Evidence for a liquid silicate layer atop the Martian core

Seismic recordings made during the InSight mission1 suggested that Mars's liquid core would need to be approximately 27% lighter than pure liquid iron2,3, implying a considerable complement of light elements. Core compositions based on seismic and bulk geophysical constraints, however, require larger quantities of the volatile elements hydrogen, carbon and sulfur than those that were cosmochemically available in the likely building blocks of Mars4. Here we show that multiply diffracted P waves along a stratified core-mantle boundary region of Mars in combination with first-principles computations of the thermoelastic properties of liquid iron-rich alloys3 require the presence of a fully molten silicate layer overlying a smaller, denser liquid core. Inverting differential body wave travel time data with particular sensitivity to the core-mantle boundary region suggests a decreased core radius of 1,675 +/- 30 km associated with an increased density of 6.65 +/- 0.1 g cm-3, relative to previous models2,4-8, while the thickness and density of the molten silicate layer are 150 +/- 15 km and 4.05 +/- 0.05 g cm-3, respectively. The core properties inferred here reconcile bulk geophysical and cosmochemical requirements, consistent with a core containing 85-91 wt% iron-nickel and 9-15 wt% light elements, chiefly sulfur, carbon, oxygen and hydrogen. The chemical characteristics of a molten silicate layer above the core may be revealed by products of Martian magmatism. Using multiply diffracted P waves and first-principles computations of the thermoelastic properties of liquid iron-rich alloys, we show that the core of Mars is smaller and denser than previously thought.