The Extraordinary Activity of the Southern Equatorial Zone of Saturn During 1996

A. Sanchez-Lavega (UPV), J. Lecacheux (Obs. Meudon), F. Colas (BDL), J.R. Acarreta (LAEFF), J.M. Gomez, A. Garcia (GEA), I. Miyazaki (Japan), D. Parker (USA)

Regular CCD imaging of Saturn from June to November 1996 (summarizing a total of 33 days), has revealed an unprecedent and intense meteorological activity in the Southern part of the Equatorial Zone. The activity manifested in form of streaks of white clouds aligned preferentially in the zonal direction and spanning from the Equator to latitude (graphic) -20 degrees. The source were two bright white "plumes" located at -12 deg. Both survived during the whole observing period (more than 4 months), moving uniformly with a zonal velocity of 274 m/s relative to System III. This value is the same as that measured in 1946 by H. Camichel on a single bright spot at the same latitude. Multiwavelength imaging of the plumes (from 400 to 900 nm) show they were bright in the continuum and in the methane bands at 619, 725 and 890 nm, indicating that they were thick and placed at a high altitude relative to theirs surroundings. The plumes were probably formed by ammonia ice injected in the upper atmosphere by apward convective motions. This convective activity was coincident with an abrupt increase in the sunlight received at these latitudes following the seasonal insolation cycle and the end of a period of shadowing produced by the rings. It could also be related to the storm activity developed in 1990 and 1994 in the Northern part of the Equatorial Zone.