Seasonal Enhancement in Upper Atmospheric D/H at Mars Driven by Both Thermospheric Temperature and Mesospheric Water

The D/H ratio in water on Mars, R water , is 4–6× the Earth ratio, signifying past water loss to space. Recently, measurements have revealed high values of the D/H ratio in hydrogen, R atomic , in the thermosphere during southern summer. Here, we use a photochemical model to explore the potential drivers of R atomic , testing three: thermospheric temperatures, excess mesospheric water, and changing insolation. We find that R atomic can achieve values between 15× the Earth ratio (due to water) and 23× the Earth ratio (due to temperature). The effects arise because H escape is diffusion‐limited, while D escape is energy‐limited. Our results underscore how R atomic reflects mesospheric dynamics, and the need for concurrent measurements of mesospheric water, thermospheric temperatures, and R atomic to understand seasonal changes in the martian water cycle and atmospheric loss.