On the Uniqueness of Saturn’s Equinox in 2010 Based on Observations in Methane Bands in 1964–2024

Abstract

Due to the inclination of Saturn’s equator to the plane of its orbit at an angle of close to 27° and due to the presence of rings that block the arrival of solar radiation to the winter hemisphere for a long time, the planet’s atmosphere undergoes significant seasonal changes. Once every 14.7 Earth years, the planet’s rings are visible edge-on to an Earth-based observer, and then the insolation conditions for both hemispheres become the same. The most favorable opportunities for such observations were in 1966, 1980, 1995, 2009–2010, and 2024. The available observational data and the results of the authors’ calculations within the framework of a two-layer model of Saturn’s atmosphere for such equinoxes were compared. They showed that the latitudinal belts of the planet, which have just emerged from the shadow of the rings, usually differ significantly from other belts in their physical characteristics under practically the same physical and orbital conditions of the planet. From the analysis of the parameter values calculated for different latitudes, the conclusion was confirmed that, for the hemisphere that until the time of receiving observational data was shielded by rings (until 1966, 1995, and 2024 in the Southern Hemisphere and until 1980 and 2009 in the Northern Hemisphere), the cloud layer is more sparse and its upper boundary is at a higher altitude than in the hemisphere that “survived” the “summer” season before. Those equatorial regions of Saturn that were closed by rings for a long time, experiencing a deficit of solar radiation inflow into the atmosphere, differ from other latitude zones in an increased amount of some strongly absorbing color impurity. However, 2009 and, partly, 1995 do not correspond to this assumption. The northern equatorial region, which had just emerged from the shadow of the rings in 2009, did not show a significant decrease in methane absorption. That is, neither high-altitude haze nor a rarefied layer of clouds formed in this part of the atmosphere. Since, as a rule, these new formations are of a photochemical nature, it can be assumed that there was not enough energy for some reason in the atmosphere to form a photochemical aerosol layer, which usually formed in the lower stratosphere (upper troposphere) of Saturn, and which reduced methane absorption and increased albedo. The reason for this could be that the equinoxes on Saturn in 1995–1996 and in 2009–2010 occurred at times close to the minimum of activity on the Sun, when the solar activity index R differed only slightly from the zero value.