The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH₄, C₂H₂, C₂H₄, C₄H₂, HCN, HC₃N and C₆H₆ between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan.     

New perspectives on Titan's upper atmosphere from a reanalysis of the Voyager 1 UVS solar occultations

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351-1359) we find N₂ densities that are 25-60% higher, CH₄ densities that are smaller by a factor of 3-7, and C₂H₂ densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153-158 K, which are approximately 20-40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information for C₂H₄, HCN, and HC₃N. Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243-256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system.     

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.     

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.     

Methane and its isotopologues on Saturn from Cassini/CIRS observations

High spectral resolution observations from the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297] are analysed to derive new estimates for the mole fractions of CH₄, CH₃D and ¹³CH₄ of (4.7±0.2)×¹⁰⁻³, (3.0±0.2)×¹⁰⁻⁷ and (5.1±0.2)×¹⁰⁻⁵ respectively. The mole fractions show no hemispherical asymmetries or latitudinal variability. The analysis combines data from the far-IR methane rotational lines and the mid-IR features of methane and its isotopologues, using both the correlated-k retrieval algorithm of Irwin et al. [Irwin, P., and 9 colleagues, 2008. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150] and a line-by-line approach to evaluate the reliability of the retrieved quantities. C/H was found to be enhanced by 10.9±0.5 times the solar composition of Grevesse et al. [Grevesse, N., Asplund, M., Sauval, A., 2007. Space Sci. Rev. 130 (1), 105-114], 2.25±0.55 times larger than the enrichment on Jupiter, and supporting the increasing fractional core mass with distance from the Sun predicted by the core accretion model of planetary formation. A comparison of the jovian and saturnian C/N, C/S and C/P ratios suggests different reservoirs of the trapped volatiles in a primordial solar nebula whose composition varies with distance from the Sun. This is supported by our derived D/H ratio in methane of (1.6±0.2)×¹⁰⁻⁵, which appears to be smaller than the jovian value of Lellouch et al. [Lellouch, E., Bézard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610-622]. Mid-IR emission features provided an estimate of , which is consistent with both the terrestrial ratio and jovian ratio, suggesting that carbon was accreted from a shared reservoir for all of the planets.     

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.     

Titan's hydrodynamically escaping atmosphere: Escape rates and the structure of the exobase region

In Strobel [Strobel, D.F., 2008. Icarus, 193, 588-594] a mass loss rate from Titan's upper atmosphere, , was calculated for a single constituent, N₂ atmosphere by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating due to CH₄ absorption. It was estimated, but not proven, that the hydrodynamic mass loss is essentially CH₄ and H₂ escape. Here the individual conservation of momentum equations for the three major components of the upper atmosphere (N₂, CH₄, H₂) are solved in the low Mach number limit and compared with Cassini Ion Neutral Mass Spectrometer (INMS) measurements to demonstrate that light gases (CH₄, H₂) preferentially escape over the heavy gas (N₂). The lightest gas (H₂) escapes with a flux 99% of its limiting flux, whereas CH₄ is restricted to ?75% of its limiting flux because there is insufficient solar power to support escape at the limiting rate. The respective calculated H₂ and CH₄ escape rates are 9.2×¹⁰²⁷ and 1.7×¹⁰²⁷ s−1, for a total of . From the calculated densities, mean free paths of N₂, CH₄, H₂, and macroscopic length scales, an extended region above the classic exobase is inferred where frequent collisions are still occurring and thermal heat conduction can deliver power to lift the escaping gas out of the gravitational potential well. In this region rapid acceleration of CH₄ outflow occurs. With the thermal structure of Titan's thermosphere inferred from INMS data by Müller-Wodarg et al. [Müller-Wodarg, I.C.F., Yelle, R.V., Cui, J., Waite Jr., J.H., 2008. J. Geophys. Res. 113, doi:10.1029/2007JE003033. E10005], in combination with calculated temperature profiles that include sputter induced plasma heating at the exobase, it is concluded that on average that the integrated, globally average, orbit-averaged, plasma heating rate during the Cassini epoch does not exceed ().     

The structure of Titan’s atmosphere from Cassini radio occultations

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34°S near the surface, and the fourth at 52.8°S. Temperature-altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal, G.F., Wood, G.E., Hotz, H.B., Sweetnam, D.N., Eshleman, V.R., Tyler, G.L. [1983]. Icarus 53, 348-363), the HASI instrument on the Huygens descent probe (Fulchignoni, M. et al. [2005]. Nature 438, 785-791), and Cassini CIRS results (Flasar, F.M. et al. [2005]. Science 308, 975-978; Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [2008b]. Icarus 194, 263-277). Sources of error in the retrieved temperature-altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34°S are about 93 K, while that at 53°S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53°S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33°S lie along a dry adiabat except within ∼200 m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34°S shows a more complex structure in the lowest 2 km, while the egress profile at 53°S is more stable.     

Saturn eddy momentum fluxes and convection: First estimates from Cassini images

We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere (). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events.     

Estimating the masses of Saturn’s A and B rings from high-optical depth N-body simulations and stellar occultations

We have completed a series of local N-body simulations of Saturn’s B and A rings in order to identify systematic differences in the degree of particle clumping into self-gravity wakes as a function of orbital distance from Saturn and dynamical optical depth (a function of surface density). These simulations revealed that the normal optical depth of the final configuration can be substantially lower than one would infer from a uniform distribution of particles. Adding more particles to the simulation simply piles more particles onto the self-gravity wakes while leaving relatively clear gaps between the wakes. Estimating the mass from the observed optical depth is therefore a non-linear problem. These simulations may explain why the Cassini UVIS instrument has detected starlight at low incidence angles through regions of the B ring that have average normal optical depths substantially greater than unity at some observation geometries [Colwell, J.E., Esposito, L.W., Srem?evi?, M., Stewart, G.R., McClintock, W.E., 2007. Icarus 190, 127-144]. We provide a plausible internal density of the particles in the A and B rings based upon fitting the results of our simulations with Cassini UVIS stellar occultation data. We simulated Cassini-like occultations through our simulation cells, calculated optical depths, and attempted to extrapolate to the values that Cassini observes. We needed to extrapolate because even initial optical depths of >4 (σ > 240 g cm⁻²) only yielded final optical depths no greater than 2.8, smaller than the largest measured B ring optical depths. This extrapolation introduces a significant amount of uncertainty, and we chose to be conservative in our overall mass estimates. From our simulations, we infer the surface density of the A ring to be , which corresponds to a mass of . We infer a minimum surface density of for Saturn’s B ring, which corresponds to a minimum mass estimate of . The A ring mass estimate agrees well with previous analyses, while the B ring is at least 40% larger. In sum, our lower limit estimate is that the total mass of Saturn’s ring system is 120-200% the mass of the moon Mimas, but significantly larger values would be plausible given the limitations of our simulations. A significantly larger mass for Saturn’s rings favors a primordial origin for the rings because the disruption of a former satellite of the required mass would be unlikely after the decay of the late heavy bombardment of planetary surfaces.     

Ultraviolet observations of Phoebe from the Cassini UVIS

Observations of Saturn's distant moon Phoebe were made at far-ultraviolet (FUV) (1100-1900 Å) and extreme-ultraviolet (EUV) (600-1100 Å) wavelengths by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during the Cassini spacecraft flyby on June 11, 2004. These are the first UV spectra of Phoebe and the first detection of water ice on a Solar System surface using FUV wavelengths. The characteristics of water ice in the FUV are presented, and Hapke models are used to interpret the spectra in terms of composition and grain size; the use of both areal and intimate mixing models is explored. Non-ice species used in these models include carbon, ice tholin, Triton tholin, poly-HCN and kerogen. Satisfactory disk-integrated fits are obtained for intimate mixtures of ∼10% H₂O plus a non-ice species. Spatially resolved regions of higher (∼20%) and lower (∼5%) H₂O ice concentrations are also detected. Phoebe does not display any evidence of volatile activity. Upper limits on atomic oxygen and carbon are 5×¹⁰¹¹ and 2×¹⁰¹² atoms/cm², respectively, for solar photon scattering. The UVIS detection of water ice on Phoebe, and the ice amounts detected, are consistent with IR measurements and contribute to the evidence for a Phoebe origin in the outer Solar System rather than in the main asteroid belt.     

The meridional phosphine distribution in Saturn's upper troposphere from Cassini/CIRS observations

The Cassini Composite Infrared Spectrometer (CIRS) has been used to derive the vertical and meridional variation of temperature and phosphine (PH₃) abundance in Saturn's upper troposphere. PH₃ has a significant effect on the measured radiances in the thermal infrared and between May 2004 and September 2005 CIRS recorded thousands of spectra in both the far (10-600 cm⁻¹) and mid (600-1400 cm⁻¹) infrared, at a variety of latitudes covering the southern hemisphere. Low spectral resolution (15 cm⁻¹) data has been used to constrain the temperature structure of the troposphere between 100 and 500 mbar. The vertical distributions of phosphine and ammonia were retrieved from far-infrared spectra at the highest spectral resolution (0.5 cm⁻¹), and lower resolution (2.5 cm⁻¹) mid-infrared data were used to map the meridional variation in the abundance of phosphine in the 250-500 mbar range. Temperature variations at the 250 mbar level are shown to occur on the same scale as the prograde and retrograde jets in Saturn's atmosphere [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243-1247]. The PH₃ abundance at 250 mbar is found to be enhanced at the equator when compared with mid-latitudes. At mid latitudes we see anti-correlation between temperature and PH₃ abundance at 250 mbar, phosphine being enhanced at 45° S and depleted at 25 and 55° S. The vertical distribution is markedly different polewards of 60-65° S, with depleted PH₃ at 500 mbar but a slower decline in abundance with altitude when compared with the mid-latitudes. This variation is similar to the variations of cloud and aerosol parameters observed in the visible and near infrared, and may indicate the subsidence of tropospheric air at polar latitudes, coupled with a diminished sunlight penetration depth reducing the rate of PH₃ photolysis in the polar region.     

Lightning storms on Saturn observed by Cassini ISS and RPWS during 2004-2006

We report on Cassini Imaging Science Subsystem (ISS) data correlated with Radio and Plasma Wave Science (RPWS) observations, which indicate lightning on Saturn. A rare bright cloud erupt at ∼35° South planetocentric latitude when radio emissions (Saturn Electrostatic Discharges, or SEDs) occur. The cloud consisting of few consecutive eruptions typically lasts for several weeks, and then both the cloud and the SEDs disappear. They may reappear again after several months or may stay inactive for a year. Possibly, all the clouds are produced by the same atmospheric disturbance which drifts West at 0.45 °/day. As of March 2007, four such correlated visible and radio storms have been observed since Cassini Saturn Orbit Insertion (July 2004). In all four cases the SEDs are periodic with roughly Saturn's rotation rate (h¹⁰m³⁹), and show correlated phase relative to the times when the clouds are seen on the spacecraft-facing side of the planet, as had been shown for the 2004 storms in [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243-1247]. The 2000-km-scale storm clouds erupt to unusually high altitudes and then slowly fade at high altitudes and spread at low altitudes. The onset time of individual eruptions is less than a day during which time the SEDs reach their maximum rates. This suggests vigorous atmospheric updrafts accompanied by strong precipitation and lightning. Unlike lightning on Earth and Jupiter, where considerable lightning activity is known to exist, only one latitude on Saturn has produced lightning strong enough to be detected during the two and a half years of Cassini observations. This may partly be a detection issue.     

Density waves in Cassini UVIS stellar occultations: 1. The Cassini Division

We analyze density waves in the Cassini Division of Saturn's rings revealed by multiple stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph. The dispersion and damping of density waves provide information on the local ring surface mass density and viscosity. Several waves in the Cassini Division are on gradients in the background optical depth, and we find that the dispersion of the wave reflects a change in the underlying surface mass density. We find that over most of the Cassini Division the ring opacity (the ratio of optical depth to surface mass density) is nearly constant and is ∼5 times higher than the opacity in the A ring where most density waves are found. However, the Cassini Division ramp, a 1100-km-wide, nearly featureless region of low optical depth that connects the Cassini Division to the inner edge of the A ring, has an opacity like that of the A ring and significantly less than that in the rest of the Cassini Division. This is consistent with particles in the ramp originating in the A ring and being transported into the Cassini Division through ballistic transport processes. Damping of the waves in the Cassini Division suggests a vertical thickness of 3–6 m. Using a mean opacity of 0.1 cm²/g we find the mass of the Cassini Division, excluding the ramp, is 3.1×¹⁰¹⁶ kg while the mass of the Cassini Division ramp, with an opacity of 0.015 cm²/g, is 1.1×¹⁰¹⁷ kg. Assuming a power-law size distribution for the ring particles, the larger opacity of the main Cassini Division is consistent with the largest ring particles there being ∼5 times smaller than the largest particles in the ramp and A ring.     

New upper limits for hydrogen halides on Saturn derived from Cassini-CIRS data

Far infrared spectra (10-600 cm⁻¹) from Cassini's Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturn's atmosphere. Three observations, comprising a total of 3088 spectra, gave 3σ upper limits on HF, HCl, HBr, and HI volume mole fractions of 8.0×¹⁰⁻¹², 6.7×¹⁰⁻¹¹, 1.3×¹⁰⁻¹⁰, and 1.4×¹⁰⁻⁹, respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCl, and HBr in Saturn's upper atmosphere—consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCl is 16 times lower than the tentative detection at 1.1×¹⁰⁻⁹ reported by Weisstein and Serabyn [Weisstein, E.W., Serabyn, E., 1996. Icarus 123, 23-36]. These observations are not sensitive to the deep halide abundance, which is expected to be enriched relative to the solar composition.     

EUV spectroscopy of the Venus dayglow with UVIS on Cassini

We analyze EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus on 24 June 1999, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We present the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 102.5 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by an airglow model previously used to analyze Venus and Mars ultraviolet spectra. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the observed intensity of the OII emission at 83.4 nm is higher than predicted by the model. An increase of the O⁺ ion density relative to the densities usually measured by Pioneer Venus brings the observations and the modeled values into better agreement. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The intensity of the OI 98.9 nm emission is well predicted by the model if resonance scattering of solar radiation by O atoms is included as a source. The calculated brightness of the NI 120 nm multiplet is larger than observed by a factor of ∼2-3 if photons from all sources encounter multiple scattering. The discrepancy reduces to 30-80% if the photon electron impact and photodissociation of N₂ sources of N(⁴S) atoms are considered as optically thin. Overall, we find that the O, N₂ and CO densities from the empirical VTS3 model provide satisfactory agreement between the calculated and the observed EUV airglow emissions.     

Far ultraviolet spectral properties of Saturn’s rings from Cassini UVIS

Spectra taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) of Saturn’s C ring, B ring, Cassini Division, and A ring have been analyzed in order to characterize ring particle surface properties and water ice abundance in the rings. UVIS spectra sense the outer few microns of the ring particles. Spectra of the normalized reflectance (I/F) in all four regions show a characteristic water ice absorption feature near 165 nm. Our analysis shows that the fractional abundance of surface water ice is largest in the outer B ring and decreases by over a factor of 2 across the inner C ring. We calculate the mean path length of UV photons through icy ring particle regolith and the scattering asymmetry parameter using a Hapke reflectance model and a Shkuratov reflectance model to match the location of the water ice absorption edge in the data. Both models give similar retrieved values of the photon mean length, however the retrieved asymmetry (g) values are different. The photon mean path lengths are nearly uniform across the B and A rings. Shortward of 165 nm the rings exhibit a slope that turns up towards shorter wavelengths, while the UV slope of 180/150 nm (reflectance outside the water absorption ratioed to that inside the absorption band) tracks I/F with maxima in the outer B ring and in the central A ring. Retrieved values of the scattering asymmetry parameter show the regolith grains to be highly backscattering in the FUV spectral regime.     

Saturn’s zonal wind profile in 2004-2009 from Cassini ISS images and its long-term variability

Five years of Cassini Imaging Science Subsystem images, from 2004 to 2009, are analyzed in this work to retrieve global zonal wind profiles of Saturn’s northern and southern hemispheres in the methane absorbing bands at 890 and 727 nm and in their respective adjacent continuum wavelengths of 939 and 752 nm. A complete view of Saturn’s global circulation, including the equator, at two pressure levels, in the tropopause (60 mbar to 250 mbar with the MT filters) and in the upper troposphere (from ∼350 mbar to ∼500 mbar with the CB filter set), is presented. Both zonal wind profiles (available at the Supplementary Material Section), show the same structure but with significant differences in the peak of the eastward jets and the equatorial region, including a region of positive vertical shear symmetrically located around the equator between the 10° < |φ_c| < 25° where zonal velocities close to the tropopause are higher than at 500 mbar. A comparison of previously published zonal wind sets obtained by Voyager 1 and 2 (1980-1981), Hubble Space Telescope, and ground-based telescopes (1990-2004) with the present Cassini profiles (2004-2009) covering a full Saturn year shows that the shape of the zonal wind profile and intensity of the jets has remained almost unchanged except at the equator, despite the seasonal insolation cycle and the variability of Saturn’s emitted power. The major wind changes occurred at equatorial latitudes, perhaps following the Great White Spot eruption in 1990. It is not evident from our study if the seasonal insolation cycle and its associated ring shadowing influence the equatorial circulation at cloud level.     

Emergence of polar-jet polygons from jet instabilities in a Saturn model

Voyager flybys of Saturn in 1980-1981 revealed a circumpolar wave at ≈78° north planetographic latitude. The feature had a dominant wavenumber 6 mode, and has been termed the Hexagon from its geometric appearance in polar-projected mosaics. It was also noted for being stationary with respect to Saturn’s Kilometric Radiation (SKR) rotation rate. The Hexagon has persisted for over 30 years since the Voyager observations until now. It has been observed from ground based telescopes, Hubble Space Telescope and multiple instruments onboard Cassini in orbit around Saturn. Measurements of cloud motions in the region reveal the presence of a jet stream whose path closely follows the Hexagon’s outline. Why the jet stream takes the characteristic six-sided shape and how it is stably maintained across multiple saturnian seasons are yet to be explained. We present numerical simulations of the 78.3°N jet using the Explicit Planetary Isentropic-Coordinate (EPIC) model and demonstrate that a stable hexagonal structure can emerge without forcing when dynamic instabilities in the zonal jet nonlinearly equilibrate. For a given amplitude of the jet, the dominant zonal wavenumber is most strongly dependent on the peak curvature of the jet, i.e., the second north-south spatial derivative of the zonal wind profile at the center of the jet. The stable polygonal shape of the jet in our simulations is formed by a vortex street with cyclonic and anticyclonic vortices lining up towards the polar and equatorial side of the jet, respectively. Our result is analogous to laboratory experiments of fluid motions in rotating tanks that develop polygonal flows out of vortex streets. However, our results also show that a vortex street model of the Hexagon cannot reproduce the observed propagation speed unless the zonal jet’s speed is modified beyond the uncertainties in the observed zonal wind speed, which suggests that a vortex street model of the Hexagon and the observed zonal wind profile may not be mutually compatible.     

UVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10¹⁵ cm⁻² in close agreement with the value provided by the VTS3 empirical atmospheric model.