Metallization of fluid hydrogen at 140 GPa (1.4 Mbar): implications for Jupiter

Abstract

Implications of recent electrical conductivity and equation-of-state measurements on hydrogen at shock pressures up to 180 GPa and temperatures up to 4000 K are discussed. The laboratory results suggest that the transition between molecular and monatomic hydrogen is continuous, rather than first-order. Thus, it is unlikely that there is a distinct boundary in Jupiter between a molecular mantle and monatomic core. Rather, molecular hydrogen begins to dissociate at ∼40 GPa and goes to completion at ∼300 GPa. The electrical conductivity of fluid hydrogen reaches the minimum conductivity of a disordered metal at 140 GPa and ∼3000 K in the middle of this complex region. This pressure corresponds to about 0.9 the outer radius of Jupiter. The surface magnetic field of Jupiter, ∼10 G, is relatively large because metallic conductivity is reached relatively close to the planet’s surface. In comparison, the magnetic field of Saturn is only 0.2 G and is made primarily at 0.5 of the outer radius of Saturn, very similar to Earth. The shallow slope of temperature versus pressure along the isentrope of hydrogen suggests that the He content might have a significant effect on convection and, thus, on the Jovian magnetic field. Higher-order components of Jupiter’s magnetic field might be affected by inhibition of convection due to properties of the continuous dissociative phase transition of hydrogen.