December 4, 2012
Cassini Data Indicates Saturn’s Moon Titan Is Icier Than Thought
Lawrence LeBlond for redOrbit.com - Your Universe Online
Titan, Saturn´s largest moon, has long been projected as an icy body with a vast ocean of liquid water underneath its crust. A recent analysis suggested that heat generated from within the natural satellite helps keep this ocean from freezing due to interactions it has with Saturn and the other moons. And now, a new analysis of the moon´s topography and gravity indicates that its icy outer crust is twice as dense as previously believed.
“Titan probably has a core that is a mixture of ice and rock,” said study author Howard Zebker, a professor of geophysics and of electrical engineering at Stanford University. He added that the rocky core is overlain by an ocean and thick icy crust. The rocky core is believed to contain radioactive elements left over from the formation of the solar system. As these radioactive elements decay, they generate heat, much like they do inside Earth. On Titan, this radioactive heat is vital to keeping the ocean from freezing solid, Zebker explained.
Zebker is part of a team interpreting radar data from Titan collected by NASA´s Cassini mission, which has been orbiting Saturn since 2004. He has been studying the topography, and has combined improved data of the moon´s surface with newly released gravity measurements to paint a new picture of the moon´s internal structure.
"The picture of Titan that we get has an icy, rocky core with a radius of a little over [1,200 miles], an ocean somewhere in the range of [135 to 180 miles] thick and an ice layer that is [120 miles] thick," he said in a statement.
Previous models of Titan estimated the icy crust to be only about 60 miles thick. With this estimation in hand, Zebker said there should be less heat generated from the core than previously believed. If there is less heat coming from the core, then it is likely the core is more icy than rocky.
While this is a plausible scenario, there is some complication involved. Titan´s shape is not truly spherical, but rather distorted by the gravitational pull of Saturn, making the moon more oblong along the equator with flattened poles. When gathering data on the gravitational field of Titan, the evidence suggests the moon should be spherical. But the new data throws that out the window, indicating more distortion than is seen in the gravitational models.
This discrepancy means that it is much more complicated getting a good idea of Titan´s internal composition.
For the moon to exert its observed gravitational pull, the average density from any given point on the moon down to its core has to be the same, Zebker explained. For the data to line up, the density of material under the poles must be slightly greater than under the equator. And since liquid is denser than ice, it would be reasonable to say the ice layer must be slightly thinner at the poles than it is at the core, with water following suit in a reverse fashion.
Zebker and his team of graduate students calculated that the thickness of the icy crust must be about 10,000 feet (nearly 2 miles) less than average at the poles and 10,000 feet greater than average at the equator. And the combination of gravity and topography further suggests that the average thickness of the icy crust is about 120 miles.
For the icy crust to vary in thickness across the surface of Titan, the heat distribution within the moon would have to vary as well. If this is the case, then it is unlikely the variation would come from the core–as heat generated there would be fairly uniform in all directions outward. Therefore, the variation in ice thickness could be the result of variation in Titan´s orbit around Saturn.
"The variation in the shape of the orbit, along with Titan's slightly distorted shape, means that there is some flexure within the moon as it orbits Saturn," said Zebker. While the other moon´s around Saturn´s orbit exert some tidal influence on Titan, the primary tidal influence comes from the planet itself.
"The tides move around a little as Titan orbits and if you move anything, you generate a little bit of heat," he added.
These tidal interactions are more concentrated at the poles than at the equator, which means there is slightly more heat generated at the poles, which in turn melts a little more ice at the bottom of the icy crust, thinning the ice in that region, Zebker said in the statement.
Zebker will present the study findings at the annual meeting of the American Geophysical Union (AGU) in San Francisco today (December 4).
He also plans to continue studying Titan for as long as the Cassini mission is active, which is scheduled to operate through 2017.