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[30.06] Latitudinal Variation in Temperature and Composition of Saturn's Upper Troposphere from Cassini/CIRS as a Tracer For Atmospheric Dynamics

L.N. Fletcher, P.G.J. Irwin, N. Teanby (Oxford), G. Orton, P. Parrish (JPL/CalTech), R. de Kok, S. Calcutt, C. Howett, P.L. Read, F.W. Taylor (Oxford), Cassini CIRS Team

The presence of the Cassini Huygens spacecraft in the Saturnian system provides an unprecedented opportunity to study the dynamics of the gas giant in more detail than ever before. Infrared spectra from Saturn's southern hemisphere have been obtained by the Composite Infrared Spectrometer (CIRS), at both a high (0.5cm^-1) and medium (2.5cm^-1) apodized spectral resolution. Latitudinal variations of temperature and composition in Saturn's upper troposphere are measured using an optimal estimation retrieval code developed in Oxford. The far infrared (10-600cm^-1) continuum is modelled by varying temperature, para-hydrogen fraction and aerosol opacity, with temperature contribution functions sensing pressures from 0.1 to 0.5atm. Abundances and fractional scale heights of ammonia and phosphine are then retrieved from rotational lines in the 10-200cm^-1 region. The mid infrared (600-1400cm^-1) provides temperature profiles from 600-700cm^-1, whilst the \nu ₄ fundamental of phosphine at 1118cm^-1 is used to retrieve a further measure of its abundance. Results are compared to global data from the Mid Infrared Spectrometer and Imager (MIRSI) instrument on NASA's Infrared Telescope Facility. Measurement from filters covering the collision induced absorption of hydrogen in the 400-600cm^-1 region and the \nu₂ phosphine fundamental near to 1000cm^-1 were obtained using the same retrieval scheme.

The distribution of phosphine and ammonia is a balance between vertical transport and photochemical destruction. Elevated abundances of these gases are used as tracers for regions of upwelling constituent-rich air from deeper within the troposphere. Variations in retrieved optical depth due to aerosols, and deviations of parahydrogen fraction from local thermal equilibrium are also presented. Using an assumed stratospheric abundance of methane, the mid infrared spectra were used to obtain preliminary measures of the [D]/[H] ratio in methane of (1.96±0.80) \times 10^-5 and a [¹²C]/[¹³C] ratio of 78.5_-14.0^+21.9.

Acknowledgement: This work is funded by the UK Particle Physics and Astronomy Research Council.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.