CLOUDS, HAZES, AND THE STRATOSPHERIC METHANE ABUNDANCE IN NEPTUNE

Analysis of high-spatial-resolution (approximately 0.8 arcsec) methane band and continuum imagery of Neptune's relatively homogeneous Equato rial Region yields significant constraints on (1) the stratospheric ga seous methane mixing ratio (f(CH4,s)), (2) the column abundances and o ptical properties of stratospheric and tropospheric hydrocarbon hazes, and (3) the wavelength-dependent single-scattering albedo of the 3-ba r opaque cloud. From the center-to-limb behavior of the 7270-angstrom and 8900-angstrom CH4 bands, the stratospheric methane mixing ratio is limited to f(CH4,s) < 1.7 x 10(-3), with a nominal value of f(CH4,s) = 3.5 x 10(-4), one to two orders of magnitude less than pre-Voyager e stimates, but in agreement with a number of recent ultraviolet and the rmal infrared measurements, and largely in agreement with the tropopau se mixing ratio implied by Voyager temperature measurements. Upper lim its to the stratospheric haze mass column abundance and 6190-angstrom and 8900-angstrom haze opacities are 0.61 mug CM-2 and 0.075 and 0.042 , respectively, with nominal values of 0.20 mug cm-2 and 0.025 and 0.0 14 for the 0.2-mum radius particles preferred by the recent Voyager PP S analysis of Pryor et al. (1992, Icarus 99, 302-316). The tropospheri c CH4 haze opacities are comparable to that found in the stratosphere, i.e., upper limits of 0.104 and 0.065 at 6190 angstrom and 8900 angst rom, respectively, with nominal values of 0.085 and 0.058. This indica tes a column abundance less than 11.0 mug cm-2, corresponding to the m ethane gas content within a well-mixed 3% methane tropospheric layer o nly 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 s tratospheric component, 6190-angstrom and 8900-angstrom imaginary indi ces 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 th e preferred value. However, conservative scattering is ruled out for t he opaque cloud near 3 bars marking the bottom of the visible atmosphe re. Specifically, we find cloud single-scattering albedos of 0.915 +/- 0.006 at 6340 angstrom, 0.775 +/- 0.012 at 7490 angstrom, and 0.803 /- 0.010 at 8260 angstrom. Global models utilizing a complete global s pectrum confirm the red-absorbing character of the 3-bar cloud. The gl obal-mean model has approximately 7.7 times greater stratospheric aero sol content then the Equatorial Region. An analysis of stratospheric h aze precipitation rates indicates a steady-state haze production rate of 0. 185-1.5 x 10(-14) g cm-2 sec-1, in agreement with recent theoret ical photochemical estimates. Finally, reanalysis of the Voyager PPS 7 500-angstrom phase angle data utilizing the f(CH4,s) value derived her e confirms the Pryor et al. result of a tropospheric CH4 haze opacity of a few tenths in the 22-30-degrees-S latitude region, several times that of the Equatorial Region or of the globe. The factor-of-10 reduct ion in f(CH4,s), below that assumed by Pryor et al. implies decreased gas absorption and consequently a decrease in the forward-scattering c omponent of tropospheric aerosols. (C) 1994 Academic Press, Inc.