Atmospheric Structure of Neptune in 1994, 1995, and 1996: HST Imaging at Multiple Wavelengths

Abstract

Hubble Space Telescope imaging of Neptune in 1994, 1995, and 1996 reveals changes in atmospheric features. Dark spots at 467 nm near latitude +31° were present in both 1994 and 1995, but absent from images in March 1996. However, a larger data set in August 1996 (Sromovskyet al., Bull. Am. Astron. Soc.28,1077, 1996) revealed a dark spot. The features resemble the Great Dark Spot present during the Voyager encounter in 1989 in size, shape, and contrast. Dark spots occur near the single brightest 619- and 889-nm features visible on the planet at that time. Methane-band clouds in the northern hemisphere were a factor of 6 brighter than the surrounding atmosphere in 1994, but only a factor of 2 brighter in 1995 and 1996. The pattern of zonal bands on Neptune was stable from 1994 to 1996, but differed from that seen by Voyager in 1989 when a prominent dark band from planetographic latitudes -40° to -68° dominated at blue wavelengths. In 1994-1996, this band was weaker and extended only from -55° to -70°, perhaps explaining the 0.06-mag brightening of Neptune between 1989 and 1994. Bright methane clouds generally appear at specific latitudes, but the active latitudes change from -25° in 1989 to -30° and -46° in 1994. The region near +30° was active in both 1989 and 1994-1996. We confirm the general shape and stability of the zonal wind profile measured by Voyager, revising previous measurements with more accurate values and filling in latitudes +38°, -30°, and -31° where no clouds had previously been seen.