Writing in the journal Nature, researchers reported an analysis of gravity and topography data obtained by NASA's Cassini spacecraft suggest the unexpected qualities of the moon's outer ice shell.
The data revealed a counter-intuitive relationship between gravity and topography, Cassini team scientists said.
"Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain," Francis Nimmo at the University of California, Santa Cruz, said. "On Titan, when you fly over a mountain, the gravity gets lower. That's a very odd observation."
One possible explanation, the researchers said, is that each bump in the topography on the surface of Titan is offset by a deeper "root" extending below the ice shell into the ocean underneath, big enough to overwhelm the gravitational effect of the bump on the surface.
In that case Cassini would detect less gravity wherever there is a big chunk of ice rather than water because ice is less dense than water, they said.
"It's like a big beach ball under the ice sheet pushing up on it, and the only way to keep it submerged is if the ice sheet is strong," lead study author Douglas Hemingway, also at UC Santa Cruz, said. "If large roots under the ice shell are the explanation, this means that Titan's ice shell must have a very thick rigid layer."