. Flowing methane could have weathered down some of the moon's ice to small grains, but the methane would have to dry up subsequently to allow the sand to blow in the wind. Alternatively, hydrocarbons may continuously rain down from Titan's hazy atmosphere. How this organic material would clump into grains is not yet known.
The Titan dunes provide a unique test of models for how dunes form here on Earth, says physicist Bruno Andreotti from the University of Paris 7. Grain size and atmospheric conditions are generally the same across our globe, so "the observations of dunes on other planets provide realizations under different working conditions," he says.
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