Rivers of Methane May Flow on Saturn's Largest Moon
for National Geographic News
|December 9, 2005|
Rivers of methane are likely flowing on Titan, Saturn's largest moon, according to observations made by the Huygens space probe, which landed on Titan in January.
But what are methane rivers like? One scientist suggests the answer may be found right here on Earth.
"If we know the underlying physics of how rivers operate on Earth, the same generalized laws ought to work on Titan," said Gary Parker, a geologist and civil engineer at the University of Illinois at Urbana-Champaign.
According to Parker, the methane rivers on Titan may be wider and deeper than Earth's rivers of water, but they might flow much the same way that Earth's rivers do.
Surface images of Titan taken by the Huygens probe show gravel-size pieces of ice resembling rounded stones in a dry riverbed.
"The differences are noticeable, but they are not so overwhelming that a gravel-based river on Titan should be unrecognizable to [people] on Earth," Parker said.
He presented his findings this week at the annual meeting of the American Geophysical Union in San Francisco.
Titan is extremely cold. The moon is about minus 290ºF (179ºC) on its surface. It has a dense atmosphere of nitrogen, methane, and hydrocarbon. At these temperatures, the rain is liquid methane.
Data from the Huygens space probe and NASA's Cassini satellite show a world with Earthlike features, including the appearance of volcanoes, mountains, and shorelines.
Although he is not a planetary researcher, Parker became interested in Titan four years ago because of speculation that there might be rivers there.
His expertise is in rivers and ocean currents on Earth, and he has collected data from rivers all over the world.
"We have a very good database on Earth for gravel-bed rivers," he said. "We saw a lot of gravel in images from Titan.
"I thought this would be a wonderful chance to take what we know from gravel-based rivers on Earth and see what they would look like on Titan if the same laws hold."
The interaction of rocks and liquid on Titan should be much like it is on Earth, Parker theorized.
"Rocks are rocks, whether made of quartz or ice, and fluids are fluids, whether liquid water or liquid methane," he said.
While the same basic laws of physics apply on Titan as they do on Earth, Parker had to take into account differences in gravity, temperature, and other conditions on Titan.
"Gary's predictions for Titan's channels are based upon well-established relationships that he has developed for terrestrial streams," said Alan Howard, a river expert at the University of Virginia in Charlottesville.
"The extrapolation to different fluids, different temperatures, different gravity, and different atmospheres is relatively straightforward."
Parker's findings show that river channels on Titan should be remarkably similar to those on Earth.
Because of smaller acceleration due to the moon's lower gravity, Titan's river channels should be 40 percent deeper and two and half times wider than they would be on Earth, Parker says. The channel slopes should also be less steep than Earth's rivers.
"Still, these numbers are sort of Earthlike, even under very different conditions," Parker said.
But there is a lot left to learn about Titan's methane rivers, experts say.
"Our knowledge of Titan's channels is rudimentary," said Howard of the University of Virginia. "Unfortunately we have a situation where theory outstrips our ability to test using observations of Titan."
"Even width and steepness of the channels can only be crudely measured under the best of circumstances," Howard added.
"But the theoretical predictions of the sort made by Gary can help us determine whether it is reasonable that flowing fluids produced the observed valley networks."
Mars is the only other planet scientists know of that has a landscape possibly sculpted by rivers.
Images from the Red Planet show what look like dry riverbeds. But scientists believe the processes that made them took place long ago and may not have been very strong.
Titan on the other hand seems highly active today.
"This is what's so exciting," Parker said. "The surface on Titan shows so little evidence of reworking by cratering [from meteors] that things must be going on today.
"Rivers aren't the only thing going on to keep the surface young all the time, but it looks like they are an important player."
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