Cassini Data Show Ice and Rock Mixture Inside Titan

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

News release: 2010-084 March 11, 2010

Cassini Data Show Ice and Rock Mixture Inside Titan

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-084&cid=release_2010-084

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 .

-end-

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