Saturn Moon Has Seas of Sand, Images Reveal
Richard A. Lovett
for National Geographic News
|May 4, 2006|
When scientists first began using powerful instruments to peer through the dense atmosphere of Titan, Saturn's largest moon, they found sprawling dark areasregions that looked remarkably like oceans.
Now the latest images from a close flyby last October by NASA's Cassini probe reveal that these dark areas are seas of sand, complete with vast complexes of startlingly Earthlike dunes.
The dunes are up to 150 meters (500 feet) tall and hundreds of kilometers long, dominating large areas of Titan's surface near the equator.
They are long, linear dunes similar to a type commonly seen in Namibia (see map), the Sahara, parts of Australia, and the Arabian Peninsula.
The discovery was surprising, says Ralph Lorenz, a researcher with the Lunar and Planetary Lab based at the University of Arizona in Tucson.
Titan was not expected to have winds strong enough to produce dunes, even in gravity that is only one-seventh that of Earth's.
That's because wind is generally driven by solar heat, and Titan receives only a thousandth as much solar energy as Earth does.
What the predictions overlooked was Titan's proximity to a massive planet, says Lorenz, who is lead author of a study appearing in tomorrow's issue of the journal Science.
Saturn's gravity (wallpaper: Saturn) stirs Titan's atmosphere in the same way our moon's gravity stirs tidal currents in Earth's oceans.
Tidal winds, in fact, exist on Earth but are so weak they are barely detectable, even on the most sensitive instruments.
Titan's winds turn out to be strong enough to blow around sand-grain-size particles.
That's actually rather surprising, says Nicholas Lancaster, director of the Center for Arid Lands Environmental Management at the Desert Research Institute in Reno, Nevada.
Lancaster is the author of a commentary, also appearing in Science, on the new Titan study.
Titan's dune-building breeze, he notes, is estimated to have a speed of only half a meter a second (about a mile an hour).
On Earth, he said, "If you were standing outside, you would regard that as calm. The low wind speed necessary to form these features on Titan was a very big surprise to me."
The process works on Titan, he said, because of the Saturn moon's low gravity and dense atmosphere, which magnify the dune-building effects of low-velocity breezes.
If these winds blew the same direction all the time, the dunes would line up crosswise to the breeze.
Instead their direction fluctuates during Titan's orbital cycle, producing long, streamer-like dunes pointing in the direction that, on average, is downwind.
Similar seasonal fluctuations produce Earth's own linear dunes.
Mars also has dunes, but they are of the crosswise sort, referred to as transverse dunes.
"This is an example of how diverse planetary surfaces are within the solar system," Lancaster said.
Titan's dunes bend around hills and upland plateaus, revealing how Titan's wind interacts with the topography.
"This is interesting, because it allows us to map out the winds," Lorenz, of the Lunar and Planetary Lab, said.
That's potentially useful for future Titan missions that might involve placing balloon-suspended probes in the atmosphere.
Also interesting is that the dunes were found at Titan's equator. Farther north in the mid-latitudes, earlier flybys had revealed features dubbed cat scratches.
These are long parallel striations that were too small to be dunes but might be low-lying streaks of sand.
One explanation for why dunes are found on the equator but not farther north, Lorenz says, is that Titan's winds appear to converge on the equator.
That would push most of the available sand in that direction, leaving only enough at the mid-latitudes to form cat scratches but not dunes.
Farther north and south, he says, methane rain may make the ground too wet for any dunes to form at all.
The source and composition of the sand grains are a bit of a mystery.
Titan is cold enough that the sand could be rock-hard ice particles. Or the grains might be composed of frozen hydrocarbons.
"Probably it's some combination of the two," Lorenz said.
As for where they come from, one possibility is that the grains rain out of the atmosphere, condensing by some unknown mechanism from the hydrocarbon smog that gives the moon a hazy orange color when viewed from space.
Another theory is that the sand is created in the same manner as Earthly sand, by erosion of bedrock.
When the European Space Agency's Huygens probe (another part of the Cassini mission) parachuted to Titan's surface in early 2005, it landed in a region of rounded cobbles near the mouth of what looked suspiciously like a river valley.
(Related news: "Huygens Sends Images of Titan.")
"Whatever it was that made up the corners of those pebbles before they got rounded off is probably crushed up into sand," Lorenz said.
"We're starting to get a picture of Titan as an active, living world," he added. "These dunes are very fresh. For all we know, they're being blown around today."
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