ATMOSPHERIC STRUCTURE OF NEPTUNE IN 1994, 1995, AND 1996 - HST IMAGING AT MULTIPLE WAVELENGTHS

Hubble Space Telescope imaging of Neptune in 1994, 1995, and 1996 reve als changes in atmospheric features. Dark spots at 467 mn near latitud e +31 degrees were present in both 1994 and 1995, but absent from imag es in March 1996. However, a larger data set in August 1996 (Sromovsky et at, Bull. Am. Astron. Sec. 28, 1077, 1996) revealed a dark spot. T he features resemble the Great Dark Spot present during the Voyager en counter in 1989 in size, shape, and contrast. Dark spots occur near th e single brightest 619- and 889-nm features visible on the planet at t hat time. Methane-band clouds in the northern hemisphere were a factor of 6 brighter than the surrounding atmosphere in 1994, but only a fac tor of 2 brighter in 1995 and 1996. The pattern of zonal bands on Nept une was stable from 1994 to 1996, but differed from that seen by Voyag er in 1989 when a prominent dark band from planetographic latitudes -4 0 degrees to -68 degrees dominated at blue wavelengths. In 1994-1996, this band was weaker and extended only from -55 degrees to -70 degrees , 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 degrees in 1989 to -30 degr ees and -46 degrees in 1994. The region near +30 degrees was active in both 1989 and 1994-1996. We confirm the general shape and stability o f the zonal wind profile measured by Voyager, revising previous measur ements with more accurate values and filling in latitudes +38 degrees, -30 degrees, and -31 degrees where no clouds had previously been seen . (C) 1997 Academic Press.