Melting of Iron at Super-Earth Core Conditions
As the number of exoplanets discovered continues to increase, the question of whether they are capable of supporting life has become one of the key scientific questions in Earth and planetary science research today. To understand the dynamics of planetary interiors that may be similar to that of Earth, an understanding of the melting behavior of iron at the pressures of “Super Earth” planetary bodies can provide crucial constraints on accretion, differentiation, and dynamics.
As part of the Discovery Science Program at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), Staff Scientist Richard Kraus and CDAC Director Russell Hemley and have recently led a dynamic compression study in which iron was compressed to terapascal pressures, nearly three times that of Earth’s core, to simulate the conditions of Super Earth cores. As the world’s most powerful laser, NIF has the capability to produce conditions unreachable with static compression or with other types of dynamic compression techniques.
Simultaneous measurements of X-ray diffraction during compression reveal that, with decreasing entropy at a constant peak pressure (as would be the case for a planet during cooling), iron undergoes a transition from a completely liquid state, to a mixed hcp-liquid, and finally to solid hcp state. This sequence of phase transformations is observed up to 1000 GPa, or 1 Terapascal (TPa), and constrains the melting behavior of iron up to four times greater pressure than previous measurements.
This work has important implications for models of solidification of planetary cores (Fig. 1) and the formation and duration of planetary dynamos. In the latter case, observations from the current work lead to the assertion that super Earth-sized planets should have a longer period during which conditions favoring habitability are possible due to magnetic shielding of cosmic radiation.
Kraus, R. G., R. J. Hemley, et al., Measuring the melting curve of iron at super-Earth core conditions. Science 375, 202-205 (2022).
For a commentary on this work from Jung-Fu Lin, a former CDAC Research Scientist and current Professor in the Department of Geological Sciences at the University of Texas at Austin, see his Perspective.
Figure 1. Time scale for solidification of planetary cores as a function of planetary mass, both relative to Earth values, based on Q(cmb), the heat flux out of the core. Dashed lines represent one-sigma uncertainties.