Long-term changes in Saturn's troposphere

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We report the results of monitoring Saturn's H 2 quadrupole and CH 4 band absorptions outside of the equatorial zone over one-half of Saturn's year. This interval covers most of the perihelion half of Saturn's elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH 4 absorption accompanied by weakly opposite changes in the H 2 absorption. Around the 1980 equinox, the H 2 and CH 4 absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturn's troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH 4 is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH 4 bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturn's south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH 4/H 2 mixing ratio of (4.2 ± 0.4) × 10 −3, a haze particle albedo of ω = 0.995 ± 0.003 , and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 Å NH 3 band made annually since the 1980 equinox. A 20 ± 4% increase in the NH 3 absorption at south temperate latitudes has occurred since 1973–1976 and the NH 3 absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH 3 crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturn's atmosphere. An apparently anomalous ortho-para H 2 ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant.