Jupiter and Saturn: chalk and cheese

Boffins question gas giants' roots

The gas giants Jupiter and Saturn must have formed in radically different ways, according to research conducted at the Los Alamos Laboratory which reveals totally different structures at the cores of the two great planets.

Dider Saumon created new computer models of the cores of both Jupiter and Saturn, based on astrophysical observations and the results of a series of shock compression experiments on hydrogen. During these experiments, hydrogen gas was subject to extreme pressures - approaching those found inside a gas giant.

After running his models, Saumon concluded that in Saturn, heavy elements like iron, silicon, carbon, nitrogen and oxygen are concentrated in the core of the planet. Although these elements are present in similar quantities in Jupiter, Saumon's models indicate that larger giant has very little central core, with the elements diffused throughout its gaseous layers. This difference suggests the two planets must have formed in very different ways, billions of years ago.

By allowing for every possible variation permitted by astrophysical observations, and the results of the shock compression experiments, Saumon developed around 50,000 models of the internal structure of both planets. "We tried to include every possible variation that might be allowed by the experimental data on shock compression of deuterium," he said.

The data generated by the shock compression of hydrogen was essential to the research. The gas giants in our solar system are composed of approximately 70 per cent hydrogen, with the other 30 per cent being mostly helium and small amounts of heavier elements. However, the equation of state for hydrogen at high pressures, how it behaves, is not well understood.

The compression experiments shone new light on the behaviour of hydrogen in high-pressure environments, reducing the uncertainties in calculations and, Saumon says, dramatically improving the models of the planets' structures.

"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."

He concluded: "Although we can't say our models are precise, we know quite well how imprecise they are." ®

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