http://www.lanl.gov/worldview/news/r...e/04-067.shtml

Los Alamos computers probe how giant planets formed
Los Alamos National Laboratory
Contact: Jim Danneskiold, (505) 667-1640 (04-067)
July 13, 2004

LOS ALAMOS, N.M., July 13, 2004 -- Nearly five billion years ago, the
giant gaseous planets Jupiter and Saturn formed, apparently in radically
different ways.

So says a scientist at the University of California's Los Alamos
National Laboratory who created exhaustive computer models based on
experiments in which the element hydrogen was shocked to pressures
nearly as great as those found inside the two planets.

Working with a French colleague, Didier Saumon of Los Alamos' Applied
Physics Division created models establishing that heavy elements are
concentrated in Saturn's massive core, while those same elements are
mixed throughout Jupiter, with very little or no central core at all.
The study, published in this week's Astrophysical Journal, showed that
refractory elements such as iron, silicon, carbon, nitrogen and oxygen
are concentrated in Saturn's core, but are diffused in Jupiter, leading
to a hypothesis that they were formed through different processes.

Saumon collected data from several recent shock compression experiments
that have showed how hydrogen behaves at pressures a million times
greater than atmospheric pressure, approaching those present in the gas
giants. These experiments - performed over the past several years at
U.S. national labs and in Russia - have for the first time permitted
accurate measurements of the so-called equation of state of simple
fluids, such as hydrogen, within the high-pressure and high-density
realm where ionization occurs for deuterium, the isotope made of a
hydrogen atom with an additional neutron.

Working with T. Guillot of the Observatoire de la Cote d'Azur, France,
Saumon developed about 50,000 different models of the internal
structures of the two giant gaseous planets that included every possible
variation permitted by astrophysical observations and laboratory
experiments.

"Some data from earlier planetary probes gave us indirect information
about what takes place inside Saturn and Jupiter, and now we're hoping
to learn more from the Cassini mission that just arrived in Saturn's
orbit," Saumon said. "We selected only the computer models that fit the
planetary observations."

Jupiter, Saturn and the other giant planets are made up of gases, like
the sun: They are about 70 percent hydrogen by mass, with the rest
mostly helium and small amounts of heavier elements. Therefore, their
interior structures were hard to calculate because hydrogen's equation
of state at high pressures wasn't well understood.

Saumon and Guillot constrained their computer models with data from the
deuterium experiments, thereby reducing previous uncertainties for the
equation of state of hydrogen, which is the central ingredient needed to
improve models of the structures of the planets and how they formed.

"We tried to include every possible variation that might be allowed by
the experimental data on shock compression of deuterium," Saumon explained.

By estimating the total amount of the heavy elements and their
distribution inside Jupiter and Saturn, the models provide a better
picture of how the planets formed through the accretion of hydrogen,
helium and solid elements from the nebula that swirled around the sun
billions of years ago.

"There's been general agreement that the cores of Saturn and Jupiter are
different," Saumon said. "What's new here is how exhaustive these models
are. We've managed to eliminate or quantify many of the uncertainties,
so we have much better confidence in the range within which the actual
data will fall for hydrogen, and therefore for the refractory metals and
other elements.

"Although we can't say our models are precise, we know quite well how
imprecise they are," he added.

These results from the models will help guide measurements to be taken
by Cassini and future proposed interplanetary space probes to Jupiter.

Los Alamos National Laboratory is operated by the University of
California for the National Nuclear Security Administration (NNSA) of
the U.S. Department of Energy and works in partnership with NNSA's
Sandia and Lawrence Livermore national laboratories to support NNSA in
its mission.

Los Alamos develops and applies science and technology to ensure the
safety and reliability of the U.S. nuclear deterrent; reduce the threat
of weapons of mass destruction, proliferation and terrorism; and solve
national problems in defense, energy, environment and infrastructure.