Seasonal evolution of Saturn’s polar temperatures and composition

•Record of seasonally evolving polar temperatures and composition reconstructed from a decade of CIRS measurements.•Warm polar cyclones within 2–3° of each pole are persistent features irrespective of season.•Southern summer stratospheric hood has disappeared; slow warming evident in the north.•Fast T changes at the pole suggest vertical motions plus stratospheric aerosol contributions to the radiative budget.•North polar minimum T 6–8 years after winter solstice, south polar maximum T 1–2 years after summer solstice. The seasonal evolution of Saturn’s polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7–16μm thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60° from northern winter/southern summer (2004, Ls=293°) through the equinox (2009, Ls=0°) to northern spring/southern autumn (2014, Ls=56°). The hot tropospheric polar cyclones that are entrained by prograde jets within 2–3° of each pole, and the hexagonal shape of the north polar belt, are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by ≈30° longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. Tropospheric temperature contrasts between the cool polar zones (near 80–85°) and warm polar belts (near 75–80°) have varied in both hemispheres, resulting in changes to the vertical windshear on the zonal jets in the upper troposphere and lower stratosphere. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75°S (by approximately −5K/yr), coinciding with a depletion in the abundances of acetylene (0.030±0.005ppm/yr) and ethane (0.35±0.1ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w≈+0.1mm/s. The upwelling appears most intense within 5° latitude of the south pole. This is mirrored by a general warming of the northern polar stratosphere (+5K/yr) and an enhancement in acetylene (0.030±0.003ppm/yr) and ethane (0.45±0.1ppm/yr) abundances that appears to be most intense poleward of 75°N, suggesting subsidence atw≈-0.15mm/s. However, the sharp gradient in stratospheric emission expected to form near 75°N by northern summer solstice (2017, Ls=90°) has not yet been observed, so we continue to await the devel