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Despite the lack of another Flagship-class mission such as Cassini–Huygens, prospects for the future exploration of Saturn are nevertheless encouraging. Both NASA and the European Space Agency (ESA) are exploring the possibilities of focused interplanetary missions (1) to drop one or more in situ atmospheric entry probes into Saturn and (2) to explore the satellites Titan and Enceladus, which would provide opportunities for both in situ investigations of Saturn’s magnetosphere and detailed remote-sensing observations of Saturn’s atmosphere. Additionally, a new generation of powerful Earth-based and near-Earth telescopes with advanced instrumentation spanning the ultraviolet to the far-infrared promise to provide systematic observations of Saturn’s seasonally changing composition and thermal structure, cloud structures and wind fields. Finally, new advances in amateur telescopic observations brought on largely by the availability of low-cost, powerful computers, low-noise, large-format cameras, and attendant sophisticated software promise to provide regular, longterm observations of Saturn in remarkable detail.
The longevity of Cassini’s exploration of Saturn’s atmosphere (a third of a Saturnian year) means that we have been able to track the seasonal evolution of atmospheric temperatures, chemistry and cloud opacity over almost every season, from solstice to solstice and from perihelion to aphelion. Cassini has built upon the decades-long ground-based record to observe seasonal shifts in atmospheric temperature, finding a thermal response that lags behind the seasonal insolation with a lag time that increases with depth into the atmosphere, in agreement with radiative climate models. Seasonal hemispheric contrasts are perturbed at smaller scales by atmospheric circulation, such as belt/zone dynamics, the equatorial oscillations and the polar vortices. Temperature asymmetries are largest in the middle stratosphere and become insignificant near the radiative-convective boundary. Cassini has also measured southern-summertime asymmetries in atmospheric composition, including ammonia (the key species forming the topmost clouds), phosphine and para-hydrogen (both disequilibrium species) in the upper troposphere; and hydrocarbons deriving from the UV photolysis of methane in the stratosphere (principally ethane and acetylene). These chemical asymmetries are now altering in subtle ways due to (i) the changing chemical efficiencies with temperature and insolation and (ii) vertical motions associated with large-scale overturning in response to the seasonal temperature contrasts. Similarly, hemispheric contrasts in tropospheric aerosol opacity and coloration that were identified during the earliest phases of Cassini’s exploration have now reversed, suggesting an intricate link between the clouds and the temperatures. Finally, comparisons of observations between Voyager and Cassini (both observing in early northern spring, one Saturn year apart) show tantalizing suggestions of non-seasonal variability. Disentangling the competing effects of radiative balance, chemistry and dynamics in shaping the seasonal evolution of Saturn’s temperatures, clouds and composition remains the key challenge for the next generation of observations and numerical simulations.