New gravity and topography data from NASA’s Cassini spacecraft reveal unexpected features of the Titan’s outer ice shell.
This image is a composite of several images taken during two separate Titan flybys on October 9 and October 25, 2004. Credit: NASA / JPL / University of Arizona.
“The best explanation for the findings is that Titan’s ice shell is rigid and that relatively small topographic features on the surface are associated with large roots extending into the underlying ocean,” explained study lead author Dr Douglas Hemingway of the University of California, Santa Cruz, and his colleagues, who reported the results in a paper published in the journal Nature.
The scientists used new data from NASA’s Cassini spacecraft. They were surprised to find a negative correlation between the gravity and topography signals on Titan.
“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 study co-author Prof Francis Nimmo, also from the University of California, Santa Cruz.
To explain that observation, the scientists developed a model in which each bump in the topography on the surface of Titan is offset by a deeper root big enough to overwhelm the gravitational effect of the bump on the surface. The root is like an iceberg extending below the ice shell into the ocean underneath it.
“Because ice is lower density than water, you get less gravity when you have a big chunk of ice there than when you have water,” Prof Nimmo said.
An iceberg floating in water is in equilibrium, its buoyancy balancing out its weight. In this model of Titan, however, the roots extending below the ice sheet are so much bigger than the bumps on the surface that their buoyancy is pushing them up against the ice sheet.
This diagram of a cross-section through Titan’s ice shell shows features that may explain the gravity anomaly: a low-density ice lens created by regional basal freezing; a rigid ice shell that resists upward deflection; and surface weathering that keeps topography small. Credit: Douglas Hemingway.
“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. If large roots are the reason for the negative correlation, it means that Titan’s ice shell must have a very thick rigid layer,” Dr Hemingway said.
The researchers calculated that, in this model, Titan’s ice shell would have to have a rigid layer at least 40 km thick. They also found that hundreds of meters of surface erosion and deposition are needed to account for the observed imbalance between the large roots and small surface topography.
The results from their model are similar to estimates obtained by geomorphologists studying the erosion of impact craters and other features on Titan.
These findings have several implications. For example, a thick rigid ice shell makes it very difficult to produce ice volcanoes, which some have proposed to explain certain features seen on the surface.
Unlike Earth’s geologically active crust, Titan’s ice shell isn’t getting recycled by convection or plate tectonics.
“It’s just sitting there, and weather and erosion are acting on it, moving stuff around and redepositing sediments. It may be like the surface of Earth would be if you turned plate tectonics off,” Prof Nimmo said.
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Bibliographic information: D. Hemingway et al. 2013. A rigid and weathered ice shell on Titan. Nature 500, 550–552; doi: 10.1038/nature12400
