Investigating metallic cores using experiments on the physical properties of liquid iron alloys

Abstract

An outstanding goal in planetary science is to understand how terrestrial cores evolved to have the compositions, thermal properties, and magnetic fields observed today. To achieve that aim requires the integration of datasets from space missions with laboratory experiments conducted at high pressures and temperatures. Over the past decade, technological advances have enhanced the capability to conduct in situ measurements of physical properties on samples that are analogs to planetary cores. These challenging experiments utilize large-volume presses that optimize control of pressure and temperature, and diamond-anvil cells to reach the highest pressures. In particular, the current experimental datasets of density, compressional velocity, viscosity, and thermal conductivity of iron alloys are most relevant to the core conditions of small terrestrial planets and moons. Here we review the physical properties of iron alloys measured in the laboratory at conditions relevant to the cores of Mars, the Moon, and Mercury. We discuss how these properties inform models of core composition, as well as thermal and magnetic evolution of their cores. Experimental geochemistry (in particular, metal-silicate partitioning experiments) provides additional insights into the nature and abundance of light elements within cores, as well as crystallization processes. Emphasis is placed on the Martian core to discuss the effect of chemistry on core evolution.

Thermoelastic Properties of Liquid Fe‐Rich Alloys Under Martian Core Conditions

“…On the other hand, how the individual LE influences the seismic properties, such as density ( ρ ) and P‐wave velocity (Vp) under relevant P‐T conditions, is poorly explored. Relative to studies on Earth’s liquid outer core (∼100s GPa) (e.g., H. Huang et al., 2013; Morard et al., 2017) and small planetary cores (typically less than 10 GPa, see (Pommier et al., 2022) for a recent review), density or compressional sound velocity measurements on liquid Fe‐X mixtures, where X is either of the cosmochemically‐relevant elements S, C, O, or H, are scarce and poorly agreed upon in the pressure range of Mars’ core (∼20–40 GPa). For example, there are five experimental studies to date on the liquid Fe(Ni)‐S system that reported either ρ or Vp data in the aforementioned pressure range without extrapolation (Balog et al., 2003; Kawaguchi et al., 2017, 2022; Morard et al., 2013; Nishida et al., 2020).…”

Seismic Detection of a 600-km Solid Inner Core on Mars

“…19 Earth radii IC (Fig. 4), implies the possibility of a much lower core temperature than previously estimated 17,[36][37][38][39] or a different composition of the IC. Two mechanisms have been proposed to form a solid IC.…”

Local Structure and Density of Liquid Fe‐C‐S Alloys at Moon's Core Conditions

“…Sulfur is the most favored light element to be alloyed with iron in the Moon's core, because of its chemical affinity to iron at Moon's core conditions (siderophile behavior), and its effectiveness in decreasing the density of pure iron. In particular, considering the eutectic point of planetary‐core‐relevant iron alloys, the relatively low temperature of the Moon's interior (T between 1300 and 1900 K, e.g., Karato, 2013; Khan et al., 2006; Pommier et al., 2022; Wieczorek et al., 2006) points to an Fe‐ and S‐rich core as the simplest explanation. Furthermore, the depletion of the lunar mantle in siderophile elements is possibly related to the presence of sulfur in the core (Rai & van Westrenen, 2014).…”