LARGE stretches of dunes seen on Saturn’s satellite Titan are providing clues to the weather on the surface of the giant moon. Because they resemble terrestrial sand dunes, it seems likely that similar weather processes formed them, though the dunes on Titan probably consist of particles of water ice or organic matter, not sand.
Dunes cover about 5 per cent of Earth’s land surface, and have also been seen on Mars and Venus. Now radar observations by the Cassini spacecraft in orbit around Saturn show dunes stretching across hundreds of kilometres on Titan. The dunes are about 150 metres high and are lined up in rows, like sand dunes in Namibia and Saudi Arabia.
“The full range of processes that modify the Earth’s surface seem to be active on Titan, too,” concludes Ralph Lorenz at the University of Arizona in Tucson, a member of the team that studied the Cassini radar images taken in October 2005. “Some process makes the sand, and something moves it around.”
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The finding was a surprise, Lorenz says. Until Cassini and its passenger probe, Huygens, arrived at Saturn last year, it was not clear how sand-like particles could have formed on Titan. Since then images from the two craft have revealed that rivers of methane once coursed across Titan, causing erosion that could have produced sand-sized particles.
The next question is what could have driven the winds needed to blow the sand into dunes. Though Titan has a dense atmosphere, it does not receive enough solar energy to generate strong winds. Recent computer models, however, suggest that Saturn’s gravity could pull strongly enough on Titan’s atmosphere to drive powerful winds.
The dunes, which run east-west, may help to map out these wind patterns, Lorenz says. “That will be very helpful for understanding Titan’s weather, and for planning future missions.” These might include balloon-borne experiments.
It is unclear what the sand is made of or how it formed. Lorenz and his colleagues offer two possibilities. In one scenario, chunks of water ice tumbled down streams of liquid methane during periodic storms on the moon. This could have broken up the ice “rocks” into sand-sized particles, which were then left behind when the streams dried up and were later blown by the wind to form dunes.
“Chunks of water ice might have tumbled down streams of liquid methane during storms on Titan, breaking them up into sand”
In the second scenario, organic compounds in the atmosphere could somehow have clumped together into sand-sized particles when they fell to the surface. Lorenz says it is not yet known what process could have made them stick together (Science, vol 312, p 702 and p 724).
It is still too early to tell which of these processes occurred, if either, but Lorenz hopes that data from the visual and infrared mapping imaging spectrometer on Cassini will reveal whether the dunes are icy or organic-rich.
