Published online 20 June 2000 | Nature | doi:10.1038/news000622-6


Great balls of metal

The image of the planet Jupiter as an onion-like mass of layers needs revising in the light of new experiments on squeezed hydrogen.

Image © NASAImage © NASA

Itis no easy thing to look inside a planet. So scientists at the Lawrence Livermore National Laboratory in California have recreated conditions like those inside Jupiter in the laboratory. In Physical Review Letters1they give a new picture of what the gas-giant planet looks like below its swirling blanket of gases.

Jupiter is made mostly of hydrogen. Its bright bands and baroque spiral markings (such as the Great Red Spot) are caused by other gases, but underneath these clouds it is almost pure hydrogen, becoming steadily denser towards the planet's centre. Gradually the hydrogen condenses into a thick fog of liquid droplets, before they coalesce completely into a hot liquid sea. Deeper still, things get even stranger.

In 1935 the physicist Eugene Wigner predicted that if hydrogen is squeezed hard enough, it will conduct electricity just like a metal. This is precisely what is thought to happen in Jupiter's depths. The circulation of the planet's core of liquid metallic hydrogen creates Jupiter's immense magnetic field, just as Earth's is the product of its core of liquid iron. Jupiter's magnetic field is the biggest thing in the Solar System -- it extends so far that the Sun would fit inside it.

Testing Wigner's idea has been a tremendous challenge. Theories suggested that the 'metallization transition' should occur at pressures of around 20 million times atmospheric pressure (20 megabars). Creating such compression in the laboratory is very difficult. Several experimenters in the 1980s and 1990s inconclusively used diamond teeth to squeeze tiny samples of hydrogen in 'diamond anvil cells'.

In 1996, William Nellis and co-workers at Livermore reported that they had at last seen the metallic state of hydrogen2. They used shock compression, in which a shock wave squeezes a sample momentarily to very high pressures. The wave also heats the sample to several thousand degrees, making the conditions still more like those in Jupiter's hot interior. The researchers saw the electrical resistance of their hydrogen sample drop dramatically between 0.9 and 1.4 megabars, suggesting that it had become metallic.

The work was impressive -- but was not convincing to everyone. One difficulty with the shock experiments was that they were literally over in a flash -- the high pressures persist for only a brief moment, during which all the data have to be gathered.

So the new measurements at Livermore, by Peter Celliers and colleagues, will be welcomed. They have used a high-power laser to generate a shock wave in a metal 'pusher' material that squeezes the sample. Instead of hydrogen itself, the researchers used deuterium-heavy hydrogen, in which the atoms are twice as massive.

Like normal metals, metallic hydrogen is expected to reflect light strongly. Celliers' group found that the reflectance of the deuterium started to increase rapidly even at pressures as low as 0.2 megabars. So not only do the new findings support the earlier experiments using a different criterion, but they hint that the metallic state starts to appear at much lower pressures than previously thought. And that the change from a non-metal to a metal is not abrupt but gradual.

The same thing should happen in Jupiter. Previous models have pictured the planet's interior as a layered, onion-like structure with sharp boundaries between the insulating and metallic forms of liquid hydrogen -- like the boundary between the Earth's rocky mantle and its liquid iron core. The new results indicate that, on the contrary, the metallic form of hydrogen unveils itself gradually over perhaps hundreds or thousands of kilometres. 

  • References

    1. Celliers,P. M. et al. Shock-induced transformation of liquid deuterium into a metallic fluid. Physical Review Letters 84, 5564 - 5567 2000. | Article | PubMed | ISI | ChemPort |
    2. Weir,S. T., Mitchell, A. C. & Nellis, W. J. Metallization of fluid molecular hydrogen. Physical Review Letters 76, 1860 1996. | Article | PubMed | ISI | ChemPort |