Lyα Emissions from Titan’s Atmosphere and Exosphere

From 2004 to 2017, the Cassini/UVIS instrument observed Titan’s Lyα airglow from Titan’s atmosphere and exosphere, helping to characterize Titan’s H population. Most Lyα observations remain unanalyzed. Using data from the T10 Titan flyby, we analyze Lyα signal profiles from observations centered at Titan’s equator. We implement a 1D radiative transfer model with Lyα resonant scattering to investigate poorly understood aspects of Titan’s atmosphere related to atomic H. Our 1D algorithm fully includes the interplanetary hydrogen (IPH) contribution in models of Titan’s Lyα dayglow. Titan’s Lyα dayglow is dominated by the direct IPH contribution above 1000 km altitude. Methane absorption of the direct IPH contribution shapes Titan’s Lyα profile between 900 and 1500 km altitude, placing strong constraints on CH₄ densities within this region. Our 1D algorithm also includes absorption by minor hydrocarbons and nitriles, which provides a small contribution to Titan’s Lyα dayglow below ∼1250 km altitude; among these species, C₂H₄ most significantly contributes to Lyα absorption. We utilize a 3D model to test the 1D approach, ensuring that 1D radiative transfer is sufficient for calculating the resonant scattering source functions on Titan. Our results demonstrate the importance of including the full Lyα line profile in the model instead of merely Doppler broadening. For the T10 equatorial observations, we calculate an exobase H density of (1.1 ± 1.0) × 10³ cm⁻³ and diffusion-limited H escape rate of (7.3 ± 6.3) × 10²⁴ s⁻¹. By placing constraints on Titan’s H population, our findings can contextualize CH₄ photochemistry and escape.