On the vertical wind shear of Saturn's Equatorial Jet at cloud level

Abstract

In this work we analyze and compare the vertical cloud structure of Saturn's Equatorial Zone in two different epochs: the first one close to the Voyagers flybys (1979–1981) and the second one in 2004, when the Cassini spacecraft entered its orbit around the planet. Our goal is to retrieve the altitude of cloud features used as zonal wind tracers in both epochs. We reanalyze three different sets of photometrically calibrated published data: ground-based in 1979, Voyager 2 PPS and ISS observations in 1981, and we analyze a new set of Hubble Space Telescope images for 2004. For all situations we reproduced the observed reflectivity by means of a similar vertical model with three layers. The results indicate the presence of a changing tropospheric haze in 1979–1981 ( P top ∼ 100 mbar , τ ∼ 10 ) and in 2004 ( P top ∼ 50 mbar , τ ∼ 15 ) where the tracers are embedded. According to this model the Voyager 2 ISS images locate cloud tracers moving with zonal velocities of 455 to 465 (±2) m/s at a pressure level of 360 ± 140 mbar. For HST observations, our previous works had showed cloud tracers moving with zonal wind speeds of 280 ± 10 m / s at a pressure level of about 50 ± 10 mbar . All these values are calculated in the same region ( 3 ° ± 2 ° N ). This speed difference, if interpreted as a vertical wind shear, requires a change of 90 −20 +50 m s −1 per scale height, two times greater than that estimated from temperature observations. We also perform an initial guess on Cassini ISS vertical sounding levels, retrieving values compatible with HST ones and Cassini CIRS derived vertical wind shear, but not with Voyager wind measurements. We conclude that the wind speed velocity differences measured between 1979–1981 and 2004 cannot be explained as a wind shear effect alone and demand dynamical processes.