Seasons on Saturn. I. Changes in reflecting characteristics of the atmosphere at 1964–2012

Abstract

Due to considerable tilting of the equator to the orbital plane, presence of rings and elongation of the orbit around the Sun, Saturn is strongly affected by seasonal changes in the solar radiation influx. Such change of the mode of irradiation of Saturn's atmosphere should affect physical properties and optical characteristics of the cloud cover and/or haze above the clouds of the planet, as well as on the vertical structure of the atmosphere at different latitudes. Once in 14,7 terrestrial years we have the equality of conditions of insolation for both hemispheres, when the planet's rings are visible edge-on to a terrestrial observer. Therefore, these moments are interest for comparative study of possible differences in atmospheric characteristics of the northern and southern hemispheres. In recent years, possibilities for such observations were in 1966, 1980, 1995 and 2009. Using the observational data on the distribution of reflective characteristics of the atmosphere on the visible planetary disk, we have compared the results of calculations in the framework of a two-layer model of Saturn's atmosphere for above moments of equinoxes. It was found that in moments of equinox in 1966, 1980 and 1995 for spring hemisphere, which had just emerged from the rings shadow, the planetary latitudinal belts differ significantly from the other zones by the values of scattering component of the optical thickness τ_c of gas layer above the clouds, the volume concentration of aerosol cloud n, and imaginary part of refractive index n_i of cloud particles. But at the time of the equinox in 2009, in the spring hemisphere the expected changes of reflective and absorptive characteristics – did not happen. There methane absorption remained at the same high level and does not form any high-altitude haze and/or rarefied cloud layer. Because they have the photochemical nature, we can assume an insufficient amount of incoming energy into the atmosphere for the possibility of the formation a photochemical aerosol layer in the lower stratosphere and/or upper troposphere of Saturn.