Coordinated 1996 HST and IRTF Imaging of Neptune and Triton. III. Neptune's Atmospheric Circulation and Cloud Structure

The first coordinated multispectral imaging of Neptune and Triton, using both the Hubble Space Telescope and the NASA Infrared Telescope Facility in August 1996 (L. A. Sromovsky et al., 149, 416-434, 435-458), provides new insights into Neptune's atmospheric circulation and cloud structure. From our 1996 data we derived cloud-tracked winds for 18 features spanning latitudes from 61°S to 38°N, finding generally close agreement with the smoothed 1989 Voyager wind profile (Sromovsky et al. 1993, Icarus105, 110-141), except for features associated with a northern Great Dark Spot (GDS), which moved with the spot rather than with the local mass flow, providing another example of GDS traveling companions. In addition to finding a GDS at ∼32°N, thought to be the same one first seen in 1994 near the same latitude (Hammel et al. 1995, Science268, 1740-1742), we also found a new kind of GDS at ∼15°N, which is unusual because of its low latitude and lack of a bright companion. From blue images we determined that the dark cloud band near 60°S latitude is significantly nonaxisymmetric, as was a similar band in 1989 Voyager images, both with a wavenumber-one zonal structure and a ∼5° latitudinal variation. We also identified a persistent pattern of clouds at 45°S, which exhibit strong variations in brightness on yearly time scales. Using HST archived observations, we determined cloud-tracked winds for June and November 1994 and September 1995, confirming and extending the 1995 result of Hammel and Lockwood (1997, Icarus129, 466-481) that most winds agreed with the Voyager profile. A significant exception we found in November 1994 is the ∼100 m/s meridional motion within the transient structure identified as the "bright complex" by H. B. Hammel and G. W. Lockwood (1997, Icarus129, 466-481). The integrated brightness of this group of GDS traveling companions is sufficient to produce a hemispheric brightness asymmetry of 0.36 mag (Sromovsky et al., 149, 435-458), comparable to the 0.44 mag associated with the 1987 "outburst" maximum (Hammel et al. 1992, Icarus99, 363-367). We determined spectral differences in cloud bands using complete global mosaics of 1996 imagery. Colocation of features in HST and groundbased near-IR images enabled us to characterize spectral variations in discrete features from the visible to 2.4 μm, from which we estimated cloud heights for discrete bright features ranging from 60 to 230 mbars. We found the equatorial atmosphere that is free of bright clouds to be very dark beyond 0.7 μm, from which we derived new upper bounds of ∼0.2 and ∼0.1 at 1.27 and 1.6 μm, respectively, on the single-scattering albedo of the putative 3.8-bar H ₂S cloud.