The photochemistry of methane in the atmosphere of Triton

The photochemistry of methane in Triton's atmosphere is driven by approximately equal contributions of solar and local Interstellar medium (LISM) Lyα and in combination with weak vertical mixing, K ∼ 4 × 10 3 in the winter hemisphere and 8 × 10 3 cm 2 s −1 in the summer hemisphere, yields the observed CH 4 scale heights of ∼ 9 km below 70 km altitude (Broadfoot et al., 1989). The hemispheric difference in K is consistent with a seasonal meridional circulation with upwelling in the summer hemisphere and downwelling in the winter hemisphere. Methane photolysis produces H 2 molecules which are removed from the photolysis region by an upward flux ( ∼ 1.5 × 10 8 cm −2 s −1 , normalized to the surface) to the ionosphere where ion reactions convert H 2 to H. This results in a hydrogen escape flux of 3 × 10 8 cm −2 s −1 . The H column density is ∼ (5−10) × 10 14 cm −2 and resonantly scatters ∼ 26 R of solar Lya consistent with the day‐night asymmetry observed by the Voyager UVS. The inferred surface mixing ratios of CH 4 indicate subsaturated conditions. The CH 4 photolysis rate is sufficient to generate a smog of condensed C 2 H 2 , C 2 H 4 , C 2 H 5 , and C 4 H 2 particles in the lowest 30 km of Triton's atmosphere with an optical depth consistent with Voyager imaging results.