Sources of hydrogen in the primordial atmosphere of Venus

Context. Understanding the hydrogen content in Venus’ primordial atmosphere is crucial for comprehending the hydrodynamic escape process that shaped its atmospheric evolution. The hydrogen originated from two main sources: molecular hydrogen (H2) from the solar nebula and water vapor (H2O) from geological degassing. The precise proportions of these sources remain uncertain, leading to different hypotheses about Venus’ atmospheric history. However, a systematic exploration of the parameter space regarding the proportions of these sources has not yet been conducted. Aims. This study aims to constrain the hydrogen content and its sources in Venus’ primordial atmosphere by conducting extensive numerical simulations of early atmospheric escape scenarios. Methods. We developed an improved energy-limited hydrodynamic escape model, integrated with a 1D radiative-convective equilibrium atmospheric model, to simulate the early atmospheric escape on Venus. Using isotopic data of Ne and Ar from the current Venusian atmosphere, we constrained the contributions of nebula-derived and degassing-derived hydrogen. Our simulations have explored over 500 000 scenarios, varying the initial H2 and H2O compositions and considering different solar extreme ultraviolet (EUV) irradiation conditions. Results. Our results, based on the isotopic ratios of 20Ne/22Ne, 36Ar/38Ar, and 20Ne/36Ar observed in Venus’ atmosphere, indicate that the primordial atmospheric water content was limited to less than 0.01 ocean equivalents of H2 (0.0004 wt%) and less than 1.4 ocean equivalents of H2O. This suggests that if Venus ever had a primary hydrogen-rich atmosphere, it was mostly lost before forming its secondary, H2O-rich atmosphere. Furthermore, our method can be applied to constrain the primordial atmospheric compositions of other terrestrial planets, providing insights into their evolutionary histories.