JUPITER, the largest planet in the solar system, may have nothing solid at its core. New models, which show Saturn with a large rocky core, suggest that Jupiter has little or none. Either something is wrong with the leading theory of giant planet formation, or hot liquids might have gouged out Jupiter鈥檚 heart.
The models, developed by Didier Saumon of Los Alamos National Laboratory in New Mexico and Tristan Guillot of Nice Observatory in France, use measurements of the gravitational fields of Jupiter and Saturn made by the Voyager and Pioneer spacecraft. But to nail down the internal structure of the planets it is also necessary to know the density of the liquid hydrogen deep within them.
Researchers have tried to measure this by subjecting hydrogen鈥檚 heavier cousin, deuterium, to pressures of up to 3 million atmospheres. Two sets of earlier experiments gave conflicting results, but now a group at the Russian Federal Nuclear Centre in Sarov has tried a third method. The team used high explosive to squash deuterium, and found the same factor of compression as in one of the previous methods. Hydrogen, the main constituent of Jupiter, ought to behave the same way.
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Plugging this information into their models, along with the gravitational fields of Saturn and Jupiter, Saumon and Guillot found that Saturn has a core with a mass more than 13 times that of the entire Earth. But Jupiter, which is three times as massive as Saturn, has at most five Earth masses of dense rock at its heart 鈥� and possibly no core at all.
So how did Jupiter end up with a soft centre? In the standard picture of the way giant planets form, a large rocky core acts as a seed, gathering an envelope of gas around it. Saturn has just the right size of core to have formed in this way, but if Jupiter formed in a similar fashion, it should have had a core of more than 20 Earth masses.
Saumon and Guillot think that Jupiter鈥檚 sea of hydrogen and helium could have ripped away most or all of the core. The temperature down there was once tens of thousands of kelvin, which they believe was hot enough to melt and then dissolve the core. Its heavy elements might then have been dredged up and mixed into the rest of the planet by convection currents. If this is what happened, it would also explain why Jupiter鈥檚 atmosphere is rich in relatively heavy elements such as carbon: they are the telltale signs of a broken heart.
But Alan Boss of the Carnegie Institution in Washington DC disagrees. 鈥淭he leading theory for giant planet formation has encountered a mortal blow, and the reaction is to put the theory on life support by invoking a physical phenomenon, core erosion, that had never before been raised.鈥�
He thinks that there was no need for a seed. Instead, Jupiter formed when part of the gas cloud that once surrounded the sun suddenly collapsed under its own gravity. In this model, Jupiter should indeed have less than six Earth masses of rocky core. Saturn鈥檚 large core, meanwhile, could be built up from captured rocky bodies.