A note on the direction of core solidification in asteroids, the iron melting curve, and phase equilibria parameterizations

Abstract

The accuracy of the pressure/temperature/composition parameterization of Buono and Walker (2011) to describe the liquidus of iron and the Fe-FeS system is examined. In the pressure range critical for asteroidal core crystallization (0- ∼2 GPa), the model predicts a shape for the iron melting curve (initially negatively sloped, and turning over near 0.7 GPa) that is inconsistent with previous experimental observations, thermodynamic constraints, and millennia of empirical metallurgical observations. Dodds et al. (2025) recently used this model to derive notable conclusions about the behavior of the solidifying cores of asteroids: the robustness of their conclusions is assessed. Two basic caveat emptor guidelines for employing parameterizations of phase equilibria data are suggested: (1) ensure that the model's fit is consistent with simple thermodynamic expectations; and (2) verify that the data used to formulate the model provide adequate coverage in the region of interest.