Phosphine on Jupiter and Saturn from Cassini/CIRS

The global distribution of phosphine (PH₃) on Jupiter and Saturn is derived using 2.5 cm⁻¹ spectral resolution Cassini/CIRS observations. We extend the preliminary PH₃ analyses on the gas giants [Irwin, P.G.J., and 6 colleagues, 2004. Icarus 172, 37-49; Fletcher, L.N., and 9 colleagues, 2007a. Icarus 188, 72-88] by (a) incorporating a wider range of Cassini/CIRS datasets and by considering a broader spectral range; (b) direct incorporation of thermal infrared opacities due to tropospheric aerosols and (c) using a common retrieval algorithm and spectroscopic line database to allow direct comparison between these two gas giants.The results suggest striking similarities between the tropospheric dynamics in the 100-1000 mbar regions of the giant planets: both demonstrate enhanced PH₃ at the equator, depletion over neighbouring equatorial belts and mid-latitude belt/zone structures. Saturn's polar PH₃ shows depletion within the hot cyclonic polar vortices. Jovian aerosol distributions are consistent with previous independent studies, and on Saturn we demonstrate that CIRS spectra are most consistent with a haze in the 100-400 mbar range with a mean optical depth of 0.1 at 10 μm. Unlike Jupiter, Saturn's tropospheric haze shows a hemispherical asymmetry, being more opaque in the southern summer hemisphere than in the north. Thermal-IR haze opacity is not enhanced at Saturn's equator as it is on Jupiter.Small-scale perturbations to the mean PH₃ abundance are discussed both in terms of a model of meridional overturning and parameterisation as eddy mixing. The large-scale structure of the PH₃ distributions is likely to be related to changes in the photochemical lifetimes and the shielding due to aerosol opacities. On Saturn, the enhanced summer opacity results in shielding and extended photochemical lifetimes for PH₃, permitting elevated PH₃ levels over Saturn's summer hemisphere.     

Global variation of the para hydrogen fraction in Jupiter's atmosphere and implications for dynamics on the outer planets

Infrared spectra obtained by the Voyager spacecraft indicate that the para hydrogen fraction near the 300-mbar pressure level on Jupiter is not in thermodynamic equilibrium. Analysis of the global mapping data sequences from Voyagers 1 and 2 shows that the para fraction is smallest at equatorial latitudes, and approaches equilibrium at high latitudes. The sampled atmospheric level is near 125°K and the equatorial para fraction would represent thermal equilibrium at about 160°K. There are small-scale variations superposed on the global pattern, and these do not correlate with albedo, flow velocity, or 5-μm brightness.Lack of correlation of cloud indicators with the para fraction suggests that catalysis of ortho-para conversion does not occur on aerosol surfaces, at least near the 300 mbar level. The fact that dynamics alters the para fraction from equilibrium while not affecting temperatures to a large degree suggests that the para hydrogen equilibration rate is slower than radiative thermal adjustment. A survey of the mechanisms for equilibration suggests that H₂H₂ paramagnetic interaction is dominant. The slow equilibration rate has dynamical implications for all the outer planets. A mixing length model is used to demonstrate that within the convective lower tropospheres of the giant planets there is very slow overturning. The mean structures are close to equilibrium para fraction, the thermal structures are equilibrium adiabats, and they are statically stable to high frequency dynamical perturbations. The para hydrogen conversion greatly increases the efficiency of convection. Within Jupiter's stably stratified upper troposphere, where the infrared spectra originate, the global variation of the para fraction appears most likely to be produced by upwelling at equatorial latitudes in response to solar heating. If this is true, there is compensating downward motion in polar regions.     

The deep water abundance on Jupiter: New constraints from thermochemical kinetics and diffusion modeling

We have developed a one-dimensional thermochemical kinetics and diffusion model for Jupiter’s atmosphere that accurately describes the transition from the thermochemical regime in the deep troposphere (where chemical equilibrium is established) to the quenched regime in the upper troposphere (where chemical equilibrium is disrupted). The model is used to calculate chemical abundances of tropospheric constituents and to identify important chemical pathways for CO-CH₄ interconversion in hydrogen-dominated atmospheres. In particular, the observed mole fraction and chemical behavior of CO is used to indirectly constrain the jovian water inventory. Our model can reproduce the observed tropospheric CO abundance provided that the water mole fraction lies in the range (0.25-6.0) × 10⁻³ in Jupiter’s deep troposphere, corresponding to an enrichment of 0.3-7.3 times the protosolar abundance (assumed to be H₂O/H₂ = 9.61 × 10⁻⁴). Our results suggest that Jupiter’s oxygen enrichment is roughly similar to that for carbon, nitrogen, and other heavy elements, and we conclude that formation scenarios that require very large (>8× solar) enrichments in water can be ruled out. We also evaluate and refine the simple time-constant arguments currently used to predict the quenched CO abundance on Jupiter, other giant planets, and brown dwarfs.     

Generation of equatorial jets by large-scale latent heating on the giant planets

Three-dimensional numerical simulations show that large-scale latent heating resulting from condensation of water vapor can produce multiple zonal jets similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). For plausible water abundances (3-5 times solar on Jupiter/Saturn and 30 times solar on Uranus/Neptune), our simulations produce ∼20 zonal jets for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the number of jets observed on these planets. Moreover, these Jupiter/Saturn cases produce equatorial superrotation whereas the Uranus/Neptune cases produce equatorial subrotation, consistent with the observed equatorial-jet direction on these planets. Sensitivity tests show that water abundance, planetary rotation rate, and planetary radius are all controlling factors, with water playing the most important role; modest water abundances, large planetary radii, and fast rotation rates favor equatorial superrotation, whereas large water abundances favor equatorial subrotation regardless of the planetary radius and rotation rate. Given the larger radii, faster rotation rates, and probable lower water abundances of Jupiter and Saturn relative to Uranus and Neptune, our simulations therefore provide a possible mechanism for the existence of equatorial superrotation on Jupiter and Saturn and the lack of superrotation on Uranus and Neptune. Nevertheless, Saturn poses a possible difficulty, as our simulations were unable to explain the unusually high speed (∼) of that planet’s superrotating jet. The zonal jets in our simulations exhibit modest violations of the barotropic and Charney-Stern stability criteria. Overall, our simulations, while idealized, support the idea that latent heating plays an important role in generating the jets on the giant planets.     

A three-dimensional model of moist convection for the giant planets II: Saturn's water and ammonia moist convective storms

Moist convective storms constitute a key aspect in the global energy budget of the atmospheres of the giant planets. Among them, Saturn is known to develop the largest scale convective storms in the Solar System, the Great White Spots (GWS) which occur rarely and have been detected once every 30 years approximately. On the average, Saturn seems to show much less convective storms than Jupiter with smaller size and reduced frequency and intensity. Here we present detailed simulations of the onset and development of storms at the Equator and mid-latitudes of Saturn. These are the regions where most of the recent convective activity of the planet has been observed. We use a 3D anelastic model with parameterized microphysics (Hueso and Sánchez-Lavega, 2001, Icarus 151, 257) studying the onset and evolution of water and ammonia moist convective storms up to sizes of a few hundred km. Water storms, while more difficult to initiate than in Jupiter, can be very energetic, arriving to the 150 mbar level and developing vertical velocities on the order of 150 m s⁻¹. Ammonia storms develop easier but with a much smaller intensity unless very large abundances of ammonia (10 times solar) are present in Saturn's atmosphere. The Coriolis forces play a major role in the morphology and properties of water based storms.     

The Structure of Jupiter, Saturn, and Exoplanets: Key Questions for High-Pressure Experiments

We give an overview of our current understanding of the structure of gas giant planets, from Jupiter and Saturn to extrasolar giant planets. We focus on addressing what high-pressure laboratory experiments on hydrogen and helium can help to elucidate about the structure of these planets.     

Characterising Saturn's vertical temperature structure from Cassini/CIRS

Thermal infrared spectra of Saturn from 10-1400 cm⁻¹ at 15 cm⁻¹ spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified.Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N.The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution.     

Deep jets on gas-giant planets

Three-dimensional numerical simulations of the atmospheric flow on giant planets using the primitive equations show that shallow thermal forcing confined to pressures near the cloud tops can produce deep zonal winds from the tropopause all the way down to the bottom of the atmosphere. These deep winds can attain speeds comparable to the zonal jet speeds within the shallow, forced layer; they are pumped by Coriolis acceleration acting on a deep meridional circulation driven by the shallow-layer eddies. In the forced layer, the flow reaches an approximate steady state where east-west eddy accelerations balance Coriolis accelerations acting on the meridional flow. Under Jupiter-like conditions, our simulations produce 25 to 30 zonal jets, similar to the number of jets observed on Jupiter and Saturn. The simulated jet widths correspond to the Rhines scale; this suggests that, despite the three-dimensional nature of the dynamics, the baroclinic eddies energize a quasi-two-dimensional inverse cascade modified by the β effect (where β is the gradient of the Coriolis parameter). In agreement with Jupiter, the jets can violate the barotropic and Charney-Stern stability criteria, achieving curvatures ^∂2u/∂y² of the zonal wind u with northward distance y up to 2β. The simulations exhibit a tendency toward neutral stability with respect to Arnol'd's second stability theorem in the upper troposphere, as has been suggested for Jupiter, although deviations from neutrality exist. When the temperature varies strongly with latitude near the equator, our simulations can also reproduce the stable equatorial superrotation with wind speeds greater than . Diagnostics show that barotropic eddies at low latitudes drive the equatorial superrotation. The simulations also broadly explain the distribution of jet-pumping eddies observed on Jupiter and Saturn. While idealized, these simulations therefore capture many aspects of the cloud-level flows on Jupiter and Saturn.     

Compressible convection in the deep atmospheres of giant planets

Fast rotating giant planets such as Jupiter and Saturn possess alternate prograde and retrograde zonal winds which are stable over long periods of time. We consider a compressible model of convection in a spherical shell with rapid rotation, using the anelastic approximation, to explore the parameter range for which such zonal flows can be produced.We consider models with a large variation in density across the layer. Our models are based only on the molecular H/He region above the metallic hydrogen transition at about 2 Mbar, and we do not include the hydromagnetic effects which may be important if the electrical conductivity is significant. We find that the convective velocities are significantly higher in the low density regions of the shell, but the zonal flow is almost independent of the z-coordinate parallel to the rotation axis. We analyse how this behaviour is consistent with the Proudman-Taylor theorem.We find that deep prograde zonal flow near the equator is a very robust feature of our models. Prograde and retrograde jets alternating in latitude can occur inside the tangent cylinder in compressible as well as Boussinesq models, particularly at lower Prandtl numbers. However, the zonal jets inside the tangent cylinder are suppressed if a no-slip condition is imposed at the inner boundary. This suggests that deep high latitude jets may be suppressed if there is significant magnetic dissipation.Our compressible calculations include the viscous dissipation in the entropy equation, and we find this is comparable to, and in some cases exceeds, the total heat flux emerging from the surface. For numerical reasons, these simulations cannot reach the extremely low Ekman number found in giant planets, and they necessarily also have a much larger heat flux than planets. We therefore discuss how our results might scale down to give solutions with lower dissipation and lower heat flux.     

On the ortho-para equilibrium of H2 in the atmospheres of the Jovian planets

The ratio for the equivalent widths for the unsaturated H₂ quadrupole transitions observed in the Jovian planets is calculated and compared with a large number of observations. The comparison indicates that equilibrium hydrogen may be present in Jupiter and Saturn, while Uranus and Nepture exhibit ratios not in accord with equilibrium hydrogen. Observations which can differentiate among the possible states of H₂ are proposed.     

Radiative-convective equilibrium models of Jupiter and Saturn

Radiative-convective equilibrium models for Jupiter and Saturn have been produced in a study centered primarily on the stratospheric energy balance and the possible role of aerosol heating. These models are compared directly to the thermal structure profiles obtained from Voyager radio occultation measurements. The method is based on a straightforward flux divergence formulation derived from earlier work (J. S. Hogan, S. I. Rasool, and T. Encrenaz 1969, J. Atmos. Sci.26, 898–905). The balance between absorbed and emitted energies is computed iteratively at each level in the atmosphere, assuming local thermodynamic equilibrium and employing a standard treatment of opacities. Results for Jupiter indicate that a dust-free model (no aerosol heating) furnishes a good mean thermal profile for the stratosphere when compared with the Voyager 1 radio occultation (RSS) measurements. These observations of the equatorial region (0° and 12°S, respectively) exhibit periodic vertical structure. Of course, among many possible complications, the Voyager profiles may not represent typical excursions from the mean. The aerosol heat depositions required to match these profiles exactly, relative to the nominal dust-free model, are reasonably consistent with independent estimates for “continuum” absorbers. Other interpretations are discussed, along with a survey of problems encountered in intercomparing the lower portions (P ? 300 mb) of the models, the RSS profiles, and a recent IRIS equatorial profile. Although aerosol heating cannot be ruled out at low latitudes on Jupiter, our results indicate that it may not be required to reproduce the Voyager 1 RSS profiles. On the other hand, heating by aerosols or some other absorber seems necessary in order to match the high-latitude Voyager 2 RSS temperature profile. The Saturn models are relatively simple and in good-to-excellent agreement with the Voyager 2 RSS profiles at all levels. Our comparisons indicate that aerosol heating played a minor role in Saturn's midlatitude stratospheric energy balance at the time of the Voyager 2 encounter. These models, however, may need to be reassessed once the hydrocarbon concentrations have been more precisely determined.     

Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions

The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H₂ mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H₂ mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H₂ flux into Titan’s surface comparable to the H₂ escape flux out of the atmosphere (∼2 × 10¹⁰ cm⁻² s⁻¹ referenced to the surface) and requires larger photochemical production rates of H₂ than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H₂ mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H₂ surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H₂ (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274-276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H₂ mixing ratio.     

Turbulent convection in rapidly rotating spherical shells: A model for equatorial and high latitude jets on Jupiter and Saturn

The origin of zonal jets on the jovian planets has long been a topic of scientific debate. In this paper we show that deep convection in a spherical shell can generate zonal flow comparable to that observed on Jupiter and Saturn, including a broad prograde equatorial jet and multiple alternating jets at higher latitudes. We present fully turbulent, 3D spherical numerical simulations of rapidly rotating convection with different spherical shell geometries. The resulting global flow fields tend to be segregated into three regions (north, equatorial, and south), bounded by the tangent cylinder that circumscribes the inner boundary equator. In all of our simulations a strong prograde equatorial jet forms outside the tangent cylinder, whereas multiple jets form in the northern and southern hemispheres, inside the tangent cylinder. The jet scaling of our numerical models and of Jupiter and Saturn is consistent with the theory of geostrophic turbulence, which we extend to include the effect of spherical shell geometry. Zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β-parameter and a jump in the Rhines length. This jump is manifest in the numerical simulations as a sharp equatorward increase in jet widths—a transition that is also observed on Jupiter and Saturn. The location of this transition gives an estimate of the depth of zonal flow, which seems to be consistent with current models of the jovian and saturnian interiors.     

Atomic and molecular hydrogen budget in Titan’s atmosphere

Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan’s atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 10⁸ cm² s⁻¹ and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H₂. Production of both H and H₂ also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 10⁹ cm⁻² s⁻¹ and the latest retrieved value of the H₂ mole fraction in the stratosphere: (1.1 ± 0.1) × 10⁻³. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H₂ is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C₄ species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C₃H₄, C₄H₂ and C₆H₆. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.     

Modeling Jupiter's cloud bands and decks: 1. Jet scale meridional circulations

We have investigated the formation of jet scale meridional circulation cells on Jupiter in response to radiative and zonal momentum forcing. In the framework of semi-geostrophic theory, the meridional streamfunction is described by an elliptic equation with a source term dependent on the sum of the latitudinal derivative of the radiative forcing and the vertical derivative of the zonal momentum forcing. Using this equation with analytic terms similar to the assumed forcing on Jupiter, we obtained two set of atmospheric circulations cells, a stratospheric and a tropospheric one. A possible shift in the overturning circulation of the high and deep atmosphere can be induced by breaking the latitudinal alignment of radiative heating with the enforced belt and zones. A series of numerical simulations was conducted with the Jovian GCM OPUS, which was initiated with observational data obtained from the Cassini CIRS temperature cross-section and a corresponding geostrophic zonal wind field. Newtonian forcing of potential temperature as well as zonal momentum was applied respectively towards latitudinally and vertically uniform equilibrium fields. In accordance with the analytic illustrations two rows of jet scale circulation cells were created. The stratospheric circulation showed the distribution of upwelling over zones and downwelling over belts, consistent with cloud observations. The tropospheric cells featured a partial reversal of the downward vertical velocity over the belts and a considerable reduction of the upward movement over the zones in the domain, consistent with recent detections of high water clouds and lightning in belts. We also used the modeled new forcing fields as source terms for the semi-geostrophic Poisson equation to attribute the origin of the modeled secondary circulation. In this analysis, the stratospheric circulation cells observed in the model are primarily generated in response to radiative forcing, while momentum forcing induces the shifted configurations in the deep atmosphere.     

Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS

We use five and one-half years of limb- and nadir-viewing temperature mapping observations by the Composite Infrared Radiometer-Spectrometer (CIRS) on the Cassini Saturn orbiter, taken between July 2004 and December 2009 (L_S from 293° to 4°; northern mid-winter to just after northern spring equinox), to monitor temperature changes in the upper stratosphere and lower mesosphere of Titan. The largest changes are in the northern (winter) polar stratopause, which has declined in temperature by over 20 K between 2005 and 2009. Throughout the rest of the mid to upper stratosphere and lower mesosphere, temperature changes are less than 5 K. In the southern hemisphere, temperatures in the middle stratosphere near 1 mbar increased by 1-2 K from 2004 through early 2007, then declined by 2-4 K throughout 2008 and 2009, with the changes being larger at more polar latitudes. Middle stratospheric temperatures at mid-northern latitudes show a small 1-2 K increase from 2005 through 2009. At north polar latitudes within the polar vortex, temperatures in the middle stratosphere show a ∼4 K increase during 2007, followed by a comparable decrease in temperatures in 2008 and into early 2009. The observed temperature changes in the north polar region are consistent with a weakening of the subsidence within the descending branch of the middle atmosphere meridional circulation.     

Saturn's south polar vortex compared to other large vortices in the Solar System

Observations made by the Imaging Science Subsystem (ISS), Visible and Infrared Mapping Spectrometer (VIMS) and the long-wavelength Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft reveal that the large, long-lived cyclonic vortex at Saturn's south pole has a 4200-km-diameter cloud-free nearly circular region. This region has a 4 K warm core extending from the troposphere into the stratosphere, concentric cloud walls extending 20-70 km above the internal clouds, and numerous external clouds whose anticyclonic vorticity suggests a convective origin. The rotation speeds of the vortex reach . The Saturn polar vortex has features in common with terrestrial hurricanes and with the Venus polar vortex. Neptune and other giant planets may also have strong polar vortices.     

Latitudinal variations in Jovian stratospheric temperature

Ground-based observations of Jupiter show that the planet's stratospheric and tropospheric thermal emission are anticorrelated. The observations can possibly be explained by latitudinal variations in cloud altitude. These variations cause differential stratospheric heating by sunlight which is reflected off the clouds and then absorbed within the stratosphere by visible and near-infrared bands of methane.     

Seasonal change on Saturn from Cassini/CIRS observations, 2004-2009

Five years of thermal infrared spectra from the Cassini Composite Infrared Spectrometer (CIRS) are analyzed to determine the response of Saturn’s atmosphere to seasonal changes in insolation. Hemispheric mapping sequences at 15.0 cm⁻¹ spectral resolution are used to retrieve the variation in the zonal mean temperatures in the stratosphere (0.5-5.0 mbar) and upper troposphere (75-800 mbar) between October 2004 (shortly after the summer solstice in the southern hemisphere) and July 2009 (shortly before the autumnal equinox).Saturn’s northern mid-latitudes show signs of dramatic warming in the stratosphere (by 6-10 K) as they emerge from ring-shadow into springtime conditions, whereas southern mid-latitudes show evidence for cooling (4-6 K). The 40-K asymmetry in stratospheric temperatures between northern and southern hemispheres (at 1 mbar) slowly decreased during the timespan of the observations. Tropospheric temperatures also show temporal variations but with a smaller range, consistent with the increasing radiative time constant of the atmospheric response with increasing pressure. The tropospheric response to the insolation changes shows the largest magnitude at the locations of the broad retrograde jets. Saturn’s warm south-polar stratospheric hood has cooled over the course of the mission, but remains present.Stratospheric temperatures are compared to a radiative climate model which accounts for the spatial distribution of the stratospheric coolants. The model successfully predicts the magnitude and morphology of the observed changes at most latitudes. However, the model fails at locations where strong dynamical perturbations dominate the temporal changes in the thermal field, such as the hot polar vortices and the equatorial semi-annual oscillation (Orton, G., and 27 colleagues [2008]. Nature 453, 196-198). Furthermore, observed temperatures in Saturn’s ring-shadowed regions are larger than predicted by all radiative-climate models to date due to the incomplete characterization of the dynamical response to the shadow. Finally, far-infrared CIRS spectra are used to demonstrate variability of the para-hydrogen distribution over the 5-year span of the dataset, which may be related to observed changes in Saturn’s tropospheric haze in the spring hemisphere.