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Sound and fury in Jupiter

Jupiter is a ball of mostly hydrogen and helium 300 times more massive than the Earth. Its surface complexion is familiar and coming into sharper focus with the myriad studies of the comet Shoemaker-Levy 9 impacts last summer and the prospect for new planetary probes next year. However, the vast interior of the planet, below the cloud tops, remains hidden from view. Because of its size, the planet is gravitationally compressed to extraordinarily high pressures and temperatures, reaching -5 TPa (50 Mbar) and -20,000 K at its center, and produces states of matter unlike anything found here on or in the Earth. Thus, even an approximate picture of the jovian interior requires knowledge of the behavior of its major components under extreme conditions. This means understanding the nature of hot, dense hydrogen, particularly in the plasma state that is theoretically predicted to occur at depth and long thought to be the source of Jupiter's strong magnetic field. As demonstrated in the reports by Nellis et al. (1) and Alavi et al. (2) elsewhere in this issue, the confluence of new experimental and theoretical studies of hydrogen at high pressure, combined with astronomical observations, is opening up a new era of such investigations. Under ambient conditions on Earth, hydrogen forms a gas of diatomic molecules, but it has been predicted since the early days of quantum mechanics to transform to a metal at sufficiently high pressures. It is a tale told by theorists: The known density and magnetic field of Jupiter arises from an interior consisting of atomic metallic hydrogen blanketed by a layer of nonconducting molecular hydrogen. Curiosity about both the jovian interior and the nature of this transition in hydrogen has spawned a large number of high-pressure experiments in recent years. Recently, Duffy et al. (3) measured the pressure dependence of the sound velocity of molecular hydrogen to record-high static pressures with a diamond-anvil cell. The results were used to construct both a sound velocity profile and a new equation of state (pressure-volume relation) for planetary modeling. Nellis et al. (I) now report new experiments to examine further the equation of state of hydrogen. In particular, they measured the