Implications for Uranus and Neptune of Electrical Conductivities of Fluid Hydrogen, Water, and Synthetic Uranus Measured Under Dynamic Quasi-Isentropic Compression up to 180 GPa and Several 1000 K

Abstract:

Electrical conductivities of metallic fluid H and ionic fluids H2O and Synthetic Uranus (SU) have been measured experimentally under dynamic quasi-isentropic compression up to 180 GPa and several 1000 K. SU is a mixture of H2O, NH3 and C3H8O with composition representative of “Ice”. Pressures P and temperatures T of the conductivity experiments were similar to P and T in interiors of Uranus and Neptune (U/N). Fluid H semiconducts at ~90 GPa and becomes a degenerate fluid metal with conductivity 2000/(ohm-cm) at 140 GPa, 0.64 g/cm3 and T/T_F~0.01, where T_F is Fermi temperature, conditions near the envelope/core boundaries of U/N. Metallization density is within a few% of the insulator-metal transition predicted by Wigner and Huntington in 1935. SU and water have conductivities of ~100/(ohm-cm) at 150 GPa. Podolak et al have shown a mixture of 75% Rock-25% Gas (by mass) behaves similarly to “pure” Ice in region that encompasses most mass of Uranus. The bandgap of water is predicted to close at 300 GPa and ~7000 K by Cavazonni et al. Models of pressure, temperature and density in U/N based on Voyager II gravity data have been developed by Helled et al. Stanley and Bloxham (SB) have developed MHD models that calculate non-dipolar and non-axisymmetric magnetic fields similar to those of U/N. The MHD models of SB assume that materials at planetary radii below the thin-shell dynamos that make the magnetic fields are stably stratified. The purpose of this paper is to develop a common picture for the deep interiors of U/N based on Voyager II gravity and magnetic data, measured electrical conductivities of planetary fluids, theoretical computations of interior conditions and the likely source of unusual magnetic fields, and extrapolation of existing experimental data for materials at 180 GPa to greater planetary depths. Main conclusions are the magnetic fields of U/N are probably made primarily by fluid metallic H at radii out to ~0.8 or more of U/N outer radii. Most of the cores of U/N might well be a mix of metallic H and Rock. At the centers of U/N pressures and temperatures are ~700 GPa and ~7000 K at which materials are metallic - either solids, as the solid-Fe inner-core of Earth, or fluids with viscosities so large that convection is effectively quenched. Thus, it is reasonable to assume the deep interiors of U/N are stably stratified.