Ly α Emissions from Titan’s Atmosphere and Exosphere
Abstract
From 2004 to 2017, the Cassini/UVIS instrument observed Titan’s Ly α airglow from Titan’s atmosphere and exosphere, helping to characterize Titan’s H population. Most Ly α observations remain unanalyzed. Using data from the T10 Titan flyby, we analyze Ly α signal profiles from observations centered at Titan’s equator. We implement a 1D radiative transfer model with Ly α 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 Ly α dayglow. Titan’s Ly α dayglow is dominated by the direct IPH contribution above 1000 km altitude. Methane absorption of the direct IPH contribution shapes Titan’s Ly α profile between 900 and 1500 km altitude, placing strong constraints on CH 4 densities within this region. Our 1D algorithm also includes absorption by minor hydrocarbons and nitriles, which provides a small contribution to Titan’s Ly α dayglow below ∼1250 km altitude; among these species, C 2 H 4 most significantly contributes to Ly α 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 Ly α 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) × 10 3 cm −3 and diffusion-limited H escape rate of (7.3 ± 6.3) × 10 24 s −1 . By placing constraints on Titan’s H population, our findings can contextualize CH 4 photochemistry and escape.
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BibTeX
@article{hoover2026emissions,
title = {{Ly α Emissions from Titan’s Atmosphere and Exosphere}},
author = {Devin Hoover and Panayotis Lavvas and Tommi Koskinen and Nathan Le Guennic and Dolon Bhattacharyya and Peter Stephenson},
journal = {The Planetary Science Journal},
year = {2026},
month = jul,
doi = {10.3847/psj/ae7a46},
url = {https://univ-reims.hal.science/hal-05835280v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Ly α Emissions from Titan’s Atmosphere and Exosphere
AU - Devin Hoover
AU - Panayotis Lavvas
AU - Tommi Koskinen
AU - Nathan Le Guennic
AU - Dolon Bhattacharyya
AU - Peter Stephenson
PY - 2026
DA - 2026/07/03
JO - The Planetary Science Journal
DO - 10.3847/psj/ae7a46
UR - https://univ-reims.hal.science/hal-05835280v1
AB - From 2004 to 2017, the Cassini/UVIS instrument observed Titan’s Ly α airglow from Titan’s atmosphere and exosphere, helping to characterize Titan’s H population. Most Ly α observations remain unanalyzed. Using data from the T10 Titan flyby, we analyze Ly α signal profiles from observations centered at Titan’s equator. We implement a 1D radiative transfer model with Ly α 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 Ly α dayglow. Titan’s Ly α dayglow is dominated by the direct IPH contribution above 1000 km altitude. Methane absorption of the direct IPH contribution shapes Titan’s Ly α profile between 900 and 1500 km altitude, placing strong constraints on CH 4 densities within this region. Our 1D algorithm also includes absorption by minor hydrocarbons and nitriles, which provides a small contribution to Titan’s Ly α dayglow below ∼1250 km altitude; among these species, C 2 H 4 most significantly contributes to Ly α 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 Ly α 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) × 10 3 cm −3 and diffusion-limited H escape rate of (7.3 ± 6.3) × 10 24 s −1 . By placing constraints on Titan’s H population, our findings can contextualize CH 4 photochemistry and escape.
SN - 2632-3338
ER -