How Gas Giants Become Giants

jimo
Prototype of nuclear-fueled Jupiter Icy Moon (JIMO) spacecraft, with its heavily finned shape
Image Credit: NASA/ JPL

Nearly five billion years ago, the giant gaseous planets Jupiter and Saturn formed, apparently in radically different ways. So says a scientist at 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 Material Science (X-7) 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.

saturn_cassini
Mysterious ring spokes first seen by Voyager. Image Credit: JPL/NASA

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. The two planets 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.

Cassini Jupiter
Giant Red Spot in background, one of Jupiter’s moon in foreground with eclipse shadow cast on gas giant Credit: NASA/JPL Cassini

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.


Related Web Pages:

Galileo Home Page
Io Blasts Mile-High
Galileo’s Spyglass
Life and Giant Planets
Jupiter: Moon Festival
Jupiter’s Perfect Storms
Salty Volcanoes on Jovian Moon
Cassini
Saturn Edition, Astrobiology Magaz.
Cassini Closes In on Saturn
Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass