The Formation of Mars' Basal Melt Layer

Recent analyses of seismic data recorded on Mars suggests a heterogeneous mantle where a global molten silicate layer lies above the core, followed by a partially crystallized layer (Samuel et al., 2023). The formation of such mantle structure is inherently link to the planet's early evolution when a global magma ocean was present and its crystallization process. Previous studies have shown that mantle overturn events during/after crystallization can produce a silicate layer enriched in iron and heat-producing elements that resides above the core-mantle boundary (CMB) (e.g., Tosi et al., 2013; Plesa et al., 2014; Samuel et al., 2021). However, processes such as melt transport, phase change, and chemical fractionation are not accounted for which are important in the describing the mantle's long-term evolution. By accounting for the aforementioned processes (Boukaré et al., 2025), we show that for the first time, a stratified melt layer can be formed and preserved over geological timescales in a self-consistent model. We observe that during the early stages of solidification, iron-rich silicates produced by chemical fractionation at the shallow mantle are delivered to the CMB. The presence of iron-rich materials at the CMB not only reduces the melting temperature of the silicates, but also produces a stably stratified melt structure at the bottom of the mantle that is resistant to chemical and thermal erosion over long timescales.