Partitioning and atmospheric loss of major volatile elements on accreting Venus: Implications for the early runaway greenhouse state

Abstract:

Due to limited information, the climate of early Venus has been poorly understood. Venus might have been in the runaway greenhouse state since its formation, in which water forms a steam atmosphere (e.g., Goldblatt et al., 2013, Hamano et al., 2013). On the other hand, a habitable climate with liquid oceans has been also suggested as another possibility (e.g., Way and Del Genio, 2020).

Here we show that the distinct ratios of the major volatile elements, carbon and nitrogen, on Venus and Earth suggest the early runaway greenhouse scenario. Present-day Venusian volatile budget shows a deficient in water by orders of magnitude and low C/N ratio by a factor of ~4 compared to the bulk silicate Earth (e.g., Catling & Kasting, 2017, Bergin et al., 2015). Hydrodynamic escape from the primordial atmosphere is thought to have contributed to the water loss, but another mechanism is needed to explain the C/N fractionation.

In this study, we adapted a numerical model of element partitioning between the atmosphere, crust, mantle (magma ocean), and core as well as impact-induced atmospheric escape (Sakuraba et al., 2019; Sakuraba et al., submitted) to accreting Venus. We estimated the final volatile inventories for both cases of the runaway greenhouse state and of Earth-like habitable condition. In the former case, all volatile species on the surface were assumed to be partitioned into the atmosphere. In the latter case, water and carbon were assumed to be partitioned into liquid oceans and sedimentary carbonates, respectively, after the magma ocean solidification.

Finally, Venusian low C/N ratio was reproduced in the runaway greenhouse case, while Earth-like high C/N ratio was obtained in the habitable case. In the former case, both C and N were mainly partitioned to the atmosphere through the accretion due to their low solubilities into the magma ocean and lack of surface reservoirs after the magma ocean solidification. Therefore, the impact-induced atmospheric erosion removed them equally. In contrast, preferential loss of atmospheric N as a result of C capture in carbonates elevated the C/N ratio in the latter case. As a conclusion, we propose that the combination of the runaway greenhouse state and the impact-induced atmospheric escape on early Venus are required to explain the present-day volatile abundances.