Sound speed and Gruneisen parameter for iron shock compressed to 3 teraPascals*

With thousands of extrasolar planets recently discovered, there is an urgent need for accurate structure and evolution models of planets. The iron equation of state (EOS) at terapascal (TPa) pressures is required to model the core of rocky planets and heavy elements in gas giants. This paper presents the first sound speed and Gruneisen parameter data for fluid iron compressed to 3 TPa (30 million atmospheres) and 20 g/cm³ on the Hugoniot. Both the sound speed and Gruneisen parameter are derivatives of the EOS, and thus tightly constrain the contours of the EOS surface. The sound speed data are systematically lower than expected from a simple extrapolation of previous data. The Gruneisen parameter shows a 30% drop at pressures and temperatures above the melt transition. Furthermore, while some models compare well with either the sound speed or Gruneisen parameter, none of today’s state-of-the-art models can explain both sets of data. Thus these new data will provide pivotal benchmarks for both future theoretical EOSs of warm dense iron and modeling planetary states and processes.