  An accurate pressure scale is a fundamental requirement to understand
planetary interiors. Here, we establish a primary pressure scale extending to
the multi-megabar pressures of the Earth's core, by combined measurement of the
acoustic velocities and the density from a rhenium sample in a diamond anvil
cell using inelastic x-ray scattering and x-ray diffraction. Our scale agrees
well with previous primary scales and shock Hugoniots in each experimental
pressure range, and reveals that previous scales have overestimated laboratory
pressures by at least 20% at 230 gigapascals. It suggests that the light
element content in the Earth's inner core (the density deficit relative to
iron) is likely to be double what was previously estimated, or the Earth's
inner core temperature is much higher than expected, or some combination
thereof.
