Clouds and circulation on Neptune: Implications of 1991 HST observations.

Abstract

Images of Neptune recorded by the Hubble Space Telescope (HST) Wide Fields/Planetary Camera in late 1991 provide a basis for the first significant comparison with the circulation results obtained from Voyager 2 observations 2 years earlier. The HST images using red (F675W) and methane band (F889N) filters contain isolated bright cloud features suitable for tracking atmospheric motions near planetocentric latitudes of 67°S, 57.5°S, 50°S, 5°N, and 27°N. Wind speeds inferred from the feature rotational periods are consistent with the latitudinal wind profile established by the 1989 Voyager observations. The latitudinal distribution of clouds also matches the Voyager distribution reasonably well, except for a notable lack of clouds between the equator and 40°S, a region where the Great Dark Spot (GDS) and its companion cloud were observed in 1989. The 1991 HST images contain no cloud feature at any latitude with an integrated brightness comparable to the GDS companion, suggesting that the GDS itself may not have survived its projected November 1990 arrival at Neptune's equator.