Methane Photochemistry on Neptune: Ethane and Acetylene Mixing Ratios and Haze Production

We have used a one-dimensional methane photochemical model to analyze Voyager observations of hydrocarbons and hazes in the stratosphere of Neptune. Vayager IRIS spectra provide information about the global average C2H2 and C2H6 mixing ratios for p > 0.1 mbar. The UVS lightcurves provide constraints on CH4 and C2H4 in addition to C2H2 and C2H6 but only at the solar occultation latitudes and for lower pressures. The model-predicted hydrocarbons are very sensitive to the height profile of the eddy diffusion coefficient (K). For both data sets K varying inversely with the atmospheric number density to some power produced poor results. Good agreement with the data requires that K be weak in the lower stratosphere (K ≃ 2 × 103 cm2 sec-1 for p⪊2 mbar) but fairly vigorous in the upper stratosphere (K > 5 × 107 cm2 sec-1 for p⪉ 0.5 mbar), i.e., a rapidly mixed upper stratosphere overlying a stagnant lower stratosphere with a rapid transition in between. The model C2H6 and C2H2 mixing ratios are also sensitive to the reaction rate constants of C2H4 + H and CH3 + C2H3. Notably, we must use the present upper limit for the C2H4 + H rate to best fit the model results to the observations. We are able to reproduce the IRIS C2H2 and C2H6 emission features well, less so the UVS occultation lightcurves. Since the transport of C2H2, C2H6, and other hydrocarbons produced from methane photolysis out of the stratosphere is by ice haze formation and sedimentation, we compared model haze predictions to PPS and IRIS observations. For solar maximum fluxes (Voyager encounter conditions) the model mass production rate is 1 × 10-14 g cm2 sec-1. C2H6 is the dominant haze component (75%), with the remainder coming from C2H2 and C3 and C4 compounds. Balancing the above haze production rate by the sedimentation rate for 0.25-μm radius particles (upper limit to particle radius from PPS observations) yields a total haze column burden slightly above the PPS upper limit. However, lifetime analysis indicates that the model haze production rate should be averaged over solar minimum and maximum conditions. Under these conditions the model haze density is consistent with the PPS data. The predicted C4H2 and C2H6 haze column densities are consistent with the lack of ice signatures in the IRIS spectra.