Near-infrared observations of Neptune's tropospheric cloud layer with the Lick Observatory adaptive optics system

We provide one of the first constraints on the combined infrared single-sca ttering albedo and opacity of Neptune's upper tropospheric cloud layer. For the observations, we used the adaptive optics system on the Lick Observato ry's 3 m Shane Telescope (Mount Hamilton, California). The cloud layer is t hought to be composed H2S of and extend up to 3.5-4.5 bars. Previously, the single-scattering albedo was measured in the range 0.2-0.94 mum and found to be extremely high (>0.8), but decreasing with increasing wavelength. Ass uming an optically thick cloud, we find the best-Dt single-scattering albed o of a 3.5 bar layer to be at 0.23(-0.08)(+0.07) at 1.27 mum and 0.18(-0.04 )(+0.03) at 1.56 mum. Uncertainties in the column density of haze above the cloud layer, and from deconvolution to remove contaminating light scattere d by the point-spread function from infrared-bright features, indicate that the cloud could be even darker, but it is unlikely to be brighter than we report. The cloud particles could be brighter than we report if the total n ear-infrared opacity of the cloud is very low or the cloud's scattering pha se function is significantly more forward-scattering at 1.2-1.6 mum than at 0.75 mum.