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[20.03] Evidence for a mesosphere on Saturn and the seasonal variation of temperature between 2002 and 2004 in Saturn's southern hemisphere.

T.K. Greathouse (Lunar and Planetary Institute), H.G. Roe (California Institute of Technology), M.J. Richter (University of California Davis)

Ground based high-resolution spectral observations of CH₄ emission features taken at the NASA IRTF using the TEXES spectrometer on September 13^th 2002 and October 14^th and 15^th 2004, L_S of 268.4^o and 296.8^o respectively, are presented. These data, taken along Saturn's central meridian, are sensitive to the stratospheric temperature between 10 and 0.01 mbar. The 1245 cm^-1 observations taken on October 15^th 2004 show distinct self-absorption cores on top of the strong CH₄ emission lines. These features indicate that there is a region overlaying the stratosphere which is cooler than the stratopause, ie. a mesosphere. This explanation is supported by detailed line-by-line radiative transfer modeling of the 2004 data.

A comparison of the 2002 and 2004 stratospheric temperature retrievals shows an enhancement of the 3 mbar thermal gradient between the equator and south pole. A 10 K increase of temperature between the equator and south pole at the 3 mbar level in 2002 has been described in Greathouse etal. 2005. By modeling the 2004 data, we infer that the equator to pole trend has increased to 15 K. These results suggest that the phase lag in the thermal response of Saturn's stratosphere is shorter than predicted by the seasonal climate model of Bézard and Gautier 1985. However, the magnitude of the equator to pole temperature variation is still within their predicted range.

The collection of multiple spectral intervals containing CH₄ emission lines during the 2004 observing run allows us to test our temperature retrieval methods, their dependence on initial guesses, and the altitude sensitivity of various spectral regions. We will discuss the level of certainty in our temperature derivations, and ways that other observations may be used to further decrease our uncertainties.

This work has been funded through the Lunar and Planetary Institute, NASA CAN-NCC5-679.

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