Saturn's zonal wind profile in 2004-2009 from Cassini ISS images and its long-term variability

摘要

Five years of Cassini Imaging Science Subsystem images, from 2004 to 2009, are analyzed in this work to retrieve global zonal wind profiles of Saturn's northern and southern hemispheres in the methane absorbing bands at 890 and 727. nm and in their respective adjacent continuum wavelengths of 939 and 752. nm. A complete view of Saturn's global circulation, including the equator, at two pressure levels, in the tropopause (60. mbar to 250. mbar with the MT filters) and in the upper troposphere (from ~350. mbar to ~500. mbar with the CB filter set), is presented. Both zonal wind profiles (available at the Supplementary Material Section), show the same structure but with significant differences in the peak of the eastward jets and the equatorial region, including a region of positive vertical shear symmetrically located around the equator between the 10° < _c < 25° where zonal velocities close to the tropopause are higher than at 500. mbar. A comparison of previously published zonal wind sets obtained by Voyager 1 and 2 (1980-1981), Hubble Space Telescope, and ground-based telescopes (1990-2004) with the present Cassini profiles (2004-2009) covering a full Saturn year shows that the shape of the zonal wind profile and intensity of the jets has remained almost unchanged except at the equator, despite the seasonal insolation cycle and the variability of Saturn's emitted power. The major wind changes occurred at equatorial latitudes, perhaps following the Great White Spot eruption in 1990. It is not evident from our study if the seasonal insolation cycle and its associated ring shadowing influence the equatorial circulation at cloud level.