H2 Chemistry and Protonation in the Dayside Venusian Upper Atmosphere

Although H2 is present in low concentrations, it critically controls the structure and composition of planetary upper atmospheres through protonation under solar extreme-ultraviolet and soft X-ray irradiation. Recent studies highlight the importance of protonation on Venus. As H2 concentrations in its upper atmosphere remain poorly constrained, we conduct a systematic model investigation of how the structure of the topside Venusian ionosphere responds to varying H2 distributions. Our model results suggest that protonated species could outnumber nonprotonated species in the topside ionosphere when ambient H2 concentrations exceed 5 ppm. Generally, increasing H2 concentrations enhances the degree of protonation, manifesting as elevated levels of most protonated species, while nonprotonated species are generally depleted due to rapid reactions with H2. These trends, however, are not universal; nonmonotonic variations among multiple species like OH+ and O 2 + emerge, driven by the complex chemical network that intricately links species distributions throughout the upper atmosphere. The results presented here provide a foundation for improved characterization of hydrogen escape on Venus during its evolutionary history.