Titan, Saturn's largest moon, icier than thought, scientists say

Dec 04, 2012 by Louis Bergeron
Artist's depiction of Saturn and its moon Titan. Stanford scientists' model suggestions that Titan's icy outer crust is likely twice as thick as previously thought. Credit: NASA/JPL-Caltech

(Phys.org)—A new analysis of topographic and gravity data from Titan, the largest of Saturn's moons, indicates that Titan's icy outer crust is twice as thick as has generally been thought.

Scientists have long suspected that a vast ocean of lies under the crust. The new study suggests that the internally generated heat that keeps that ocean from freezing solid depends far more on 's interactions with Saturn and its other moons than had been suspected.

Howard Zebker, a professor of and of at Stanford University, will present the findings at the annual meeting of the (AGU) in San Francisco on Tuesday (Dec. 4).

Zebker is part of the team interpreting of Titan acquired by 's , which has been orbiting Saturn since 2004. He has been studying the topography of Titan, and has combined improved of the moon's surface with newly released to make the new analysis.

Titan has long intrigued scientists because of its similarities to the Earth. Like Earth, Titan appears to have a layered structure, crudely similar to the concentric layers of an onion, albeit far less edible.

"Titan probably has a core that is a mixture of ice and rock," said Zebker. The core is overlain by the ocean and icy crust.

The rock in the core is thought to contain left over from the formation of the solar system. As in Earth's core, when those elements decay, they generate heat. On Titan, that heat is crucial to keeping its ocean from freezing solid.

As Titan orbits Saturn, Titan is slowly spinning on its axis, one spin for each trip around Saturn. Still, that spin is enough for the gravity instrument onboard Cassini to measure the resistance of Titan to any changes in its spin – also called the moment of inertia.

"The moment of inertia depends essentially on the thickness of the layers of material within Titan," Zebker said. Thus, he and his graduate students were able to use that data to calculate the moon's internal structure.

"The picture of Titan that we get has an icy, rocky core with a radius of a little over 2,000 kilometers, an ocean somewhere in the range of 225 to 300 kilometers thick and an ice layer that is 200 kilometers thick," he said.

Previous models of Titan's structure estimated the icy crust to be approximately 100 kilometers thick. So if there is more ice, then there should be less heat from the core than had been estimated. One way to account for less heat being generated internally is for there to be less rock and more ice in the core than previous models had predicted.

That all seems simple enough, but there is a complication. Titan is not a true sphere. Its shape is distorted by the gravitational pull of Saturn, making the moon sort of oblong along its equator and a little flattened at the poles.

From measurements of the observed gravitational field of Titan, one can compute what the shape of Titan ought to be. But the new data show that Titan's shape is much more distorted than would be predicted by a simple gravitational model.

That discrepancy means the internal structure of Titan isn't quite so simple.

For Titan to exert its observed gravitational pull, the average density from any given point on the moon down to the center of the core has to be the same, said Zebker.

But that's not the case, since Titan is somewhat squashed. For the data to line up, the density of material under the poles must be slightly greater than it is under the equator.

Since liquid water is denser than ice, Zebker's team reasoned that the ice layer must be slightly thinner at the poles than at the core, and the layer of water correspondingly thicker.

The team members calculated that the thickness of the icy crust is about 3,000 meters less than average at the poles and 3,000 meters greater than average at the equator. And the combination of gravity and topography further suggests that the average thickness of the icy layer is about 200 km.

For the icy crust to vary in thickness across Titan's surface, the heat distribution within the moon must vary as well. But that variation is not likely to come from the moon's core – heat generated there would be fairly uniform in all directions.

Zebker said the variation in ice thickness could be a result of variation in the shape of Titan's around Saturn, which is not perfectly circular.

"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. The planet's other moons also exert some tidal influence on Titan as they all follow their different orbits, but the primary tidal influence is Saturn.

"The tides move around a little as Titan orbits and if you move anything, you generate a little bit of heat."

For example, if you take a thin strip of metal and flex it, it will begin to weaken and eventually you can break it. That weakening is the result of heat being generated as you flex the metal.

The tidal interactions tend to be more concentrated at the poles than the equator, which means that there is slightly more heat generated at the poles, which in turn melts a little bit of ice at the bottom of the ice layer, thinning the ice in that region in comparison to other parts of the planet, Zebker said.

The Cassini mission was recently given funding to continue operating through 2017, which means about five more years of data will be acquired that can contribute to further refinements of Zebker's model of Titan.

Explore further: Successful engine test enables SpaceX Falcon 9 soar to space station in Jan. 2015

More information: On Tuesday, Dec. 4, from 1:40 – 1:55 p.m. PT, Howard Zebker will present "Titan's Figure Fatter, Flatter Than Its Gravity Field" at the AGU fall meeting in Room 2005, Moscone Center West, 800 Howard St., San Francisco.

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kevinrtrs
1.3 / 5 (14) Dec 04, 2012
OK, so the obvious question then is just how much heat is lost to space per second and how much heat is generated by the gravitational interactions with Saturn?
This would give a fair indication of how long a body 2000km in radius would be able to sustain a fluid ocean of water in the absence of another more substantial heat source.
Furthermore, it would then point to just how much radioactive heat generation would be required to sustain the ocean for billions of years.

Somehow, I think the researchers are in for a major, mind-numbing surprise in the next five years.
NOM
4.6 / 5 (10) Dec 04, 2012
So what permanently numbed yours?
cantdrive85
1.3 / 5 (16) Dec 04, 2012
But the new data show that Titan's shape is much more distorted than would be predicted by a simple gravitational model.


Maybe the use of the "simple gravitational model" should be deposited in the dust bin along with epicycles. The EM forces involved are far more powerful than gravity and can easily account for the observations of the odd shape, just as those same forces cause the Sun to be so round. Clearly the "simple gravitational model" needs to be replaced by a theory that is better at predicting the observations rather than resorting to huge underground oceans and miles thick glaciers.
jsdarkdestruction
4.3 / 5 (6) Dec 04, 2012
So what permanently numbed yours?

the bible and a young earth creationist agenda.
Torbjorn_Larsson_OM
4.7 / 5 (12) Dec 04, 2012
Creationists shouldn't comment on science. It is hilarious, and making people think and then flee religion (see Dawkins' Converts' Corner).

Here the GW atmosphere of methane is at most ~ 0.5 billion years old. So any such simple model as proposed by our creationist troll isn't going to predict observations.

And of course the same goes for EU/PC religion. The article describes precisely the null hypothesis of known mechanisms. No sufficiently energetic EM field to heat Titan has been measured by Cassini. Anyone studying Cassini and EM for real would know that.
rubberman
4.3 / 5 (6) Dec 05, 2012
CD85....please, read other stuff. EM isn't resposible for everything. If it is your goal of furthering the understanding of EM and it's role in the universe, you have to at least appear to comprehend how it works. When you post as you do and try to pin everything on it, you hinder your goal. If your goal is simply to troll...what's the point?
cantdrive85
1 / 5 (5) Dec 05, 2012
CD85....please, read other stuff. EM isn't resposible for everything. If it is your goal of furthering the understanding of EM and it's role in the universe, you have to at least appear to comprehend how it works. When you post as you do and try to pin everything on it, you hinder your goal.

What's the point to blame gravity for everything? Clearly the "simple gravitational model" isn't working on any level, but here they are tweeking every aspect of the model to make sure the gravity only model stays intact.
There is no mention of the electromotive potential created by a spinning charged body within an active plasma environment, something which is completely ignored by standard theorists and dogmatic believers such as bjorn larsson. Anyone studying Cassini data readily knows there is an highly complex active plasma environment around Saturn, it's the deniers and ignoramuses who claim differently.

http://www.thunde...e-way-2/
GSwift7
3 / 5 (4) Dec 05, 2012
The observational data is interesting, but their interpretation seems to rely on quite a bit of interpretational analysis. I haven't seen the numbers, but when dealing with 'big' stuff like planetary density and such, a small deviation in an approximate value can produce very different results.

One major unknown here is the density of the internal ocean water. There's no way to know how much stuff is disolved in that water, or what exactly is disolved in it. It could be anywhere from nearly pure water to super-saturated. There could also be layers of accumulated condensate at the bottom, which they apparently haven't considered here.

Our own ocean demonstrates quite a bit of variability in density due to variations in salinity and temperature, and methane clathrates do condense on our ocean floor, amongst other things.

I would take the above theory with a grain of salt (pun intended).

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