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Description
We present a study of the seasonal evolution of Titan's thermal field and distributions of haze, C_2_H_2_, C_2_H_4_, C_2_H_6_, CH_3_C_2_H, C_3_H_8_, C_4_H_2_, C_6_H_6_, HCN, and HC_3_N from March 2015 (Ls=66{deg}) to September 2017 (Ls=93{deg}) (i.e., from the last third of northern spring to early summer). We analyzed thermal emission of Titan's atmosphere acquired by the Cassini Composite Infrared Spectrometer (CIRS) with limb and nadir geometry to retrieve the stratospheric and mesospheric temperature and mixing ratios pole-to-pole meridional cross sections from 5mbar to 50ubar (120-650km). The southern stratopause varied in a complex way and showed a global temperature increase from 2015 to 2017 at high-southern latitudes. Stratospheric southern polar temperatures, which were observed to be as low as 120K in early 2015 due to the polar night, showed a 30K increase (at 0.5mbar) from March 2015 to May 2017 due to adiabatic heating in the subsiding branch of the global overturning circulation. All photochemical compounds were enriched at the south pole by this subsidence. Polar cross sections of these enhanced species, which are good tracers of the global dynamics, highlighted changes in the structure of the southern polar vortex. These high enhancements combined with the unusually low temperatures (<120K) of the deep stratosphere resulted in condensation at the south pole between 0.1 and 0.03mbar (240-280km) of HCN, HC_3_N, C_6_H_6_ and possibly C4H2 in March 2015 (Ls=66{deg}). These molecules were observed to condense deeper with increasing distance from the south pole. At high-northern latitudes, stratospheric enrichments remaining from the winter were observed below 300km between 2015 and May 2017 (Ls=90{deg}) for all chemical compounds and up to September 2017 (Ls=93{deg}) for C_2_H_2_, C_2_H_4_, CH_3_C_2_H, C_3_H_8_, and C_4_H_2_. In September 2017, these local enhancements were less pronounced than earlier for C_2_H_2_, C_4_H_2_, CH_3_C_2_H, HC_3_N, and HCN, and were no longer observed for C_2_H_6_ and C_6_H_6_, which suggests a change in the northern polar dynamics near the summer solstice. These enhancements observed during the entire spring may be due to confinement of this enriched air by a small remaining winter circulation cell that persisted in the low stratosphere up to the northern summer solstice, according to predictions of the Institut Pierre Simon Laplace Titan Global Climate Model (IPSL Titan GCM). In the mesosphere we derived a depleted layer in C_2_H_2_, HCN, and C_2_H_6_ from the north pole to mid-southern latitudes, while C_4_H_2_, C_3_H_4_, C_2_H_4_, and HC_3_N seem to have been enriched in the same region. In the deep stratosphere, all molecules except C_2_H_4_ were depleted due to their condensation sink located deeper than 5mbar outside the southern polar vortex. HCN, C_4_H_2_, and CH_3_C_2_H volume mixing ratio (VMR) cross section contours showed steep slopes near the mid-latitudes or close to the equator, which can be explained by upwelling air in this region. Upwelling is also supported by the cross section of the C_2_H_4_ (the only molecule not condensing among those studied here) volume mixing ratio observed in the northern hemisphere. We derived the zonal wind velocity up to mesospheric levels from the retrieved thermal field. We show that zonal winds were faster and more confined around the south pole in 2015 (Ls=67-72{deg}) than later. In 2016, the polar zonal wind speed decreased while the fastest winds had migrated toward low-southern latitudes.
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