Exoplanet atmospheres

Abstract:

The Cassini spacecraft observed Titan from 2004 to 2017, capturing key atmospheric features, including the tilt of the middle atmosphere and the formation and breakup of winter polar vortices. We analyze low spectral resolution infrared observations from Cassini’s Composite Infrared Spectrometer (CIRS), which provide excellent spatial and temporal coverage and the best horizontal spatial resolution of any of the CIRS observations. With approximately 4 times higher meridional resolution than previous studies, we map the stratospheric temperature for almost half a Titan year. We determine the evolution of Titan’s stratospheric tilt, finding that it is most constant in the inertial frame, directed 120° ± 6° west of the Titan–Sun vector at the northern spring equinox, with seasonal oscillations in the tilt magnitude between around 2 .° 5 and 8°. Using the high meridional resolution temperature field, we reveal finer details in the zonal wind and potential vorticity. In addition to the strong winter zonal jet, a weaker zonal jet in Titan’s summer hemisphere is observed, and there is a suggestion that the main winter hemisphere jet briefly splits into two. We also present the strongest evidence yet that Titan’s polar vortex is annular for part of its life cycle.