Photochemistry of Pluto's atmosphere and ionosphere near perihelion

Abstract

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 N2+ and CH4 significantly reduces the production of atomic nitrogen. The most abundant photochemical products are C2H2 (3×1017), C4H2 (1017), HCN (6×1016), H2 (4×1016), C2H4 (4×1016), HC3N (3.4×1016), C2H6 (2×1016), C3H2 (9×1015), and C3H4 (8×1015, all in cm-2). In addition to the parent N2, CH4, 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 C2H2, C4H2, HC3N, HCN, C2H6, and C2H4 (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 H2, H, C2H2, C2H4, HCN, and N (2×1026, 1.4×1026, 6×1024, 3.6×1024, 2.3×1024, and 1.8×1024 s-1, respectively). The electron density reaches a maximum of 800 cm-3 at 2250 km. The most abundant ions are HCNH+, C3H3+, and C3H5+. Some of the photochemical products might be detected using the technique of UV solar occultation spectroscopy from a spacecraft flyby. Copyright 1999 by the American Geophysical Union.