Methane photochemistry and haze production on Neptune

Abstract

A numerical model was used to study methane photochemistry in the stratosphere of Neptune. The observed mixing ratio of methane, 2%, forces photolysis to occur near the CH4 homopause. For an assumed nominal value of the eddy mixing coefficient of 106 cm2 sec-1 at the CH4 homopause, the predicted average mixing ratios of C2H6 and C2H2, 1.5 x 10-6 and 6 x 10-7, respectively, agree well with observations in the infrared. The acetylene and ethane abundances are weakly dependent upon the strength of the eddy mixing and directly proportional to it. Haze production from methane photochemistry results from the formation of hydrocarbon ices and polyacetylenes. The calculated mixing ratios of C2H6, C2H2 are large enough to cause condensation to their respective ices near the tropopause. These hazes are capable of providing the necessary aerosol optical depth at the appropriate pressure levels required by observations of Neptune in the visible and near IR. Polyacetylene formation from C2H2 photolysis is limited by the low quantum yield of dissociation for acetylene, efficient recycling of its photolysis products by the other hydrocarbons, and the greatly reduced solar flux at Neptune. Comparisons of model predictions to Uranus show both a lower ratio of polyacetylene production to hydrocarbon ice and a lower likelihood of UV postprocessing of the acetylene ice to polymers on Neptune compared to Uranus. This is in agreement with the observed difference in the single scattering albedo of the stratospheric aerosols in the visible between Uranus and Neptune, with the aerosols on Neptune being brighter.