Earth and Mars Interior Structures Set by Remelting of the First Solid Mantle

Abstract Magma-ocean crystallization sets up the early structure and long-term evolution of terrestrial planets. Recent seismic evidence signals the presence of a silicate layer at the base of Mars’s mantle. Magma-ocean crystallization and subsequent overturn has been invoked as a hypothesis for this layer’s origin. However, while a magma ocean existed on both Earth and Mars, there is no seismic evidence for a basal layer in present-day Earth. In this study, we apply a parameterized-convection model to study whether the effect of partial melting in the growing mantle on the overlying magma-ocean composition can explain this discrepancy. Melts from the mantle buffer the crystallizing magma ocean, limiting progressive differentiation, iron enrichment, and the density anomaly of the overturned layer. This buffering is more efficient for larger planets with more vigorous mantle convection and for planets that are originally less enriched in iron. Consequently, a shallow magma ocean is more iron enriched and denser on Mars than on Earth, providing an explanation for the Mars–Earth difference in the present-day structure of the mantle. We also predict a dichotomy in terrestrial-exoplanet interior structures, with a population with small, stratified mantles and another with large, mostly homogeneous mantles.