This Moon Continues to Surprise Us

Everybody knows (or should know) about the Cassini Mission that has been out at Saturn for several years now studying the ringed planet and its collection of moons and rings.  I suspect the mission has collected enough data to keep scads of graduate (and probably undergraduate) research students and their advisors busy for decades.  Not to mention the work for the mission scientists who planned and executed this project.

Of particular interest has been Titan. It’s Saturn’s largest moon. At -179 degrees Celsius, it’s a very cold place. Darned cold.

Before the Cassini mission we knew it was covered with a hazy hydrocarbon smog and that its surface was solid ice.  The Voyager missions provided us with some up-close looks and some interesting chemistry of the clouds.  Cassini’s Huygens lander showed us what the surface looked like, and repeated studies by the mother ship as it loops past Titan have given us a continual look at this once-mysterious world.

Titan has a very thick icy crust, thicker than scientists thought before the Cassini Mission began radar mapping this moon. Radar signals give scientists a way to chart and measure landforms on a surface.

Long before the Cassini Mission arrived, Titan had attracted scientists as a place to study. This is because it actually some similarities to Earth. For example, Titan appears to have a layered structure, just as our planet does.  It very likely has a core that is a mixture of ice and rock.  Scientists suspect the rock is rich with radioactive elements left over from the time when the planets and moons were forming.  Earth has a similar radiogenic inventory at its core. And, those radioactives generate heat as they decay.  Above the core is a watery ocean, which is heated by the radioactive heat from inside.  Capping off Titan is a frozen crust.

So, how do planetary scientists know that Titan’s crust is so thick?  Take a look at Titan’s orbit.  As this moon goes around Saturn, it spins on its axis (just as Earth does while at the same time orbiting the Sun).  Titan spins once around for each orbit it makes around Saturn.  There’s a gravity instrument onboard Cassini to measure the resistance of Titan to any changes in its spin – also called the moment of inertia, or MOI.  The MOI is affected by the thickness of Titan’s internal layers. MOI data allow scientists to  calculate the moon’s internal structure. That’s exactly what Stanford University professor Howard Zebker and his students did. Their work is described in detail in a Stanford press release. 

“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.

This is actually more ice than scientists expected. So, if there is more ice, then there should be less heat from the core to melt the ice than estimated. So, what’s happening? 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 exactly spherical. It’s actually more of an oblate (flat) ball. It gets this shape because Saturn’s gravity is pulling on it, making it look oblong along its equator and a little flattened at the poles.

So, this means that we can compute a correct shape for Titan based on models of its layers and  models of Saturn’s gravitational pull, right?  Well, yes, except the team’s data suggest that Titan is more distorted than it should be, and THAT implies that Titan’s internal structure may be more complex than everybody thought.

For one thing, the density of material under Titan’s 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. These are not the kinds of thicknesses you’d see if the simple layered model and gravitational-pull model were used to figure out Titan’s internal structure. So, there has to be something else at work.

Zebker said the variation in ice thickness could be a result of variation in the shape of Titan’s orbit 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,” he said.  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.”

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 Titan. More studies should help scientists like Zebker pin this one down.

Stargazing Anyone?

Speaking of cold places, it’s getting close to winter for northern hemisphere stargazers.  Cold weather doesn’t mean you get to stay inside all the time. The December skies are gorgeous, so it’s well worth bundling up and checking them out.  Of course, our friends in the southern hemisphere are starting to get that nice summertime weather, which makes stargazing even MORE delightful. So, get out there and look up!  To find out what’s up and what’s happening, check out December edition of Our Night Sky at Astrocast.TV.