Photochemistry of Pluto's atmosphere and ionosphere near perihelion

We consider Pluto's photochemistry using a background model for a hydrodynamically escaping atmosphere by Krasnopolsky [1999]. Some adjustments are made in the basic continuity equation and in the boundary conditions to account for hydrodynamic flow in the atmosphere. We model the photochemistry for 44 neutral and 23 ion species. Because of the high methane mixing ratio, Pluto's photochemistry is more similar to that of Titan than that of Triton. Charge exchange between N 2 + and CH 4 significantly reduces the production of atomic nitrogen. The most abundant photochemical products are C 2 H 2 (3×10 17 ), C 4 H 2 (10 17 ), HCN (6×10 16 ), H 2 (4×10 16 ), C 2 H 4 (4×10 16 ), HC 3 N (3.4×10 16 ), C 2 H 6 (2×10 16 ), C 3 H 2 (9×10 15 ), and C 3 H 4 (8×10 15 , all in cm −2 ). In addition to the parent N 2 , CH 4 , and CO molecules which absorb photons with λ<145 nm, these products absorb almost completely photons with λ<185 nm, therefore significantly increasing the number of dissociation events. Photochemical losses of the parent species are much smaller than their escape. Precipitation rates are the highest for C 2 H 2 , C 4 H 2 , HC 3 N, HCN, C 2 H 6 , and C 2 H 4 (65, 58, 23, 14, 9, and 6 g cm −2 Byr −1 , respectively, reduced by a factor of 3 to account for seasonal variations). Escape of photochemical products is highest for H 2 , H, C 2 H 2 , C 2 H 4 , HCN, and N (2×10 26 , 1.4×10 26 , 6×10 24 , 3.6×10 24 , 2.3×10 24 , and 1.8×10 24 s −1 , respectively). The electron density reaches a maximum of 800 cm −3 at 2250 km. The most abundant ions are HCNH + , C 3 H 3 + , and C 3 H 5 + . Some of the photochemical products might be detected using the technique of UV solar occultation spectroscopy from a spacecraft flyby.