The relative influence of H2O and CO2 on the primitive surface conditions and evolution of rocky planets

How the volatile content influences the primordial surface conditions of terrestrial planets and, thus, their future geodynamic evolution is an important question to answer. We simulate the secular convective cooling of a 1‐D magma ocean (MO) in interaction with its outgassed atmosphere. The heat transfer in the atmosphere is computed either using the grey approximation or using a k‐correlated method. We vary the initial CO2 and H2O contents (respectively from 0.1 × 10−2 to 14 × 10−2 wt % and from 0.03 to 1.4 times the Earth Ocean current mass) and the solar distance—from 0.63 to 1.30 AU. A first rapid cooling stage, where efficient MO cooling and degassing take place, producing the atmosphere, is followed by a second quasi steady state where the heat flux balance is dominated by the solar flux. The end of the rapid cooling stage (ERCS) is reached when the mantle heat flux becomes negligible compared to the absorbed solar flux. The resulting surface conditions at ERCS, including water ocean's formation, strongly depend both on the initial volatile content and solar distance D. For D > DC, the “critical distance,” the volatile content controls water condensation and a new scaling law is derived for the water condensation limit. Although today's Venus is located beyond DC due to its high albedo, its high CO2/H2O ratio prevents any water ocean formation. Depending on the formation time of its cloud cover and resulting albedo, only 0.3 Earth ocean mass might be sufficient to form a water ocean on early Venus. Plain Language Summary Early in their history, Earth‐like planets are impacted by small rocky bodies, and the energy brought by the impactors heats the planet. Giant impactors can even remove the atmosphere and melt a large and deep fraction of the planet, leading to the formation of an “ocean” of molten rocks. From this initial stage, cooling and solidification proceed, expelling volatiles to rebuild an atmosphere. Varying the initial CO2 and H2O contents for planets located at different distances from the star, we study their influence on the planet evolution and on the surface temperature and pressure. These will condition the formation of a water ocean and the tectonic regime of the solid‐state planet. From our calculations, we derived simple relations to forecast water ocean formation. They suggest that a water ocean might have formed on Venus early in its history. Key Points Magma ocean and atmospheric coupled modeling during the first million years