Thermal Evolution of the Sulfur‐Rich, Small Terrestrial Planetary Core as Inferred From the Experimental Study of the Fe‐S‐O‐H System

Abstract The cores of rocky planets, including the Earth, are believed to contain light elements such as silicon, oxygen, sulfur, hydrogen, and carbon. Amongst them, sulfur appears to be rich in the cores of small terrestrial bodies like Mars and Ganymede. To understand the evolution of sulfur‐rich cores in the presence of other light elements, we have performed in‐situ high‐temperature and high‐pressure experiments on the Fe‐S‐O‐H system in the range of 18 to 44 GPa and 1260 to 2980 K to simulate the cooling of Mars‐sized planetary core using FeS and Mg(OH) 2 . Our results show crystallizations of FeS 2 , (Fe,Mg)O, together with FeSH x (IV), upon the decomposition of Mg(OH) 2 , indicating that in a sulfur‐rich environment, hydrogen tends to be selectively incorporated into the FeS structure. Based on the observed density contrast of the phases formed in the Fe‐S‐O‐H system, we conjecture a buoyancy overturn of the Fe‐O layer, initially formed in the topmost part of the core, by the lighter Fe‐S‐H layer. Our proposed stratification scheme would impact thermal and chemical convection of the core to dictate the generation and/or cessation of geodynamo in sulfur‐rich terrestrial planets.