Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov
Jia-Rui C. Cook
Jet Propulsion Laboratory, Pasadena,
Calif.
818-354-0850
jccook@jpl.nasa.gov
WASHINGTON -- Data from NASA's Cassini
spacecraft have revealed Saturn's moon Titan likely harbors a layer of liquid
water under its ice shell.
Researchers saw a large amount of
squeezing and stretching as the moon orbited Saturn. They deduced that if Titan
were composed entirely of stiff rock, the gravitational attraction of Saturn
would cause bulges, or solid "tides," on the moon only 3 feet (1
meter) in height. Spacecraft data show Saturn creates solid tides approximately
30 feet (10 meters) in height, which suggests Titan is not made entirely of
solid rocky material. The finding appears in today's edition of the journal
Science.
"Cassini's detection of large tides
on Titan leads to the almost inescapable conclusion that there is a hidden
ocean at depth," said Luciano Iess, the paper's lead author and a Cassini
team member at the Sapienza University of Rome, Italy. "The search for
water is an important goal in solar system exploration, and now we've spotted
another place where it is abundant."
Titan takes only 16 days to orbit
Saturn, and scientists were able to study the moon's shape at different parts
of its orbit. Because Titan is not spherical but slightly elongated like a
football, its long axis grew when it was closer to Saturn. Eight days later,
when Titan was farther from Saturn, it became less elongated and more nearly
round. Cassini measured the gravitational effect of that squeeze and pull.
Scientists were not sure Cassini would
be able to detect the bulges caused by Saturn's pull on Titan. By studying six
close flybys of Titan from Feb. 27, 2006, to Feb. 18, 2011, researchers were
able to determine the moon's internal structure by measuring variations in the
gravitational pull of Titan using data returned to NASA's Deep Space Network
(DSN).
"We were making ultrasensitive
measurements, and thankfully Cassini and the DSN were able to maintain a very
stable link," said Sami Asmar, a Cassini team member at NASA's Jet
Propulsion Laboratory (JPL) in Pasadena, Calif. "The tides on Titan pulled
up by Saturn aren't huge compared to the pull the biggest planet, Jupiter, has
on some of its moons. But, short of being able to drill on Titan's surface, the
gravity measurements provide the best data we have of Titan's internal
structure."
An ocean layer does not have to be huge
or deep to create these tides. A liquid layer between the external, deformable
shell and a solid mantle would enable Titan to bulge and compress as it orbits
Saturn. Because Titan's surface is mostly made of water ice, which is abundant
in moons of the outer solar system, scientists infer Titan's ocean is likely
mostly liquid water.
On Earth, tides result from the
gravitational attraction of the moon and sun pulling on our surface oceans. In
the open oceans, those can be as high as two feet (60 centimeters). While water
is easier to move, the gravitational pulling by the sun and moon also causes
Earth's crust to bulge in solid tides of about 20 inches (50 centimeters).
The presence of a subsurface layer of
liquid water at Titan is not itself an indicator for life. Scientists think
life is more likely to arise when liquid water is in contact with rock, and
these measurements cannot tell whether the ocean bottom is made up of rock or
ice. The results have a bigger implication for the mystery of methane
replenishment on Titan.
"The presence of a liquid water
layer in Titan is important because we want to understand how methane is stored
in Titan's interior and how it may outgas to the surface," said Jonathan
Lunine, a Cassini team member at Cornell University. "This is important
because everything that is unique about Titan derives from the presence of
abundant methane, yet the methane in the atmosphere is unstable and will be
destroyed on geologically short timescales."
A liquid water ocean, "salted"
with ammonia, could produce buoyant ammonia-water liquids that bubble up
through the crust and liberate methane from the ice. Such an ocean could serve
also as a deep reservoir for storing methane.
The Cassini-Huygens mission is a
cooperative project of NASA, the European Space Agency and the Italian Space
Agency. The mission is managed by JPL for NASA's Science Mission Directorate in
Washington. DSN, also managed by JPL, is an international network of antennas
that supports interplanetary spacecraft missions and radio and radar astronomy
observations for the exploration of the solar system and the universe. The
network also supports selected Earth-orbiting missions. Cassini's radio science
team is based at Wellesley College in Massachusetts.
For more information about the mission,
visit http://www.nasa.gov/cassini.
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