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

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 1997-10, Vol.129 (2), p.466-481
• Main Authors: Hammel, H.B., Lockwood, G.W.
• Format: Article
• Language: English
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1006/icar.1997.5764

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.