Seasonal evolution of Saturn’s polar temperatures and composition

Abstract

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 µm thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60• from northern winter/southern summer (2004, L s = 293The 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 -5 K/yr), coinciding with a depletion in the abundances of acetylene (0.030 ± 0.005 ppm/yr) and ethane (0.35 ± 0.1 ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w ≈ +0.1 mm/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 (+5 K/yr) and an enhancement in acetylene (0.030 ± 0.003 ppm/yr) and ethane (0.45 ± 0.1 ppm/yr) abundances that appears to be most intense poleward of 75• N, suggesting subsidence at w ≈ −0.15 mm/s. However, the sharp gradient in stratospheric emission expected to form near 75• N by northern summer solstice (2017, L s = 90has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. The peak stratospheric warming in the north occurs at lower pressure levels (p < 1 mbar) than the peak stratospheric cooling in the south (p > 1 mbar). Vertical motions are derived from both the temperature field (using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models) and hydrocarbon distributions (solving the continuity equation). Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller, except within the tropospheric polar cyclones where w ≈ ±0.02 mm/s. 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), consistent with the expectations of radiative climate models. The influence of dynamics implies that the coldest winter temperatures occur in the 75 − 80• region in the stratosphere, and in the cool polar zones in the troposphere, r...

Stratospheric benzene and hydrocarbon aerosols detected in Saturn’s auroral regions

“…Our modelled temperature profiles suggest that auroral-induced aerosols confined in the polar hood could significantly contribute to this polar warming, although we cannot rule out that other processes, such as adiabatic heating due to subsidence, also play a part in the warming. This hypothesis has been suggested in previous studies (Orton & Yanamandra-Fisher 2005;Sánchez-Lavega et al 2006;Fletcher et al 2008Fletcher et al , 2015, but this is the first time that this effect is estimated.…”

“…• S (Fletcher et al 2008(Fletcher et al , 2015. Future measurements of the temperature and haze opacities in the polar regions, in particular the monitoring of the formation -or not -of a northern counterpart of the south polar hood, will be extremely valuable for the study of the chemistry and radiative budget in Saturn's polar stratosphere.…”

Faint Luminescent Ring Over Saturn’s Polar Hexagon

“…The hexagonal shape of the structure suggests that it originates at tropospheric altitudes: in fact, the temperatures retrieved in Saturn polar regions from data by the Composite Infrared Spectrometer on board Cassini (Fletcher et al 2008(Fletcher et al , 2015 showed a circular stratospheric thermal field, not giving any hint of a possible hexagonal shape. Even stellar occultation data from VIMS at high latitude do not observe any 3700 nm doublet signature in the stratosphere (Kim et al 2012).…”

Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

“…In particular, we use four different sets of wind profiles at cloud level: (i) June 2013 (early northern summer) from latitudes 68° to 90° (all latitudes in this study are planetographic, Antuñano et al, ), (ii) December 2008 (late northern winter) from latitudes 70° to 84° (García‐Melendo et al, ), (iii) October/December 2008 from latitudes −68° to −90° (Antuñano et al, ), and (iv) October 2006 (southern summer) from latitudes −68° to −90° (Sánchez‐Lavega et al, ), and four data sets of zonally averaged temperature profiles for pressures between 1 and 1 bar. Due to the lack of simultaneous measurements of winds and temperatures during the Cassini mission, the temperature fields for the months of interest (i.e., June 2013, December 2008, and October 2006) were obtained by interpolating the existing temperature measurements over the whole Cassini mission, and extracting only the months where wind measurements were available (see Fletcher et al, , ). Temperatures were retrieved from CIRS focal plane 1 (10–600 cm −1 ), sensing the 80–800‐mbar range, and focal plane 4 (1,100–1,400 cm −1 ), sensing the 0.5–5.0‐mbar range.…”

“…In order to estimate the error introduced in the QGPV due to errors on the zonal wind profiles (with a mean standard deviation of 7 m s −1 ; Antuñano et al, ), and the temperature profiles (around 2 K at the stratosphere above and 0.5 K at the troposphere; Fletcher et al, ), we have added random perturbations of a magnitude equal to the error to the zonal wind and temperature profiles, respectively, and analyzed the variations in QGPV caused by these perturbations. Results are discussed in section .…”