Reduced water loss due to atmospheric photochemistry under a runaway greenhouse condition on Venus

Venus likely lost a significant fraction of its initial water during an early runaway greenhouse (RG) phase (e.g., Hamano et al., 2013). Under the RG condition, surface water fully evaporates, creating an H₂O-dominated atmosphere. Intense ultraviolet (UV) irradiation from the young Sun drives H₂O photolysis, and the liberated hydrogen subsequently escapes to space (Kasting, 1988). Previous studies suggest that RG Venus could have lost several tens of terrestrial oceans (1.4 × 10²¹ kg; hereafter TO) within 1 Gyr (e.g., Gillmann et al., 2009).

Recently, two atmospheric photochemical processes—H₂O reproduction and UV shielding by O₂—were shown to suppress water loss in an H₂O-dominated atmosphere of a terrestrial exoplanet orbiting an M dwarf (Kawamura et al., 2024). However, the impact of these processes on water loss around G-type stars like Venus, and their dependence on atmospheric composition, remains poorly understood.

Here, we quantify these effects using a one-dimensional photochemical model based on PROTEUS (Nakamura et al., 2023). The model simulates vertical profiles of H₂O–CO₂ atmospheres under RG conditions by solving chemical reactions and diffusion. It solves 51 reactions including H₂O, CO₂, and their photolysis products (H, OH, H₂, O(¹D), O₃, O₂, O, HO₂, H₂O₂, CO, and HOCO) following Chaffin et al. (2017). To estimate the water loss rate, we impose diffusion-limited hydrogen escape as the upper boundary condition. Additionally, we consider intense UV irradiation conditions characteristic of the active young Sun (Claire et al., 2012). We considered a variety of atmospheric parameters, with H₂O inventories ranging from 0.1 to 10 TO and CO₂ inventories from 1 to 50 times the mass of the current Venusian Atmospheric Carbon Dioxide (4.69 × 10²⁰ kg; hereafter VACD).

Our results show that the water loss rate on the RG Venus is significantly suppressed by the two previously identified photochemical processes and by additional UV shielding from CO₂ and O₃. In an atmosphere with 5 TO of H₂O and 1 VACD of CO₂, these combined effects reduce the loss rate to 5.4 TO Gyr^-1, substantially below previous estimates. At 50 VACD of CO₂, enhanced UV shielding by CO₂ further decreases the rate. These findings suggest that Venus retained substantial water for much longer than previous studies suggested. Moreover, if the RG phase duration is controlled by the water loss rate (Hamano et al., 2013), it may have persisted for several Gyr.