  Precise information about the composition of the Earth's core is critical to
understand planetary evolution and for discussing current hot topics in
geodynamic behavior, such as core-mantle boundary heat flow. However, samples
from deep in the Earth's interior are not available, so our knowledge is based
on comparison of laboratory measurements with seismological observations,
informed by meteorite composition, and indications of the Earth's core
temperature. One of the most interesting results of such work has been the
suggestion that Earth's inner core must contain light elements because the
density of the core, as determined from seismological measurements, is lower
than the density of pure iron, its main constituent, as determined from
laboratory measurements and/or theoretical work: the density deficit is now
considered to be ~4%. However, this conclusion relies critically on having an
accurate pressure scale to relate lab generated pressures to geological
pressures. Establishing such a scale has been the subject of intensive research
but still involves significant extrapolation and approximations, especially at
higher pressures. Further, a pressure scale to the multi-megabar pressures is
indispensable for discussing super-Earth planets. Here we establish the first
primary pressure scale extending to the multi-megabar pressures of Earth's core
by measuring acoustic phonon velocities using inelastic scattering from a
rhenium sample in a diamond anvil cell. Our new pressure scale agrees with
previous primary scales at lower pressures and also shock compression
experiments, but is significantly different from previous secondary and
theoretical scales at Earth's core pressures: previous scales have
overestimated, by at least 20%, laboratory pressures at 230 gigapascals. Our
new scale suggests the density deficit of the inner core is ~9%, doubling the
light-element content of the core.
