How tidal tomography and thermal constraints can probe the existence of a Martian basal molten layer

By computing the tidal deformations of Mars, we investigated its spherically symmetric internal structure, and specifically the nature (liquid, partially melted or both) of the interface between the mantle and the liquid core. Through an evaluation of their compatibility with diverse geophysical observations, we demonstrated that, despite the short excitation periods, tidal deformation (tidal dissipation induced by Phobos and tidal quality factor at the Phobos excitation frequency) provides an effective means to constrain Mars's internal structure. Our analysis yielded independently density and thickness estimates for the Martian lithosphere, mantle, core–mantle boundary layers, and core, which were consistent with previous results from other methods. Additionally, we derived new viscosity estimates for these layers. Notably, we showed that geodetic observations, combined with thermal constraints, are particularly sensitive to the presence of a two-layered interface at the top of the liquid core in the deep Martian mantle. This interface comprises two layers with similar densities but very different viscosities and rheologies. The layer directly atop the liquid core follows a Newtonian constitutive equation (Newtonian Basal Layer or NBL), while the overlying layer at the base of the mantle has an Andrade rheology (Andrade Basal Layer or ABL), characterized by a viscosity approximately 10 orders of magnitude greater than that of the Newtonian layer. Our results indicate that the presence of this two-layered interface significantly affects the viscosity profiles of both the mantle and lithosphere. Specifically, models incorporating the two-layered interface show small viscosity contrast between the mantle and the lithosphere, preventing mechanical decoupling between these layers. This would support a stagnant lid regime, consistent with the current absence of Earth-like plate tectonics on Mars. Finally, our findings suggest that the presence of a liquid Newtonian layer atop the liquid core is incompatible with the existence of a solid inner core on Mars.