The density deficit of Earth's core with respect to a pure iron composition has been a long-standing problem since the core was discovered. Lack of knowledge about molten alloys of iron with other light elements at high pressures is responsible for such a persistent puzzle. Taking advantage of synchrotron radiation sources at national laboratories, this project has developed state-of-the-art techniques to tackle the issue of density and local structure of melts at high pressures. The objectives for the current period of the project include a) measuring sulfur-content dependence of EOS (equation of state) of Fe-FeS melt system and b) investigating structure of Fe-FeS melts at high pressures. While many light elements, e.g. S, O, Si, C and H, have been considered as candidates to account for the density deficit, sulfur is one of the most favorable candidates from geochemical and cosmochemical points of view. However, current available density data on the iron-sulfur alloy system remain a huge uncertainty. This project applies the newly developed technique of in situ synchrotron x-ray measurement at high pressures, and continues improving accuracy of experimental data on high pressure melts to address the multidisciplinary challenge from the mineral physics point of view. Integration of multi anvil press and diamond anvil cell techniques in this project offers not only complementary data in different pressure ranges but also serves a cross check for both newly developed experimental approaches. Success of this research will provide very useful information for understanding the composition and structure of the Earth's core, and the convection driven by compositional buoyancy in the Earth's outer core. The project actively integrates research and educational training including both undergraduate and graduate levels, which will enhance the awareness of modern technology among college students, especially minority students at Florida International University, and produce a new generation of scientists who are working at the frontier of advanced research. The project enhances the experimental capability of a community facility at national synchrotron x-ray sources, and therefore generates broader contributions to the scientific research in Earth science community and general material science.