Seasonal Changes in Middle-Atmosphere Circulation at Titan's Poles

Saturn's moon Titan, is one of only four terrestrial atmospheres in the Solar System. In many ways Titan's atmosphere is similar to the Earth's - with nitrogen as the main component, a similar temperature structure, comparable axial tilt, and polar vortices. However, the temperatures are much colder, the day and year are much longer, and the atmosphere awash with products of nitrogen-methane photochemistry. These differences allow us to test our understanding of atmospheric dynamics under vastly different conditions to those on the Earth, but in a regime that is still relevant. Over the past eight years the Cassini spacecraft has been orbiting Saturn and has returned detailed information on the temperature and composition of Titan's atmosphere. Therefore, even though Titan's year lasts 29.5 Earth years, we now have enough data to begin to probe the atmosphere's seasonal evolution.

Here we use measurements of temperature and trace gas composition measured by Cassini's Composite Infra Red Spectrometer to determined changes in the atmospheric structure and large-scale circulation patterns of the middle-atmosphere (stratosphere and mesosphere). Early in the mission the north pole was experiencing winter and the middle-atmosphere circulation comprised a single south to north meridional cell and a north polar vortex with a strong mixing barrier between polar and non-polar airmasses. This structure allowed the accumulation of large quantities of trace gas within the vortex and analogies have been drawn with exotic chemistry in the Earth's Antarctic vortex. However, Titan's northern spring equinox occurred in mid-2009 and this circulation pattern is now changing. The north polar region is now exposed to sunlight for the first time in 15 years and the south pole is entering southern winter. We will present observations of the recent changes from Cassini and discuss their implications in terms of understanding the fundamental atmospheric processes at work.