Sound Speed in Shock-Compressed Iron up to 3.7 TPa

The formation of terrestrial planets involves high-velocity impacts of iron-rich planetesimals in the later stages of accretion. The distribution of iron and other highly siderophile elements within the resulting planets depends on the thermodynamic evolution of the heated material after pressure release from the shock states produced by the impact. Sound speed is a critical thermodynamic property that provides information about how a material evolves after isentropic release from the shock state and is directly connected to other thermodynamic derivatives such as the bulk modulus, Grüneisen parameter, and specific heat. We report sound speed measurements in iron shocked to various pressures from slightly above melt near 250 GPa up to 3700 GPa. Steady shocks were produced simultaneously in the iron sample and a sound speed-reference (alpha-quartz) using a pedestal laser pulse shape. Pressure perturbations were then launched simultaneously through the sample and reference by specially-tailored laser power modulations. Sound speed was deduced using the arrival time of the perturbation sequence at the shock front after emerging into a transparent window using high-precision velocimetry. Our results show the sound speed in shocked iron is less than 10% higher than recent isentropic measurements of the sound speed at 20 g/cc.