Seasonal changes in Saturn's stratosphere from Cassini/CIRS data
Abstract
Saturn undergoes important seasonal variations in solar insolation because of its large obliquity (26.7°). Hence, we expect significant changes in the atmospheric temperature, photochemistry and possibly in large scale circulation. To measure the seasonal variations of the stratospheric thermal structure, we analyze limb spectra acquired by Cassini/CIRS (Composite InfrarRed Spectromer) in September 2010 and January 2012, during the spring in the northern hemisphere and we compare them to the previous observations of March 2005 and August 2006 during winter. The CIRS data of September 2010 span from 25°N to 80°N (before the large northern storm) whereas the data of January 2012 probe the latitudes from 30°S to 70°S. Their limb viewing geometry and the two wavelengths ranges used (from 7 µm to 9 µm and from 9 µm to 17 µm) allow us to measure the temperature from the lower to the upper stratosphere (i. e. from 20 hPa to 0.001 hPa) with a resolution of 1.5 times the scale height. We use a forward radiative transfer model coupled to a constrained linear inverse method to retrieve temperature profiles at the different observed latitudes. In the northern hemisphere, the temperatures increased by 10 K at 1 hPa consistent with the results of Fletcher et al. (2010) from CIRS nadir data. Nevertheless, in the upper stratosphere at 0.1 hPa, the temperature do not show any significant variation within 2 K. This was not predicted by radiative equilibrium models such as Greathouse et al. (2008). This suggests that the temperature is not simply governed by the radiative heating and cooling by the atmospheric minor constituents but that other processes such as large scale dynamics or wave breaking are at play. We also present seasonal variations of ethane abundance in the stratosphere. Guerlet et al. (2009) found that ethane displayed meridional variations not accounted for by the seasonal 1-D photochemical models of Moses et al. (2005), hence showing its sensitivity to atmospheric dynamics. Following this work, we will study its seasonal variations between 2005 and 2012 in order to provide constraints on Saturn's stratospheric seasonal circulation.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #45
- Pub Date:
- October 2013
- Bibcode:
- 2013DPS....4550606S