The interior of the earth has always been of great interest to geoscientists. Due to the inaccessibility of samples from deep earth, we have been relying upon other scientific methods and procedures to explore the earth’s interior. Cosmochemical and geochemical studies of samples from shallow earth suggest that the core is mainly consisted of Fe, Ni, and one or more lighter elements. Previous investigations from seismic data and mineral physics data indicate that the outer core’s density is about 6% to 10% less than that of pure Fe at the outer core’s pressure and temperature conditions, and thus there must be a significant amount of a light element or various kinds of light elements existing in the outer core. The light element candidates in the outer core include C, H, O, S, Si, and the caused density variation might play a critical role in the liquid outer core convection. Using the x-ray absorption radiograph system, we have successfully measured the density of liquid phase FeS at 1673K and up to 5.6GPa in pressure. Our self-developed absorption image fitting program has proved to be reliable in determining the density of liquid FeS. The 15.4 GPa isothermal bulk modulus of liquid FeS at 1673K derived from the density compression curve provides information in constraining the sulfur content in the liquid outer core, which is one of the strong light element candidates that might be responsible for the density deficit in the outer core. To further understand the liquid behavior under extreme condition, we used the pair distribution function (PDF) method to study the structure of an elemental liquid—gallium and its atomic structure change due to compression. Diffuse scattering data were collected over the whole pressure range of liquid state (0.1-2GPa) at ambient temperature. The PDF results show that the first nearest neighbor peak position did not change with pressure increasing, while the farther peaks positions in the intermediate distance range decreased with pressure increasing. This leads to a conclusion of the possible existence of “locally rigid units” in the liquid. With the addition of a series of reverse Monte Carlo modeling of the liquid structure, we have observed that the coordination number in the local rigid unit increases with pressure. The bulk modulus of liquid gallium derived from the volume compression curve at ambient temperature is 12.1(6) GPa.