Composition and Haze Layering in Titan's Winter Polar Vortex

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Cassini first observed discrete haze layers in Titan's atmosphere in 2004 with a series of images taken close to the equator by the Imaging Science Sub-system (ISS) instrument (Porco et al 2005, Science). These mysterious layers had unknown origin although atmospheric waves were suggested as a possible cause. To gain a better understanding of the layers' distribution and cause we have surveyed three years of Cassini data at a wide range of latitudes. We used UV/visible images from ISS and additionally infra-red spectra measured by the Composite Infra-Red Spectrometer (CIRS). This multi-instrument approach allows us to compare layering in the hazes (ISS), with layering in composition of trace gases (CIRS) - giving additional clues as to the cause.
At high northern latitudes, near the winter polar vortex, layering is very strong and both ISS (haze) and CIRS (composition) independently show fine layered structures in the 150--450 km altitude range, with a preferred vertical wavelength of around 50 km. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth's polar vortices.
Some weaker and more regular layers exist at more southerly latitudes, but are only visible in the haze-sensitive ISS images. These may have a different formation mechanism, perhaps gravity waves or atmospheric tides.

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