by ANDY FLEMING
Saturn's enigmatic moon Titan, as imaged from the NASA's Cassini' spacecraft.
Scientists have long suspected that a vast ocean
of liquid water lies under the crusty exterior of Titan, Saturn's largest moon.
New analysis suggests that the internally generated heat that keeps that ocean
from freezing relies on the moon's interactions with Saturn and its other
moons.
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 liquid water lies under the crust. The new study suggests that the
internally generated heat that keeps that ocean from freezing solid depends far
more on Titan's interactions with Saturn and its other moons than had been
suspected.
Howard Zebker, a professor of geophysics and of
electrical engineering at Stanford University, will present the findings at the
annual meeting of the American Geophysical Union (AGU) in San Francisco on
Tuesday (Dec. 4) at 1:40 p.m. PST in Room 2005 of Moscone Centre West.
Zebker is part of the team interpreting radar
data of Titan acquired by NASA's Cassini spacecraft, which has been orbiting
Saturn since 2004. He has been studying the topography of Titan, and has
combined improved radar measurements of the moon's surface with newly released
gravity measurements 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
radioactive elements 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 on-board 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 kilometres, an ocean
somewhere in the range of 225 to 300 kilometres thick and an ice layer that is
200 kilometres thick," he said.
Previous models of Titan's structure estimated
the icy crust to be approximately 100 kilometres 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 centre
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 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," 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.
Source: Stanford University
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