An analysis of gravity and topography data from the Saturnian moon Titan obtained by NASA's Cassini spacecraft suggests there could be something unexpected about the moon's outer ice shell. The findings, published on Aug. 28, suggest that Titan's ice shell could be rigid, and that relatively small topographic features on the surface could be associated with large ice "roots" extending into the underlying ocean. The study was led by planetary scientists Douglas Hemingway and Francis Nimmo. The researchers were surprised to find a counterintuitive relationship between gravity and topography.
"Normally, if you fly over a mountain, you expect to see an increase in
gravity due to the extra mass of the mountain, on Titan, when you fly over a
mountain, the gravity gets lower. That's a very odd observation," said
One potential explanation is that each bump in the topography on the surface of
Titan is offset by a deeper "root" that is big enough to overwhelm
the gravitational effect of the bump on the surface. The root could act like an
iceberg extending below the ice shell into the ocean underneath it. In this
model, Cassini would detect less gravity wherever there is a big chunk of ice
rather than water because ice is less dense than water.
"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," said
Hemingway. "If large roots under the ice shell are the explanation, this
means that Titan's ice shell must have a very thick rigid layer."
If these findings are correct, a thick rigid ice shell makes it very difficult
to have ice volcanoes, which some scientists have proposed to explain other
features seen on the surface. They also suggest that convection or plate
tectonics are not recycling Titan's ice shell, as they do with Earth's
geologically active crust.