Seasonal Evolution of Saturn's Polar Temperatures and Composition

The seasonal evolution of Saturn's polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7-16 $\mu$m thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60$^\circ$ from northern winter/southern summer (2004, $L_s=293^\circ$) through the equinox (2009, $L_s=0^\circ$) to northern spring/southern autumn (2014, $L_s=56^\circ$). The hot tropospheric polar cyclones and the hexagonal shape of the north polar belt are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by $\approx30^\circ$ longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75$^\circ$S, coinciding with a depletion in the abundances of acetylene and ethane, and suggestive of stratospheric upwelling with vertical wind speeds of $w\approx+0.1$ mm/s. This is mirrored by a general warming of the northern polar stratosphere and an enhancement in acetylene and ethane abundances that appears to be most intense poleward of 75$^\circ$N, suggesting subsidence at $w\approx-0.15$ mm/s. However, the sharp gradient in stratospheric emission expected to form near 75$^\circ$N by northern summer solstice (2017, $L_s=90^\circ$) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. North polar minima in tropospheric and stratospheric temperatures were detected in 2008-2010 (lagging one season, or 6-8 years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1-2 years after summer solstice). [Abridged]