Clouds, Hazes, and the Stratospheric Methane Abundance in Neptune

Abstract

Analysis of high-spatial-resolution (∼0.8 arcsec) methane band and continuum imagery of Neptune's relatively homogeneous Equatorial Region yields significant constraints on (1) the stratospheric gaseous methane mixing ratio (ƒCH4,3), (2) the column abundances and optical properties of stratospheric and tropospheric hydrocarbon hazes, and (3) the wavelength-dependent single-scattering albedo of the 3-bar opaque cloud. From the center-to-limb behavior of the 7270-Å and 8900-Å CH4 bands, the stratospheric methane mixing ratio is limited to ƒCH4,3 < 1.7 × 10-3, with a nominal value of ƒCH4,3 = 3.5 × 10-4, one to two orders of magnitude less than pre-Voyager estimates, but in agreement with a number of recent ultraviolet and thermal infrared measurements, and largely in agreement with the tropopause mixing ratio implied by Voyager temperature measurements. Upper limits to the stratospheric haze mass column abundance and 6190-Å and 8900-Å haze opacities are 0.61 μg cm-2 and 0.075 and 0.042, respectively, with nominal values of 0.20 μg cm-2 and 0.025 and 0.014 for the 0.2-μm radius particles preferred by the recent Voyager PPS analysis of Pryor et al. (1992, Icarus 99, 302-316). The tropospheric CH4 haze opacities are comparable to that found in the stratosphere, i.e., upper limits of 0.104 and 0.065 at 6190 Å and 8900 Å, respectively, with nominal values of 0.085 and 0.058. This indicates a column abundance less than 11.0 μg cm-2, corresponding to the methane gas content within a well-mixed 3% methane tropospheric layer only 0.1 cm think near the 1.5-bar CH4 condensation level. Constraints on the single-scattering albedos of these hazes include (1) for the stratospheric component, 6190-Å and 8900-Å imaginary indices of refraction less than 0.047 and 0.099, respectively, with 0.000 (conservative scattering) being the nominal value at both wavelengths, and (2) CH4 haze single-scattering albedos greater than 0.85 and 0.50 at these two wavelengths, with conservative scattering again begin the preferred value. However, conservative scattering is ruled out for the opaque cloud near 3 bars marking the bottom of the visible atmosphere. Specifically, we find cloud single-scattering albedos of 0.915 ± 0.006 at 6340 Å, 0.775 ± 0.012 at 7490 Å, and 0.803 ± 0.010 at 8260 Å. Global models utilizing a complete global spectrum confirm the red-absorbing character of the 3-bar cloud. The global-mean model has ∼7.7 times greater stratospheric aerosol content then the Equatorial Region. An analysis of stratospheric haze precipitation rates indicates a steady-state haze production rate of 0.185-1.5 × 10-14 g cm-2 sec-1, in agreement with recent theoretical photochemical estimates. Finally, reanalysis of the Voyager PPS 7500-Å phase angle data utilizing the ƒCH4,3 value derived here confirms the Pryor et al. result of a tropospheric CH4 haze opacity of a few tenths in the 22-30°S latitude region, several times that of the Equatorial Region or of the globe. The factor-of-10 reduction in ƒCH4,3 below that assumed by Pryor et al. implies decreased gas absorption and consequently a decrease in the forward-scattering component of tropospheric aerosols.