Ethane, acetylene and propane distribution in Saturn's stratosphere from Cassini/CIRS limb observations
We present an analysis of limb observations performed by the Composite InfraRed Spectrometer (CIRS) instrument onboard the Cassini spacecraft, in order to investigate the temperature and composition of Saturn's stratosphere. Spectra were acquired at twenty different latitudes between 80° S and 45° N from March 2005 to January 2008, which corresponds to mid-summer in the southern hemisphere. We used a line-by-line radiative transfer model, coupled to an iterative retrieval algorithm, to determine the vertical temperature profile of the stratosphere at each latitude. Then, we derived the volume mixing ratio vertical profiles of various hydrocarbons: ethane, acetylene and propane. Our results show an equatorial maximum of the abundance of acetylene at 1 mbar, almost twice as high as expected from photochemical models, which can be explained by a descent of air at the equator. As already observed in 2002 and 2004 by Greathouse et al. (2005) and Howett et al. (2007) at 2 mbar, we note that the volume mixing ratio of ethane slightly increases towards the South pole, which is evidence for a meridional circulation. As for propane, its meridional distribution suggests that its chemical lifetime in Saturn's stratosphere may be shorter than predicted.
- Pub Date:
- November 2008