Methane

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 (faa,s), (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 clou~./ \ From the center-to-limb behavior of 7270- ~ and 8900- ~ CH4 bands, the stratospheric methane mixing ratio is limited to f=~,s < 1.7103, with a nominal value fmd,s = 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- ~ and 8900- ~ haze opacities are 0.61 pg cm-2 and 0.075 and 0.042, respectively, with nominal values of 0.195 pg cm-2 and 0.0245 and 0.0137 for the 0.2-pm radius particles preferred by the recent Voyager PPS analysis of Pryor et al. (1992, Icarus 99, 302-316), The tropospheric CH4 haze opacities ar