Antonangeli, Daniele ; Rivoldini, Attilio ; Badro, James ; Morard, Guillaume ; Plesa, Ana-Catalina ; Collinet, Max ; Lognonné, Philippe Henri ; Boccato, Silvia ; Gendre, Héloïse ; Xu, Fang

Poster presented at AGU Fall Meeting 2021 on 2021-12-15

Abstract: InSight’s seismic data allowed to confirm the liquid state of the Martian core and constrain its radius to 1790-1870 km. Together with constraints on the bulk chemistry, the thermal state, and using mantle compositions proposed in literature, mass conservation implies a mean core density between 5.7 and 6.3 g/cm3. Such a low density calls for a liquid iron core rich in light elements. In order to refine the nature and abundances of light elements in the Martian core we use continuous core formation models based on high-temperature metal-silicate partitioning experiments and core-mantle equilibration, together with a thermodynamic parametrization of the liquid metallic alloy forming the core. Core compositions compatible with the chemistry of the mantle and depletion of siderophile elements in the bulk silicate Mars are down selected. For each mantle and core compositions, whole planet interior structure models are constructed using state of the art equations of state for the liquid core alloy and solid mantle materials. The predicted geodetic quantities and arrival times of core reflected shear waves are then compared to observations. The P-T and redox conditions required to match the abundance of siderophile elements in bulk silicate Mars indicate that its core cannot contain a significant amount of silicon. The abundance of oxygen is limited to ~3wt% and tightly coupled to that of sulfur, the major light element. To match the core density inferred by geophysical data, a S fraction in excess of 20wt% is required if the core contains 3wt% of O, which is well above the upper limit imposed by cosmochemical considerations (<~17wt%). Additional light elements such as carbon and hydrogen, both siderophile at Mars’ core conditions, are necessary at wt% level to decrease the amount of S within acceptable bounds. The required amounts of C and H for a core density of ~5.7 g/cm3 are however beyond what is expected according to cosmochemical models. Alternatively, the amount of light elements can be reduced if the fraction of FeO in the mantle is lower than what it is currently assumed by most of the standard composition models. We find that by reducing the FeO content of the mantle by a few wt% the amount of light elements in the core can be brought to within acceptable bounds while accounting for geophysical observations.

Keyword(s): Mars, core, composition, formation
Funding: 3PRODPLANINT/3PRODPLANINT/3PRODPLANINT