THE DISTRIBUTION OF HYDROCARBONS IN NEPTUNE UPPER-ATMOSPHERE

Data from the Voyager UVS solar occultation experiment are analyzed to determine the distribution of hydrocarbons in the upper atmosphere of Neptune. Objective inversion techniques are used to infer densities from the transmission properties of the atmosphere measured by the UVS. Densities of H2, CH4, and C2H6 are determined over limited altitude ranges and constraints are placed on the abundances of C2H2 and C2H4. The inferred densities when used in conjunction with models for the transport of CH4 and C2H6 imply that the CH4 mole fraction in the stratosphere is between 6 × 10-4 and 5 × 10-3, i.e., more than one order of magnitude larger than expected if the tropopause cold trap is operating. This conclusion is independent of assumptions about the temperature profile in the stratosphere. C2H6 is produced from CH4 photolysis with an efficiency of 30-50%. The eddy diffusion coefficient has a value of 1.0 ± 0.3 × 105 cm2 sec-1 at and below 300 km, rising exponentially to a value of 2.4+0.7-0.5 × 106 cm2sec-1 at 550 km. The C2H6 densities determined from this study are consistent with results from groundbased and Voyager infrared observations. The H2 densities are in excellent agreement with results from the Voyager radio occultation. © 1993 by Academic Press, Inc.