Timescale and morphology of Martian mantle overturn immediately following magma ocean solidification

Energy of accretion in terrestrial planets is expected to create liquid silicate magma oceans. Their solidification processes create silicate differentiation and set the initial mantle structure for the planet. Solidification may result in a compositionally unstable density profile, leading to cumulate Rayleigh-Taylor overturn if a sluggish rather than stagnant lithosphere existed in the early stages of planetary history. The pattern and timescale of overturn, in which cold, dense surface material sinks to the core-mantle boundary, have implications for core dynamo production, volatile escape, and fundamental differences between differently sized bodies. Our fully spherical mantle models reaffirm previous work suggesting that harmonic degree of overturn is dependent on viscosity contrast and layer thickness. We find that cumulate overturn would likely have occurred with short wavelengths. In an isoviscous model, thermal convection ensues rapidly after overturn; however, when viscosity is temperature dependent, compositional stability in the mantle suppresses the onset of whole-mantle thermal convection. For a viscosity of 10¹⁸ Pa s, the mantle could fully overturn in as little as 3 Ma. Key Points Magma ocean crystallization would likely not cause degree-one overturn in Mars Post-overturn entrainment enables diverse reservoirs to participate in melting Thermal diffusion plays a key role in overturn timescales