Number 240 (Story #2), September 18, 1995 by Phillip F. Schewe and Ben Stein JUPITER HAS A TRANSITION ZONE in its interior where an envelope of mostly molecular hydrogen (H2) gives way to a deeper mantle of atomic (unpaired) hydrogen. Some scientists believe that perhaps most of the hydrogen at this lower level is metallic in nature, a fact which could account for Jupiter's strong magnetic field. Several new studies, attempting to simulate a small sample of Jupiter here on earth, suggest that current theories of the Jovian interior may have to be revised. The terrestrial work tries to match the conditions of pressure (thousands and millions of atm.) and temperature (thousands of K) prevailing inside Jupiter. Experiments with high-pressure diamond anvil cells and with high velocity guns---sending shock waves through containers of liquid hydrogen (W.J. Nellis et al., Science, 1 Sept)---and computer simulations of the interactions among liquid hydrogen molecules (Ali Alavi et al., same issue of Science) all have sought to calculate the speed of sound through hydrogen under extreme conditions. The new studies are at odds with velocity estimates derived from observations of oscillation modes in Jupiter's surface; e.g., the shock experiment finds that molecular hydrogen dissociation occurs at lower pressures than predictions based on the oscillation data. Further work is needed because of the astrophysical importance of hydrogen, which forms the bulk of stars and some planets.