Tracking Deep Mantle Melting on Mars Using Phobos Tides

Abstract

We study the deep internal structure and rheological state of Mars by analyzing its response to tidal forces exerted by Phobos and by introducing a melt fraction in the mantle layers. Motivated by recent seismic evidence for a basal molten layer near the base of the Martian mantle, we explore the distribution of melt within the mantle and its influence on the tidal response. Assuming a 3-layers mantle structure, tidal deformations alone provide a radially dependent melt fraction profile, with partial melting concentrated in the deepest layer. Adding thermal constraints from recent solidus and liquidus determinations refines these limits, confirming that only the deepest part of the mantle might exhibit significant melting. The inferred densities for the mantle and core align with previous studies and point toward iron-rich mantle layers and a core enriched in light elements. The excess density of the deepest layer favors iron enrichment rather than hydration. Viscosity reduction profiles suggest a transition from solid to partially molten behavior above the core-mantle boundary, consistent with the presence of a thin basal molten layer enriched in iron and radiogenic elements. However, the rheological parameters $\alpha$ and $\beta$ deviate from laboratory estimates for terrestrial materials, suggesting that the anelastic behavior of the Martian mantle may differ from terrestrial analogs. This highlights the need for experimental studies on iron-rich, melt-bearing compositions representative of Mars.