Effect of Silicon Alloying on the Structure of Exoplanetary Cores

Abstract:

The composition of cores of terrestrial planets are expected to be broadly similar to that of Earth in that they are comprised of a Fe-Ni alloy with variable amounts of light elements such as O, Si, C, S, and H. With the increasing number of discoveries of Super-Earths (rocky planets many times the mass of our own), the properties of terrestrial phases at ultrahigh pressures are required to understand and interpret the variability of large-scale exoplanet observations. The properties of the cores of these bodies are important for understanding the bulk chemistry, thermal evolution, magnetic fields, and, ultimately, habitability of a planet.

Recent diamond anvil cell studies interrogating the structure of iron generally agree that Fe should be hcp at core pressures and temperatures, although other structures have been proposed. At higher pressures and with the addition of light elements, the structure is less understood. The addition of large amounts of Si, for example, stabilizes the cubic B2 structure with respect to hcp at outer core pressures. Our goal in this study is to explore the effect of Si-alloying at inner core and exoplanetary-core pressures.

Dynamic compression experiments were carried out at the Omega Laser at the Laboratory for Laser Energetics, University of Rochester. High pressures were achieved by focusing laser drives onto target packages containing Fe-Si alloys. Pressures within the sample were determined by monitoring the velocity history at the sample/window interface. Quasi-monochromatic X-rays, timed with maximum compression of the Fe-alloy sample, were generated via laser irradiation of iron or germanium foils arranged in a backlighter configuration and collected on image plates lining the inner walls of a box attached to the target package.

In this presentation we will report on the effect of Si-alloying on the structure and density of Fe over the pressure range 100–1000 GPa. We find that while Fe with 7 wt.% Si remains in the hcp structure, Fe with 15wt.% Si is cubic to very high pressures. We present our experimental findings and discuss the implications for exoplanet interiors.