[S-IT21] Planetary cores: Structure, formation, and evolution

[SIT21-04] Density deficit of the inner core revealed by a new rhenium primary pressure scale

*Eiji Ohtani¹, Daijo Ikuta¹, Hiroshi Fukui^2,3, Tatsuya Sakamaki¹, Daikuke Ishikawa^2,3, Alfred Q.R. Baron^2,3 (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 2.RIKEN, 3.Japan Synchrotron Radiation Research Institute)

Keywords:primary pressure scale, rhenium, inner core, density deficit, inelastic x-ray scattering

Our knowledge on the Earth’s core is based on comparison of laboratory measurements with geophysical and geochemical observations. One of the most important results on the Earth’s inner core is that it must contain light elements because the density of the core determined by seismological observations, is lower than the density of pure iron determined by laboratory measurements and/or theoretical calculations. However, this conclusion relies critically on an accurate pressure scale to relate laboratory generated pressures to geological pressures. Establishing such a pressure scale has been the subject of intensive research but still involves significant extrapolation and approximations, especially at higher pressures of the Earth’s core. Further, a pressure scale to multi-megabar pressures is indispensable for discussing super-Earth planets. Here, we report the first primary pressure scale extending to the multi-megabar pressures of Earth’s core by measuring acoustic phonon velocities of rhenium using inelastic X-ray scattering combined with a diamond anvil cell. Our new pressure scale agrees with previous primary scales at lower pressures and also shock compression experiments, however, it is lower than previous secondary and theoretical scales at Earth’s core pressures: previous scales overestimated laboratory pressures by at least 20% at 230GPa. Our new scale suggests the density deficit of the inner core is ~9% assuming the ICB temperature of 6000 K, doubling of the light-element content of the inner core of 3-5% estimated by previous pressure scales.