Linked magma ocean solidification and atmospheric growth for Earth and Mars

Early in terrestrial planet evolution energetic impact, radiodecay, and core formation may have created one or more whole or partial silicate mantle magma oceans. The time to mantle solidification and then to clement surface conditions allowing liquid water is highly dependent upon heat flux from the planetary surface through a growing primitive atmosphere. Here we model the time to clement conditions for whole and partial magma oceans on the Earth and Mars, and the resulting silicate mantle volatile compositions. Included in our calculations are partitioning of water and carbon dioxide between solidifying mantle cumulate mineral assemblages, evolving liquid compositions, and a growing atmosphere. We find that small initial volatile contents (0.05 wt.% H₂O, 0.01 wt.% CO₂) can produce atmospheres in excess of 100 bars, and that mantle solidification is 98% complete in less than 5 Myr for all magma oceans investigated on both Earth and Mars, and less than 100,000 yr for low-volatile magma oceans. Subsequent cooling to clement surface conditions occurs in five to tens of Ma, underscoring the likelihood of serial magma oceans and punctuated clement conditions in the early planets. Cumulate mantles are volatile-bearing and stably stratified following solidification, inhibiting the onset of thermal convection but allowing for further water and carbon emissions from volcanoes even in the absence of plate tectonics. Models thus produce a new hypothetical starting point for mantle evolution in the terrestrial planets.