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Titan's stratosphere exhibits significant seasonal changes, including breakup and formation of polar vortices. Here we present the first analysis of midinfrared mapping observations from Cassini's Composite InfraRed Spectrometer to cover the entire mission (L-s=293-93 degrees, 2004-2017)-midnorthern winter to northern summer solstice. The north polar winter vortex persisted well after equinox, starting breakup around L-s similar to 60 degrees and fully dissipating by L-s similar to 90 degrees. Absence of enriched polar air spreading to lower latitudes suggests large-scale circulation changes and photochemistry control chemical evolution during vortex breakup. South polar vortex formation commenced soon after equinox and by L-s similar to 60 degrees was more enriched in trace gases than the northern middle-winter vortex and had temperatures similar to 20 K colder. This suggests that early-winter and middle-winter vortices are dominated by different processes-radiative cooling and subsidence-induced adiabatic heating respectively. By the end of the mission (L-s=93 degrees) south polar conditions were approaching those observed in the north at L-s=293 degrees, implying seasonal symmetry in Titan's vortices.

Plain Language Summary The Cassini spacecraft observed Saturn's largest moon, Titan, during its 13-year tour of the Saturn system. This allowed temperature and gas composition to be measured for almost half a Titan year, which lasts 29.46 Earth years. Spectra measured by Cassini's infrared spectrometer show that Titan's winter poles are much colder and significantly more enriched in trace gas species than more equatorial latitudes. These observations can be explained by the presence of winter polar vortices, where sinking air enriches the composition of the lower atmosphere and isolation by strong vortex winds allows enhanced cooling in the winter darkness. The coldest temperatures and most extreme trace gas concentrations were seen at Titan's southern pole during early winter and vortex formation.