Titan's middle-atmospheric temperatures and dynamics observed by the Cassini Composite Infrared Spectrometer

Abstract: The Composite Infrared Radiometer–Spectrometer (CIRS) instrument, on the NASA Cassini Saturn orbiter, has been acquiring thermal emission spectra from the atmosphere of Titan since orbit insertion in 2004. Observation sequences for measuring stratospheric temperatures have been obtained using both a nadir mapping mode and a limb viewing mode. The limb observations give better vertical resolution, and give information from higher altitudes, while the nadir observations provide more complete longitude coverage. Because the scale height of Titan''s atmosphere is large enough so that emission from a grazing ray is influenced by horizontal temperature variations in the atmosphere, we have developed a two-dimensional temperature retrieval algorithm for reducing the limb spectra, which solves simultaneously for meridional and vertical temperature variations. The analyzed nadir mapping data have sampled nearly all longitudes at latitudes from about 90° S to 60° N, providing temperatures between pressure levels of about 5 to 0.2 mbar. The limb data covers latitudes between about 75° S and 85° N, and yields temperatures between about 1 and 0.005 mbar, at a small number of longitudes. The retrieved temperatures are consistent with early results from nadir observations [Flasar, F.M., and 44 colleagues, 2005. Science 308, 975–978] between 0.5 and 5 mbar where both results are valid, with the warmest temperatures at the equator, and much stronger meridional temperature gradients in the northern (winter) hemisphere than in the southern. At higher altitudes not probed by nadir viewing, the limb data reveal that the stratopause is nearly 20 K warmer in the northern polar regions than at the equator and southern hemisphere, and that the altitude of the stratopause shifts from ≈0.1 mbar (300 km) near the equator to 0.01 mbar (400 km) poleward of about 40° N. When the gradient wind equation is used to construct a zonal mean wind, the reversal in sign of the temperature leads to capping of the winter westerly flow. The core of the resulting jet is about 190 m s−1 in magnitude, spans between 30° N and 60° N, and peaks near 0.1 mbar. Estimates of the radiative heating associated with the radiative disequilibrium lead to a meridional overturning timescale of about three Earth years. [Copyright &y& Elsevier]