In situ observation of the Rayleigh-Taylor instability of liquid Fe and Fe-Si alloys under extreme conditions: Implications for planetary core formation

Rayleigh-Taylor (RT) instability, which occurs when a heavy fluid overlies a light fluid in a gravitational field, is an important scenario for planetary core formation, especially beneath the planetary magma ocean. This process has been discussed based on numerical simulations and experiments using analog materials. However, experiments on the RT instability using the core-forming melt have not been performed at high pressures. In this study, we perform in situ observation of the RT instability of liquid Fe and Fe-Si (Si = 10 and 20 wt. %) alloys under high pressure using a high-power laser-shock technique. The observed perturbation on the Fe-Si surface grows exponentially with time, while there is no obvious growth of perturbations on the Fe in the measured time range. Therefore, the growth rate of the RT instability increases with Si content. The timescale of the initial growth of the RT instability in planetary interiors is likely to be much faster (by more than two orders of magnitude) than the 30-40 × 106 year timescale of planetary core formation.