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Titan's icy shell is tougher than previously thought, scientists say

August 29, 2013|By Melissa Pandika
  • NASA's Cassini spacecraft captured this photo of Titan, Saturn's largest moon, on Jan, 29, 2008. New research finds that Titan's icy shell is thicker and more rigid than previously thought.
NASA's Cassini spacecraft captured this photo of Titan, Saturn's… (Associated Press )

You don’t need to worry about walking on thin ice on Titan. The icy shell that encapsulates Saturn’s largest moon is tougher and thicker than previously thought, according to a study published Wednesday in the journal Nature.

Like Earth’s crust, Titan’s icy surface floats atop a dense ocean. Earlier studies have suggested this outer layer is thin and flexible. But new data from NASA’s Cassini spacecraft point to an extremely thick, rigid shell with massive underwater “roots.”

While analyzing Cassini’s latest gravity and topography measurements, astrophysicists at UC Santa Cruz and other institutions  noticed something unusual. Normally, the gravitational pull is strong in high elevations due to the added mass of hills and mountains, and weak where the elevation is low. But researchers saw the opposite on Titan.

“It was very bizarre,” said Doug Hemingway, an astrophysicist at UC Santa Cruz.

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To explain the strange pattern, Hemingway and his colleagues developed a model in which each “bump” on Titan’s surface was offset by a large “root” that extended into the ocean below, much like the bottom of an iceberg.  Since ice is less dense than water, the  gravitational pull of the roots had to be weak. To be stable with weak gravity in high elevations, the roots also had to be large enough to counteract the gravitational fields of their corresponding bumps.

What’s more, the ice sheet had to be extremely rigid and about 30 to 125 miles thick to keep the roots submerged; otherwise their low density would have caused them to float upward. Although the roots would have still pushed against the ice sheet, the resulting bumps on the surface would have been smaller than if the sheet was thin.

But the size of the bumps predicted by the model was still larger than what Cassini had measured. That meant Titan’s rains and winds must have eroded about 655 feet from its surface, the researchers said.

The gravity measurements also revealed an odd feature of Titan’s interior. In most large celestial bodies, including Earth, the densest material sinks to the center, while less-dense material “floats” to the surface, forming separate differentiated layers.

However, Titan’s mass wasn’t concentrated in its center, suggesting that the silicates, ice and rock that make up its core are jumbled together, much like “a dirty snowball,” said Catherine Neish, an astrophysicist at the Florida Institute of Technology, who wasn’t involved in the study. That composition is more typical of comets and asteroids, she said.

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How Titan ended up this way is still a mystery, Neish said.

The new findings “complicate the puzzle” of Titan’s geological activity, said Hemingway, who led the study. Earlier radar measurements have shown features resembling volcanoes on Titan. But a thick, rigid shell would make it difficult for volcanoes to emerge. Titan “probably isn’t geologically active,” Hemingway said. “It’s very cold and dead.”

Cassini has been orbiting Saturn since 2004, collecting data from Titan as it passes by. As a result, its measurements of Titan are low-resolution, which makes them tricky to interpret, Neish said.  The resolution might improve as the spacecraft brings back more data but won’t be nearly as high as what multiple lunar orbiters have achieved for the moon, for example.

“What we really need is a Titan orbiter,”  Hemingway said. “Then we could resolve some of these open-ended questions.”

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Twitter: @mmpandika

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