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The Saturnian ribbon feature—A baroclinically unstable model

1986; Elsevier BV; Volume: 68; Issue: 2 Linguagem: Inglês

10.1016/0019-1035(86)90026-6

1090-2643

David Godfrey, V. Moore,

Geology and Paleoclimatology Research

Images of Saturn from the two Voyager spacecraft showed few discrete atmospheric features in the low-contrast latitudinal banding over the planet. One unusual feature, however, was a narrow, dark, oscillatory structure encircling the planet in the light zone at 45° to 50° north. This paper presents a description of this dark structure, which we call the ribbon feature. A wavelength of 5710 ± 260 km is obtained from moderate resolution images which were used to produce a 360° image of the feature. Higher resolution image pairs were used to examine the local circulation which shows counterclockwise rotation north of the ribbon and clockwise rotation south of it. The zonal circulation measurements show that it is located in a 120 m sec−1 westerly jet but with a small (≤5 m sec−1) meridional velocity. The infrared spectrometer results from Voyager showed the greatest thermal wind sheer in this latitude region. The regions adjacent to the ribbon are of similar brightness except for the UV (3400 Å) filter images where the northern part of this zone is lighter than the southern section. From this observation we have estimated that the disturbance associated with the ribbon feature extends to at least the 200-mbar level. A study of a 120° section of the ribbon feature and the associated latitudinal thermal gradient by L. A. Sromovsky, H. E. Revercomb, R. J. Krauss, and V. E. Suomi (1983, J. Geophys. Res. 88, 8650–8666) led them to suggest that it was produced by some form of baroclinic instability. This proposal is considered here, with other alternative types of instability. We conclude that the ribbon feature could arise from a baroclinic instability. We then develop a numerical model of such an instability that is consistent with the observed wavelength, the vertical extent of the ribbon, and the local latitudinal thermal profile.