The photochemistry of methane in Triton's atmosphere is driven by approximately equal contributions of solar and local interstellar medium (LISM) Ly&agr; and in combination with weak vertical mixing, K~4¿103 in the winter hemisphere and 8¿103 cm2 s-1 in the summer hemisphere, yields the observed CH4 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 downwellling in the winter hemisphere. Methane photolysis produces H2 molecules which are removed from the photolysis region by an upward flux (~1.5¿108 cm-2 s-1, normalized to the surface) to the ionosphere where ion reactions convert H2 to H. This results in a hydrogen escape flux of 3¿108 cm-2 s-1. The H column density is ~5--10)-1014 cm-2 and resonantly scatters ~26 R of solar Ly&agr;, consistent with the day-night asymmetry observed by the Voyager UVS. The inferred surface mixing ratios of CH4 indicate subsaturated conditions. The CH2 photolysis rate is sufficient to generate a smog of condensed C2H2, C2H4, C2H6, and C4H2 particles in the lowest 30 km of Triton's atmosphere with an optical depth consistent with Voyager imaging results. ¿The American Geophysical Union 1990