Thermal Effects of Acoustic-gravity Waves in Titan’s Upper Atmosphere and Their Implications for Atmospheric Jeans Escape

Abstract Early observations from the Cassini spacecraft revealed the presence of acoustic-gravity waves in Titan’s upper atmosphere. However, the impacts of wave modes with different temporal and spatial scales on the thermal structure and atmospheric escape have not been fully investigated. In this study, we use a linearized full-wave model to examine the wave heating/cooling effects and the resulting Jeans escape flux of molecular hydrogen in Titan’s atmosphere. The results show that wave-induced heating/cooling is primarily governed by the sensible heat flux and viscous heating rate. Acoustic waves consistently heat the upper atmosphere, with maximum heating rates of up to 80 K day −1 and exobase temperature increases of up to 22 K, leading to enhancements of the Jeans escape flux by 3%–24%. Gravity waves with high phase speeds (>69 m s −1 ) also contribute to thermospheric heating, producing heating rates of up to 30 K day −1 , increasing the exobase temperature by 4–5 K and elevating the escape flux by 2%–6%. In contrast, gravity waves with lower phase speeds predominantly cool the upper atmosphere, with peak cooling rates reaching 10 K day −1 , reducing the exobase temperature by up to 3.5 K and decreasing the escape flux by up to 4%. These findings highlight the important role of acoustic-gravity waves in governing Titan’s upper atmospheric thermal structure and regulating atmospheric escape and provide broader insights into the contribution of these waves to the thermal structure of upper atmospheres of different planets and moons.