The structure and dynamics of Titan's middle atmosphere

Abstract

Titan's middle atmosphere is characterized by cyclostrophic winds and strong seasonal modulation. Cassini CIRS observations, obtained in northern winter, indicate that the stratosphere near 1 mbar is warmest at low latitudes, with the South Pole a few degrees colder and the North Pole approximately 20 K colder. Associated with the cold northern temperatures are strong circumpolar winds with speeds as high as 190 m s K1 . Within this vortex, the mixing ratios of several organic gases are enhanced relative to those at low latitudes. Comparison with Voyager thermal infrared measurements, obtained 25 years ago in northern spring, suggests that the enhancement currently observed will increase as the winter progresses. The stratopause height increases from 0.1 mbar near the equator to 0.01 mbar near the North Pole, where it is the warmest part of the atmosphere, greater than 200 K. This implies subsidence at the pole, which is consistent with the enhanced organics observed. Condensate features, several still not identified, are also apparent in the infrared spectra at high northern latitudes. In many ways, the winter vortex observed on Titan, with cyclostrophic winds, resembles the polar winter vortices on the Earth, where the mean winds are geostrophic.

Spatial and seasonal variations in C3H hydrocarbon abundance in Titan’s stratosphere from Cassini CIRS observations

“…5. We see variation in the stratospheric temperature and the shape and altitude of the stratopause as described in Flasar and Achterberg (2009). While the stratopause increases in altitude towards the north winter pole, we do not see the full extent of the stratopause, since our data is only sensitive to an altitude of 0.02 mbar.…”

The structure of Titan’s atmosphere from Cassini radio occultations: Occultations from the Prime and Equinox missions

“…There are also broad features that remain unidentified. One of the strongest at high latitudes occurs near 221 cm À1 , or 45 lm de Kok et al, 2007;Flasar and Achterberg, 2009). A condensate cloud situated between the altitudes above the local temperature maximum near 80 km and the local minimum (near 100 km in the 80°N profile) could effect the observed destabilization if the optical thickness was J 1.…”

The structure of Titan’s atmosphere from Cassini radio occultations

“…There is a discrepancy between HASI and the CIRS retrievals at low latitudes that is not understood. The HASI profiles are ∼ 10 K warmer in the upper troposphere, and calculated synthetic spectra using these profiles produce infrared spectra that have radiances near 7 µm that are much too high compared to CIRS observations Ferri et al, 2006;Flasar and Achterberg, 2009). …”