Cassini Data Show Ice and Rock Mixture Inside Titan

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Cassini Data Show Ice and Rock Mixture Inside Titan
Jet Propulsion Laboratory
March 11, 2010

PASADENA, Calif. -- By precisely tracking NASA's Cassini spacecraft on
its low swoops over Saturn's moon Titan, scientists have determined the
distribution of materials in the moon's interior. The subtle
gravitational tugs they measured suggest the interior has been too cold
and sluggish to split completely into separate layers of ice and rock.

The finding, to be published in the March 12 issue of the journal
Science, shows how Titan evolved in a different fashion from inner
planets such as Earth, or icy moons such as Jupiter's Ganymede, whose
interiors have split into distinctive layers.

"These results are fundamental to understanding the history of moons of
the outer solar system," said Cassini Project Scientist Bob Pappalardo,
commenting on his colleagues' research. Pappalardo is with NASA's Jet
Propulsion Laboratory in Pasadena, Calif. "We can now better understand
Titan's place among the range of icy satellites in our solar system."

Scientists have known that Titan, Saturn's largest moon, is about half
ice and half rock, but they needed the gravity data to figure out how
the materials were distributed. It turns out Titan's interior is a
sorbet of ice studded with rocks that probably never heated up beyond a
relatively lukewarm temperature. Only in the outermost 500 kilometers
(300 miles) is Titan's ice devoid of any rock, while ice and rock are
mixed to various extents at greater depth.

"To avoid separating the ice and the rock, you must avoid heating the
ice too much," said David J. Stevenson, one of the paper's co-authors
and a professor of planetary science at the California Institute of
Technology in Pasadena. "This means that Titan was built rather slowly
for a moon, in perhaps around a million years or so, back soon after the
formation of the solar system."

This incomplete separation of ice and rock makes Titan less like
Jupiter's moon Ganymede, where ice and rock have fully separated, and
perhaps more like another Jovian moon, Callisto, which is believed to
have a mixed ice and rock interior. Though the moons are all about the
same size, they clearly have diverse histories.

The Cassini measurements help construct a gravity map, which may help
explain why Titan has a stunted topography, since interior ice must be
warm enough to flow slowly in response to the weight of heavy geologic
structures, such as mountains.

Creating the gravity map required tracking minute changes in Cassini's
speed along a line of sight from Earth to the spacecraft as it flew four
close flybys of Titan between February 2006 and July 2008. The
spacecraft took paths between about 1,300 to 1,900 kilometers (800 to
1,200 miles) above Titan.

"The ripples of Titan's gravity gently push and pull Cassini along its
orbit as it passes by the moon and all these changes were accurately
recorded by the ground antennas of the Deep Space Network within 5
thousandths of a millimeter per second [0.2 thousandths of an inch per
second] even as the spacecraft was over a billion kilometers [more than
600 million miles] away," said Luciano Iess, a Cassini radio science
team member at Sapienza University of Rome in Italy, and the paper's
lead author. "It was a tricky experiment."

The results don't speak to whether Titan has an ocean beneath the
surface, but scientists say this hypothesis is very plausible and they
intend to keep investigating. Detecting tides induced by Saturn, a goal
of the radio science team, would provide the clearest evidence for such
a hidden water layer.

A Cassini interdisciplinary investigator, Jonathan Lunine, said of his
colleagues' findings, "Additional flybys may tell us whether the crust
is thick or thin today." Lunine is with the University of Rome, Tor
Vergata, Italy, and the University of Arizona, Tucson. "With that
information we may have a better understanding of how methane, the
ephemeral working fluid of Titan's rivers, lakes and clouds, has been
resupplied over geologic time. Like the history of water on Earth, this
is fundamental to a deep picture of the nature of Titan through time."

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a division of
Caltech, manages the project for NASA's Science Mission Directorate in
Washington. The Cassini orbiter was designed, developed and assembled at
JPL. Cassini's radio science subsystem has been jointly developed by
NASA and the Italian Space Agency (ASI).

More Cassini information is available, at http://www.nasa.gov/cassini
and http://saturn.jpl.nasa.gov .

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui...@jpl.nasa.gov

2010-084