Coordinated 1996 HST and IRTF Imaging of Neptune and Triton. II. Implications of Disk-Integrated Photometry

Near-IR groundbased observations coordinated with Wide Field Planetary Camera 2 (WFPC2) HST observations (Sromovsky et al.Icarus149, 416-434, 459-488) provide new insights into the variations of Neptune and Triton over a variety of time scales. From 1996 WFPC2 imaging we find that a broad circumpolar nonaxisymmetric dark band dominates Neptune's lightcurve at 0.467 μm, while three discrete bright features dominate the lightcurve at longer wavelengths, with amplitudes of 0.5% at 0.467 μm and 22% at 0.89 μm, but of opposite phases. The 0.89-μm modulation in 1994, estimated at 39%, is close to the 50% modulation observed during the 1986 "outburst" documented by Hammel et al. (1992, Icarus99, 363-367), suggesting that the unusual 1994 cloud morphology might also have been present in 1986. Lightcurve amplitudes in J-K bands, from August 1996 IRTF observations, are comparable to those observed in 1977 (D. P. Cruikshank 1978, Astrophys. J. Lett.220, 57-59) but significantly larger than the 1981 amplitudes of M. J. S. Belton et al. (1981, Icarus45, 263-273). The 1996 disk-integrated albedos of Neptune at H-K wavelengths are 2-7 times smaller than the 1977 values of U. Fink and S. Larson (1979, Astrophys. J.233, 1021-1040), which can be explained with about 1/2-1/4 of the upper level cloud opacity being present in 1996. A simplified three-layer model of cloud structure applied to CCD wavelengths implies ∼7% reflectivity at 1.3 bars (at λ=0.55 μm, decreasing as λ ^-0.94) and ∼1% at 100-150 mbars. To fit the WFPC2 observations and those of E. Karkoschka (1994, Icarus111, 174-192), the putative H ₂S cloud between 3.8 and 7-9 bars must have a strong decrease in reflectivity between 0.5 and 0.7 μm, as previously determined by K. H. Baines and W. H. Smith (1990, Icarus85, 65-108). To match our 1996 IRTF results, this cloud must have another substantial drop in reflectivity at near-IR wavelengths, to a level of 0-5%, corresponding to single-scattering albedos of ∼0-0.3. The model that fits our near-IR observations on 13 August 1996 can reproduce the magnitudes of the dramatic 1976 "outburst" (R. R. Joyce et al. 1977, Astrophys. J.214, 657-662) by increasing the upper cloud fraction to 6% (from ∼1%) and lowering its effective pressure to ∼90 mbars (from 151 mbars). Triton's disk-integrated albedo from HST imagery at 11 wavelengths from 0.25 to 0.9 μm are consistent with previous groundbased and Voyager measurements, thus providing no evidence for the albedo decrease suggested by Triton's recent warming (J. L. Elliot et al. 1998 Nature393, 765-767). Triton's lightcurve inferred from 1994-1996 WFPC2 observations has about twice the amplitude inferred from 1989 Voyager models for the UV to long visible range (J. Hillier et al. 1991, J. Geophys. Res.96, 19,211-19,215).