A Joint Experimental‐Modeling Investigation of the Effect of Light Elements on Dynamos in Small Planets and Moons

Abstract

We present a joint experimental‐modeling investigation of core cooling in small terrestrial bodies. Significant amounts of light elements (S, O, Mg, Si) may compose the metallic cores of terrestrial planets and moons. However, the effect of multiple light elements on transport properties, in particular, electrical resistivity and thermal conductivity, is not well constrained. Electrical experiments were conducted at 10 GPa and up to 1850 K on high‐purity powder mixtures in the Fe‐S‐O(±Mg, ±Si) systems using the multianvil apparatus and the four‐electrode technique. The sample compositions contained 5 wt.% S, up to 3 wt.% O, up to 2 wt.% Mg, and up to 1 wt.% Si. We observe that above the eutectic temperature, electrical resistivity is significantly sensitive to the nature and amount of light elements. For each composition, thermal conductivity‐temperature equations were estimated using the experimental electrical results and a modified Wiedemann‐Franz law. These equations were implemented in a thermochemical core cooling model to study the evolution of the dynamo. Modeling results suggest that bulk chemistry significantly affects the entropy available to power dynamo action during core cooling. In the case of Mars, the presence of oxygen would delay the dynamo cessation by up to 1 Gyr compared to an O‐free, Fe‐S core. Models with 3 wt% O can be reconciled with the inferred cessation time of the Martian dynamo if the core‐mantle boundary heat flow falls from >2 TW to ~0.1 TW in the first 0.5 Gyr following core formation.

Influence of Thermal Stratification on the Structure and Evolution of the Martian Core

“…These experiments were conducted at 8–10 GPa, lower than the 20–40 GPa in the Martian core. Extrapolation of these results to 20–40 GPa suggests that reduces by approximately 10% (Pommier et al., 2020). Since wt% of sulfur is predicted based on density estimates in the Martian core (Rivoldini et al., 2011; Khan et al., 2018), significantly lower values of than 20 W may be expected.…”

“…Due to the large amount of FeO in the Martian mantle (Wänke & Dreibus, 1994), significant amounts of oxygen may have dissolved into the core (Tsuno et al., 2007). The presence of 0.5 wt% oxygen added to Fe‐5 wt%S is expected to drastically reduce to W at 2000 K (Pommier et al., 2020). Furthermore, nickel is present in metallic cores, and can significantly reduce as well as the melting temperature (Gilfoy & Li, 2020).…”

“…Estimates on typically span the range Pa s (Breuer & Spohn, 2006; Fraeman & Korenaga, 2010) and so we include with this range in our parameter search. As discussed in the introduction, dynamo operation is heavily dependent upon the core thermal conductivity, and so, we consider a range from 5 to 40 W , based on laboratory‐based electrical resistivity results for Fe‐S and Fe‐S‐O‐Mg‐Si alloys (Pommier, 2018, 2020; Pommier et al., 2020).…”

“…In this study, we focus on three factors that have not been previously considered by Martian core‐mantle evolution studies. First, recent experimental studies on a variety of iron alloys at conditions relevant to the core of Mars have yielded values for the thermal conducitivity, , from as low as 5 W to around 30 W , depending on the iron alloy composition in the Fe‐S and Fe‐S‐O‐Mg‐Si systems (Pommier, 2018; Pommier et al., 2020). These experiments span a lower range of conductivities than previous models of Mars have considered, typically 30–120 W (Davies & Pommier, 2018; Hemingway & Driscoll, 2021; Nimmo & Stevenson, 2000; Williams & Nimmo, 2004).…”

“…These experiments span a lower range of conductivities than previous models of Mars have considered, typically 30–120 W (Davies & Pommier, 2018; Hemingway & Driscoll, 2021; Nimmo & Stevenson, 2000; Williams & Nimmo, 2004). Given is proportional to , low values will affect the ability of the core to generate a magnetic field (Pommier et al., 2020), impacting which evolutionary histories are consistent with magnetic observations.…”