[S-IT21] Planetary cores: Structure, formation, and evolution

[SIT21-11] The oxygen and sulfur enriched Martian big core and Martian dynamo

*Eiji Ohtani¹, William F McDonough^1,2, Takashi Yoshizaki¹ (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 2.Department of Geology, University of Maryland)

Keywords:Martian core, Oxygen, Sulfur, miscibility, immiscibility, dynamo

Recent seismic analyses revealed a large Martian core with a radius from 1810 to 1860 km with a mean density around 6 g/cm³ [1]. This discovery of a large Martian core provided a new constraint for the properties and composition of the Martian core. The density of the Martian core was less dense than that considered previously. We reevaluated the composition of the Martian core based on the EOS of the liquid Fe, FeO, FeS with the assumption of ideal mixing, and phase relations of the Fe-(Ni)-S-O system at high pressure and temperature.
Our analyses constrain the composition of the Martian core containing large amounts of sulfur and oxygen, 8.5wt% and 15.5wt%, respectively [2]. Phase relations of Fe-O, Fe-O-S, and Fe-Ni-O-S systems [3, 4, 5] indicated existence of a large region of the liquid immiscibility in these systems at least up to 27 GPa. The bulk Martian core composition estimated above locates in the field of a liquid immiscibility coexisting with an oxygen-rich ionic liquid and metallic iron liquid. Therefore, the present Martian core has a stratification of the oxygen-rich liquid outer core and a small metallic liquid (or solid) inner core separated during cooling through the liquid immiscibility field.
The early Martian dynamo [6] might have been generated by thermal convection of the miscible liquid core. However, the dynamo activity ceased during cooling and gravitational stratification of the core, and formation of the O-rich ionic liquid with a low thermal and electrical conductivity. The present model of the Fe-S-O Martian core reveals the cooling and change from miscible to immiscible liquid in the Martian core provided a strong effect for the formation and disappearance of the Marian magnetic field in the early Martian history.
The present model of the liquid immiscibility in the Fe-O-S system provides better explanation for the evolution of the Martian core and its magnetic field compared with that of the immiscible Fe-H-S Martian core [7].

References
[1] Stähler et al. (2021) Science, 373, 433-448 (2021).
[2] Yoshizaki and McDonough, Geochim Cosmochim. Acta 273, 137-162 (2020).
[3] Urakawa et al. TERRAPUB/AGU, Tokyo/Washington, D.C., pp. 95–111 (1987).
[4] Tsuno et al. (2007) Physics of the Earth and Planetary Interiors 160, 75–85 (2007)
[5] Tsuno et al. Phys Chem Minerals, 36:9–17 (2009)
[6] Mittelholz et al., Sci. Adv. 6: eaba0513 (2020)
[7] Yokoo et al. Nature Comm., 13:644, (2022).