CCD imaging of Neptune at methane-band wavelengths

Abstract

Ground-based CCD imaging of Neptune in the near IR clearly shows discrete cloud features. The brightest cloud features on Neptune are confined to latitudes from about 30° to 50° (in both hemispheres). Fainter enhanced regions are sometimes seen at even higher latitudes. Imaging obtained prior to 1986 showed bright clouds in both northern and southern mid-latitudes, giving Neptune the appearance of having a dark equatorial belt. But since 1986, only the southern hemisphere has shown clouds; no bright features have been seen in the northern hemisphere. The equatorial region is not significantly darker than the northern mid-latitudes. Discrete features are most obvious in the strongest methane band (8900 Å), but brightness asymmetry is seen in weaker methane bands (7270 and 6190 Å). Disk-integrated photometry obtained from the 1986 and 1987 imaging was calibrated absolutely with the same flux standard. In both years, variability at 8900 Å was dominated by scattered light from a single bright cloud feature. The amplitude of the variability was a factor of two higher in 1987. The brightness of the less active hemisphere appears relatively constant between the two years, although some variability is seen on this hemisphere in 1987.