Shock Wave Studies of Liquids in Earth's Core and Mantle

California Institute Of Technology, Pasadena CA

Investigators

Abstract

This award will support experimental shock wave measurements of molten rocks and metals at the pressure-temperature conditions of the deep Earth. Shock wave experiments use controlled high-speed collisions to generate high pressure and temperature for a short time, allowing unique measurements of the response of Earth materials to damage, high-speed deformation, and conditions like those in the lower mantle and core of the Earth. These data are needed to support next-generation models of lower mantle structure, the deep carbon cycle, evolution of an early terrestrial magma ocean, and the composition of the Earth's core. Molten materials are central to the understanding of planetary evolution and structure, but our understanding of them lags our understanding of solids because liquids are more complex and more difficult to study. Data from these shock wave experiments is often the only experimental data available at extreme pressure for validation of atomistic models of liquid structure and dynamics. This work leverages collaboration with Lawrence Livermore National Laboratory to exploit the expertise in the Department of Energy's shock physics group for geophysical problems and, in turn, the measurements made and methods developed in this work will have wide impact not only in experimental geophysics but also in condensed matter physics and the broader shock compression community. The project includes training of young scientists and outreach through television specials. The project incorporates technical advances that enable new experiments, more extreme conditions, higher precision results, and faster turn-around time. Advances in shock temperature measurement technique, reducing uncertainty from ~500 K to ~30 K, will be used to look at temperatures of shocked silicate liquids. The one experimental constraint on the heat capacity of a silicate liquid (pure SiO2) at high compression shows great complexity; measuring shock temperature in liquid defines this heat capacity and completes the description of the equation of state. Experiments that measure shock temperature can also be configured to measure the sound speed in the shocked liquid state, which defines high-order constraints on the equation of state and allows a key test of the Mie-Gruneisen formulation for silicate liquids. Measurement of sound speeds and densities of liquid Fe alloys along actual outer core adiabats will use a combination of shock-and-release and shock-and-ramp loading paths to bring molten Fe and Fe-alloy samples to the outer core isentrope without the need for pre-heating or extrapolation. Isentropic experiments will make the well-defined observed gradient in sound speed with depth in the outer core a new constraint on its composition. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.