Hydrocarbons in Neptune's stratosphere from Voyager infrared observations

Emission from the acetylene and ethane bands at 729 and 822 cm-1detected in the Voyager infrared spectra of Neptune has been analyzed. A large selection of low-spatial resolution spectra was used to derive the disk-averaged abundances of C2H2 and C2H6. Under the assumption of uniform vertical distributions above the saturation region, a C2H2 mixing ratio of 6−4+14 x 10 −8 and a C2H6 mixing ratio of 1.5−0.5+2.5 x 10−6 were inferred. The accuracy of the retrievals is limited by the large uncertainty in the stratospheric temperature structure. The maximum contribution to the observed C2H2 and C2H6emission comes from the 0.2- and 0.7-mbar regions, respectively. Mixing ratio profiles derived from photochemical modeling, which are not constant with height above the saturation region, indicate that the hydrocarbon emission is most sensitive to the assumed eddy diffusion coefficient in the millibar region. Either the C2H2 or the C2H6 emission can be reproduced by the photochemical model to within the accuracy of the retrievals, but not both simultaneously. Best fits to both emission features simultaneously occur with C2H2 mixing ratios a factor of 2 too high and C2H6 mixing ratios a factor of 2 too low. We consider this agreement satisfactory considering the unknowns in the chemical and photolytic processes. A set of Voyager spectra at higher spatial resolution was used to study the latitudinal variation of the C2H2 emission between 30°N and 80°S. Zonal mean radiances at the C2H2 peak show a minimum near 50°–60°S and maxima near the south pole and equator. This behavior is similar to that observed at 350 and 250 cm−1, where the lower stratosphere and troposphere are sounded. The mid-latitude minimum can be explained by a fivefold depletion of acetylene or a temperature decrease of about 15 K (or any combination of the two effects) in the 0.03- to 2-mbar region. The latitude variation in the C2H2emission could result from a circulation pattern forced from deep levels, with upwelling at mid-latitudes and subsidence at low and high latitudes.

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