Using Vulcan to Recreate Planetary Cores

Abstract

An accurate equation of state (EOS) for planetary constituents at extreme conditions is the key to any credible model of planets or low mass stars. However, experimental validation has been carried out on at high pressure (>few Mbar), and then only on the principal Hugoniot. For planetary and stellar interiors, compression occurs from gravitational force so that material states follow a line of isentropic compression (ignoring phase separation) to ultra-high densities. An example of the predicted states for water along the isentrope for Neptune is shown in a figure. The cutaway figure on the left is from Hubbard, and the phase diagram on the right is from Cavazzoni et al. Clearly these states lie at quite a bit lower temperature and higher density than single shock Hugoniot states but they are at higher temperature than can be achieved with accurate diamond anvil experiments. At extreme densities, material states are predicted to have quite unearthly properties such as high temperature superconductivity and low temperature fusion. High density experiments on Earth are achieved with either static compression techniques (i.e.diamond anvil cells) or dynamic compression techniques using large laser facilities, gas guns, or explosives. A major thrust of this work is to develop techniques to create and characterize material states that exists primarily at the core of giant planets and brown dwarf stars. Typically, models used to construct planetary isentropes are constrained by only the planet radius, outer atmospheric spectroscopy, and space probe gravitational moment and magnetic field data. Thus any data, which provide rigid constraints for these models will have a significant impact on a broad community of planetary and condensed matter scientists. Recent laser shock wave experiments have made great strides in recreating material states that exist in the outer 25% (in radius) of the Jovian planets and at the exterior of low-mass stars. Large laser facilities have been used to compressed materials to ultra-high pressures and characterize their thermodynamic and transport properties (plastic Hugoniot to 40 Mbar, deuterium Hugoniot to 3 Mbar, metallization of ''atomic'' deuterium on the Hugoniot). To probe materials properties at these high pressures, several experimental techniques were developed high resolution radiography, optical reflectance, pyrometry, and velocity/displacement sensitive interferometry are some of the diagnostics currently used in laser-generated shock EOS experiments. During our experiments at Vulcan we developed and tested precompressed and multiple shock experimental techniques which allowed us to recreate the extreme core states of giant plants. These experiments compressed water to densities higher than accessible by single shock Hugoniot techniques and showed that the metal-insulator transition of shocked precompressed water is suppressed significantly as compared to uncompressed water. Further, as predicted the temperature of shocked precompressed water is lower than the temperature of uncompressed water enabling us to determine the metallization mechanism for water near the Hugoniot.