The Venusian Atmospheric Oxygen Ion Escape : Extrapolation to the Early Solar System

The present atmosphere of Venus contains almost no water, but recent measurements indicate that in its early history, Venus had an Earth-like ocean. Understanding how the Venusian atmosphere evolved is important not only for Venus itself but also for understanding the evolution of other planetary atmospheres. In this study, we quantify the escape rates of oxygen ions from the present Venus to infer the past of the Venusian atmosphere. We show that an extrapolation of the current escape rates back in time leads to the total escape of 0.02-0.6 m of a global equivalent layer of water. This implies that the loss of ions to space, inferred from the present state, cannot account for the loss of an historical Earth-like ocean. We find that the O+ escape rate increases with solar wind energy flux, where more energy available leads to a higher escape rate. Oppositely, the escape rate decreases slightly with increased extreme ultraviolet radiation (EUV) flux, though the small variation of EUV flux over the measured solar cycle may explain the weak dependency. These results indicate that there is not enough energy transferred from the solar wind to Venus' upper atmosphere that can lead to the escape of the atmosphere over the past 3.9 billion years. This means that the Venusian atmosphere did not have as much water in its atmosphere as previously assumed or the present-day escape rates do not represent the historical escape rates at Venus. Otherwise, some other mechanisms have acted to more effectively remove the water from the Venusian atmosphere. Plain Language Summary Today, Venus only has small amounts of water in its atmosphere. In its early history, Venus presumably contained an Earth-like ocean of several meters. The evolution of the atmosphere may have been caused by escape of atmospheric content to space. In this study, we investigate how much the escape of oxygen ions to space could have affected the atmospheric evolution for Venus from measurements of the present-day escape rates. Using measurements of oxygen ions in the vicinity of Venus, we show that the amount of energy available in the solar wind to be transferred to the upper atmosphere of Venus determines how much of the atmosphere escapes. From the evolution of the energy in the solar wind over the past 3.9 billion years, together with the relation between the solar wind energy and oxygen ion escape, we show that in total, about 0.02-0.6 m of water depth, if spread equally over the entire Venusian surface, was lost. This indicates that either Venus did not have as much water as previously assumed or the current escape rates are not representative of the historical escape rates. Otherwise, some other mechanisms must have acted to more effectively remove the water from Venus