Researchers mimic high-pressure form of ice found in giant icy moons(Forwarded)

Public Affairs
Lawrence Livermore National Laboratory

Contact: Anne M. Stark
Phone: (925) 422-9799

FOR IMMEDIATE RELEASE: March 2, 2006

NR-06-03-01

Researchers mimic high-pressure form of ice found in giant icy moons

LIVERMORE, Calif. -- That everyday ice you use to chill your glass of
lemonade has helped researchers better understand the internal structure
of icy moons in the far reaches of the solar system.

A research team has demonstrated a new kind of "creep," or flow, in a
high-pressure form of ice by creating in a laboratory the conditions of
pressure, temperature, stress, and grain size that mimic those in the deep
interiors of large icy moons.

High-pressure phases of ice are major components of the giant icy moons of
the outer solar system: Jupiter's Ganymede and Callisto, Saturn's Titan,
and Neptune's Triton. Triton is roughly the size of our own moon; the
other three giants are about 1.5 times larger in diameter. Accepted theory
says that most of the icy moons condensed as "dirty snowballs" from the
dust cloud around the Sun (the solar nebula) about 4.5 billion years ago.
The moons were warmed internally by this accretionary process and by
radioactive decay of their rocky fraction.

The convective flow of ice (much like the swirls in a hot cup of coffee)
in the interiors of the icy moons controlled their subsequent evolution
and present-day structure. The weaker the ice, the more efficient the
convection, and the cooler the interiors. Conversely, the stronger the
ice, the warmer the interiors and the greater the possibility of something
like a liquid internal ocean appearing.

The new research reveals in one of the high-pressure phases of ice ("ice
II") a creep mechanism that is affected by the crystallite or "grain" size
of the ice. This finding implies a significantly weaker ice layer in the
moons than previously thought. Ice II first appears at pressures of about
2,000 atmospheres, which corresponds to a depth of about 70 km in the
largest of the icy giants. The ice II layer is roughly 100 km thick. The
pressure levels at the centers of the icy giant moons eventually reach the
equivalent of 20,000 to 40,000 Earth atmospheres.

Researchers from Lawrence Livermore National Laboratory (LLNL), Kyushu
University in Japan and the U.S. Geological Survey conducted creep
experiments using a low-temperature testing apparatus in the Experimental
Geophysics Laboratory at LLNL. They then observed and measured ice II
grain size using a cryogenic scanning electron microscope. The group found
a creep mechanism that dominates flow at lower stresses and finer grain
sizes. Earlier experiments at higher stresses and larger grain size
activated flow mechanisms that did not depend on grain size.

The experimentalists were able to prove that the new creep mechanism was
indeed related to the size of the ice grains, something that previously
had only been examined theoretically.

But the measurement was no easy feat. First, they had to create ice II of
very fine grain size (less than 10 micrometers, or one-tenth the thickness
of a human hair). A technique of rapid cycling of pressure above and below
2,000 atmospheres eventually did the trick. Adding to that, the team
maintained a very steady 2,000 atmospheres of pressure within the testing
apparatus to run a low-stress deformation experiment for weeks on end.
Finally, to delineate the ice II grains and make them visible in the
scanning electron microscope, the team developed a method of marking the
grain boundaries with the common form of ice ("ice I"), which appeared
different from ice II in the microscope. Once the boundaries were
identified, the team could measure ice II's grain size.

"These new results show that the viscosity of a deep icy mantle is much
lower than we previously thought," said William Durham, a geophysicist in
Livermore's Energy and Environment Directorate.

Durham said the high-quality behavior of the test apparatus at 2,000
atmospheres pressure, the collaboration with Tomoaki Kubo of Kyushu
University, and success in overcoming serious technical challenges made
for a fortuitous experiment.

Using the new results, the researchers conclude that it is likely the ice
deforms by the grain size-sensitive creep mechanism in the interior of icy
moons when the grains are up to a centimeter in size.

"This newly discovered creep mechanism will change our thinking of the
thermal evolution and internal dynamics of medium- and large-size moons of
the outer planets in our solar system," Durham said. "The thermal
evolution of these moons can help us explain what was happening in the
early solar system."

The research appears in the March 3 issue of the journal Science.

Founded in 1952, Lawrence Livermore National Laboratory has a mission to
ensure national security and apply science and technology to the important
issues of our time. Lawrence Livermore National Laboratory is managed by
the University of California for the U.S. Department of Energy's National
Nuclear Security Administration.

More Information:

* LLNL's Public Affairs Office
http://www.llnl.gov/pao/
* Cassini-Huygens observations show how Titan compares with the Earth
LLNL News Release, May 12, 2005
http://www.llnl.gov/pao/news/news_re...-05-05-04.html
* Recreating the extreme state of water found on giant planets
LLNL Science Feature, April 1, 2005
http://www.llnl.gov/pao/news/news_re...-05-04-01.html

IMAGE CAPTION:
[http://photojournal.jpl.nasa.gov/jpeg/PIA03456.jpg]
This May 2001 photo of Jupiter's moon Callisto, taken by NASA's Galileo
spacecraft, is the only complete global color image of Callisto obtained
by Galileo, which has been orbiting Jupiter since December 1995.
Scientists believe the brighter areas are mainly ice and the darker areas
are highly eroded, ice-poor material. (Jet Propulsion Laboratory/NASA)