Just Gas, Part III (Electric Version)

Tuesday, October 01, 1996
Jupiter has an enormous magnetic field--ten times as powerful as Earth’s. To account for it, astronomers have speculated that tremendous pressure at the giant planet’s core squeezes hydrogen there into a metal, and like all metals, this hydrogen could conduct electricity and generate a magnetic field. The only problem with this theory is that until recently, there was no proof that hydrogen could exist in this guise. No one had ever observed metallic hydrogen anywhere--not even in the lab, let alone on Jupiter.

Hydrogen isn’t a very likely candidate for a metal--it remains a nonconducting gas under all but the most exotic conditions. Previous attempts to create metallic hydrogen involved crushing a sample of supercold, solid hydrogen with a vise-like diamond anvil. The theory was that under high pressure, hydrogen atoms would be squeezed together so closely that their electrons, like those in normal metals such as copper or iron, would become part of a collective electron pool. These electrons would belong to no single atom but to the entire lump of metal. Free to move around, they could conduct electricity. But the theory remained just that: even after squeezing the hydrogen with pressures of more than 2 million atmospheres--or 29.4 million pounds per square inch--no one had been able to create the necessary electron soup.

Last March, though, physicist Bill Nellis and his colleagues at the Lawrence Livermore National Laboratory in Livermore, California, announced that they’d succeeded where everyone else had failed. And they used neither solid hydrogen nor a diamond anvil. Instead they placed a thin layer of liquid hydrogen in what they call a two-stage gas gun. A gunpowder charge drives a piston down a gas-filled tube; the gas pressure builds until it ruptures a seal and propels a smaller piston down a narrow barrel at a speed of 15,750 miles per hour. When the piston smacks into the walls of the hydrogen container, it creates pressures of up to 1.8 million atmospheres and temperatures of up to 5000 degrees. Nellis found that under these pressures and in the fluid state, the hydrogen sample began conducting electricity just like any other metal, though only briefly.

Most physicists and astronomers had thought that pressures of 3 million atmospheres or more might be required to create metallic hydrogen and that metallic hydrogen would probably be solid. So how did Nellis succeed with liquid hydrogen? It’s probably easier, he says, to knock electrons free from liquid hydrogen’s atoms than from solid hydrogen’s atoms, which are locked in a crystalline grid. The discovery could force astronomers to revise some of their models for Jupiter. Metallic hydrogen, says Nellis, may lie a mere 4,500 miles below Jupiter’s cloud tops, not 9,000. The existence of a larger current-carrying hydrogen core may help explain Jupiter’s enormous magnetic field.

Despite the effort to create metallic hydrogen, it doesn’t seem to have extraordinary properties. Says Nellis: Liquid metallic hydrogen turns out to be a rather ordinary material.
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