H+/O+ Escape Rate Ratio in the Venus Magnetotail and its Dependence on the Solar Cycle

Abstract

A fundamental question for the atmospheric evolution of Venus is how much water-related material escapes from Venus to space. In this study, we calculate the nonthermal escape of H+ and O+ ions through the Venusian magnetotail and its dependence on the solar cycle. We separate 8 years of data obtained from the ion mass analyzer on Venus Express into solar minimum and maximum. The average escape of H+ decreased from 7.6 · 1024 (solar minimum) to 2.1 · 1024 s−1 (solar maximum), while a smaller decrease was found for O+: 2.9 · 1024 to 2.0 · 1024 s−1. As a result, the H+/O+ flux ratio decreases from 2.6 to 1.1. This implies that the escape of hydrogen and oxygen could have been below the stoichiometric ratio of water for Venus in its early history under the more active Sun.