Mantle differentiation and thermal evolution of Mars, Mercury, and Venus

Abstract

Thermal evolution models for the terrestrial planets Mars, Mercury, and Venus with core and mantle chemical differentiation, lithosphere growth, and volcanic heat transfer have been calculated. The mantle differentiates by forming a crust and the core differentiates by inner core solidification. Continued volcanic activity for billions-of-years is found to be possible even on small terrestrial planets if crust growth is limited by lithosphere growth during the early evolution. Later, crust formation may be limited by the declining vigor of mantle convection. The thicknesses of the crust and lithosphere are found to depend mainly on planet size, on the bulk concentration of radiogenic elements in the planet, and on the ratio between volcanic and conductive heat transfer through the lithosphere. Two end-member models have been calculated and the concentration of radiogenics in the planet has been varied. In the first model, heat transfer from the mantle to the surface occurs via heat conduction through the lithosphere, while in the second model, mantle heat is advected via volcanic vents. Geologic evidence for volcanism on Mars and Mercury for at least 3.5 Ga and up to 1 Ga, respectively, the absence of a magnetic field on Mars, and the presence of such a field on Mercury suggest that heat transfer in these planets was dominated by heat conduction through the lithosphere for most of their thermal history. The present crust of Mercury is estimated to be a few tens of kilometers thick and about 10% of the mantle initial inventory of heat sources is fractionated into the crust. The Martian crust may be 50–100 km thick, possibly constituting more than a third of the lithosphere. Volcanic heat piping may have been an important heat transfer mechanism on Venus and volcanic activity may continue to the present day. Venus may have a crust that may constitute almost the entire lithosphere but crustal thickness may be limited by the basalt-eclogite phase transformation to 60 to 80 km. It is estimated that the present mantles of Mars and Venus are similarly depleted of about 20 to 40% of their initial heat source inventory.