Seasonal changes in Saturn's stratosphere inferred from Cassini/CIRS limb observations

We present temperature and hydrocarbons abundances (C2H6, C2H2, C3H8) retrieved from Cassini/CIRS limb spectra, acquired during northern spring in 2010 (LS = 12 °) and 2012 (LS = 31 °). We compare them to the previous limb measurements performed by Guerlet et al. (Guerlet, S. et al. [2009]. Icarus 203, 214-232) during northern winter. The latitudinal coverage (from 79°N to 70°S) and the sensitivity of our observations to a broad range of pressure levels (from 20 hPa to 0.003 hPa) allow us to probe the meridional and vertical structure of Saturn's stratosphere during northern spring. Our results show that in the northern hemisphere, the lower stratosphere (1 hPa) has experienced the strongest warming from northern winter to spring (11 ±0.91.1 K), while the southern hemisphere exhibits weak variations of temperature at the same pressure level. We investigate the radiative contribution in the thermal seasonal evolution by comparing these results to the radiative-convective model of Guerlet et al. (Guerlet, S. et al. [2014]. Icarus 238, 110-124). We show that radiative heating and cooling by atmospheric minor constituents is not always sufficient to reproduce the measured variations of temperature (depending on the pressure level). The measurements of the hydrocarbons abundances and their comparison with the predictions of the 1D photochemical model of Moses and Greathouse (Moses, J.I., Greathouse, T.K. [2005]. J. Geophys. Res. (Planets) 110, 9007) give insights into large scale atmospheric dynamics. At 1 hPa, C2H6, C2H2, and C3H8 abundances are remarkably constant from northern winter to spring. At the same pressure level, C2H6 and C3H8 exhibit homogeneous meridional distributions unpredicted by this photochemical model, unlike C2H2. This is consistent with the existence of a meridional circulation at 1 hPa, as suggested by previous studies.