Constraints on core composition from nuclear resonant scattering and x-ray diffraction studies on iron-light-element compounds The light element composition of the Earth's core has been a long-standing mystery in the study of the Earth's interior. The presence of light elements was first inferred from the density deficit and velocity excess of the core relative to that of pure iron under corresponding conditions. In order to test competing core composition models, we need accurate knowledge of the effects of various light elements on the density and velocities of iron as a function of pressure and temperature. Previous work has placed sulfur and carbon among the leading candidates for the principal light element in the core, even though density and sound velocity data for iron-sulfur and iron-carbon alloys are limited to room temperature and moderate pressures. The investigators will extend the data coverage into the Mbar regime and up to 1700 K, over the same pressure and temperature range of the existing measurements on pure iron. X-ray diffraction and nuclear resonant scattering - both established synchrotron radiation techniques - will be applied to determine the phase stability, equation-of-state, and partial phonon density-of-state of iron-rich compounds containing amounts of sulfur and carbon that are within the estimated range for the Earth's core. The new data will permit the team to explore the effects of sulfur and carbon on the density and sound velocities of iron in a previously uncharted pressure-temperature sector. They will be able to conduct stringent tests of candidate models describing sulfur-bearing or carbon-bearing core scenarios. This research will provide fundamental new knowledge about the properties of iron-rich alloys at high pressures and temperatures and the nature of planetary cores.
