Saturn's vertical and latitudinal cloud structure 1991-2004 from HST imaging in 30 filters

We analyzed 134 images of Saturn taken by the Hubble Space Telescope between 1991 and 2004. The images cover wavelengths between 231 and 2370 nm in 30 filters. We combined some 10 million calibrated reflectivity measurements into 18,000 center-to-limb curves. We used the method of principal component analysis to find the main latitudinal and temporal variations in Saturn's atmosphere and their spectral characteristics. The first principal variation is a strong latitudinal variation of the aerosol optical depth in the upper troposphere. This structure shifts with Saturn's seasons, but the structure on small scales of latitude stays constant. The second principal variation is a variable optical depth of stratospheric aerosols. The optical depth is large at the poles and small at mid- and low latitudes with a steep gradient in-between. This structure remains essentially constant in time. The third principal variation is a variation in the tropospheric aerosol size, which has only shallow gradients with latitude, but large seasonal variations. Thus, aerosol sizes and their phase functions inferred at a particular season are not representative of Saturn's atmosphere at other seasons. Aerosols are largest in the summer and smallest in the winter. The fourth principal variation is a feature of the tropospheric aerosols with irregular latitudinal structure and fast variability, on the time scale of months. Spherical aerosols do not display the spectral characteristic of that feature. We suspect that variations in the shape of aerosols may play a role. We found a spectral feature of the imaginary index of aerosols, which darkens them near 400 nm wavelength. While we can describe Saturn's variations quite accurately, our presented model of Saturn's average atmosphere is still uncertain due to possible systematic offsets in methane absorption data and limitations of the knowledge about the shape of aerosols. In order to compare our results with those from comparable investigations, which used less than 30 filters, we fit models to spectral subsets of our data. We found very different best-fitting models, depending on the subset of filters, indicating a high sensitivity of results on the spectral sampling.     

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.     

Ion winds in Saturn's southern auroral/polar region

We present profiles of the line-of-sight (l.o.s.) ionospheric wind velocities in the southern auroral/polar region of Saturn. Our velocities are derived from the measurement of Doppler shifting of the H₃⁺ν₂Q(1,0⁻) line at 3.953 microns. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the night of February 6, 2003 (UT). The l.o.s. velocity profiles finally derived are consistent with an extended region of the upper atmosphere sub-corotating with the planet: the ion velocities in the inertial reference are only 1/3 of those expected for full planetary corotation. We discuss the results in the light of recent proposals for the kronian magnetosphere, and suggest that, in this region, Saturn's ion winds may be under solar wind control.     

BVR photometry of Hyperion near the time of the 2005 Cassini encounter

Six nights of BVR photometry and three nights of R photometry were collected over a month-long period shortly after the Cassini encounter with Hyperion on September 24 2005. Our observations were designed to help constrain the rotational state of the chaotically rotating satellite. Fourier analysis of our lightcurve data yields three possible periods: 10.2±0.2, 13.9±0.2, and 19.7±0.4 days. Our B-V and V-R colors agree well with previous ground-based and Voyager 2 measurements.     

A close look at Saturn's rings with Cassini VIMS

Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H₂O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring.     

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.     

Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32, L14S07, doi:10.1029/2005GL022653] to L∼10 along with also providing a more comprehensive study of the interrelationship of the various fluid parameters. Coincidence data are sub-divided into protons and water group ions. Our revised analysis uses an improved convergence algorithm which provides a more definitive and independent estimate of the spacecraft potential Φ_SC for which we enforce the protons and water group ions to co-move with each other. This has allowed us to include spacecraft charging corrections to our fluid parameter estimations and allow accurate estimations of fluctuations in the fluid parameters for future correlative studies. In the appendix we describe the ion moments algorithm, and minor corrections introduced by not weighting the moments with sinθ term in Sittler et al. [2005] (Correction offset by revisions to instruments geometric factor). Estimates of the spacecraft potential and revised proton densities are presented. Our total ion densities are in close agreement with the electron densities reported by Moncuquet et al. [2005. Quasi-thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: electron temperatures and density. Geophys. Res. Lett. 32, L20S02, doi:10.1029/2005GL022508] who used upper hybrid resonance (UHR) emission lines observed by the radio and plasma wave science (RPWS) instrument. We show a positive correlation between proton temperature and water group ion temperature. The proton and thermal electron temperatures track each with both having a positive radial gradient. These results are consistent with pickup ion energization via Saturn's rotational electric field. We see evidence for an anti-correlation between radial flow velocity V_R and azimuthal velocity V_φ, which is consistent with the magnetosphere tending to conserve angular momentum. Evidence for MHD waves is also present. We show clear evidence for outward transport of the plasma via flux tube interchange motions with the radial velocity of the flow showing positive radial gradient with functional dependence for 4<L<10 (i.e., if we assume to be diffusive transport then D_LL∼D₀L¹¹ for fixed stochastic time step δt). Previous models with centrifugal transport have used D_LL∼D₀L³ dependence. The radial transport seems to begin at Enceladus’ L shell, L∼4, where we also see a minimum in the W⁺ ion temperature . For the first time, we are measuring the actual flux tube interchange motions in the magnetosphere and how it varies with radial distance. These observations can be used as a constraint with regard to future transport models for Saturn's magnetosphere. Finally, we evaluate the thermodynamic properties of the plasma, which are all consistent with the pickup process being the dominant energy source for the plasma.     

Numerical models of Saturn's long-lived anticyclones

New measurements of the dynamical properties of the long-lived Saturn's anticyclonic vortex known as “Brown Spot” (BS), discovered during the Voyager 1 and 2 flybys in 1980-1981 at latitude 43.1° N, and model simulations using the EPIC code, have allowed us to constrain the vertical wind shear and static stability in Saturn's atmosphere (vertically from pressure levels from 10 mbar to 10 bars) at this latitude. BS dynamical parameters from Voyager images include its size as derived from cloud albedo gradient (6100 km East-West times 4300 km North-South), mean tangential velocity ( at 2400 km from center) and mean vorticity (4.0±1.5×¹⁰⁻⁵ s⁻¹), lifetime >1 year, drift velocity relative to Voyager's System III rotation rate, mean meridional atmospheric wind profile at cloud level at its latitude and interactions with nearby vortices (pair orbiting and merging). An extensive set of numerical experiments have been performed to try to reproduce this single vortex properties and its observed mergers with smaller anticyclones by varying the vertical structure of the zonal wind and adjusting the static stability of the lower stratosphere and upper troposphere. Within the context of the EPIC model atmosphere, our simulations indicate that BS's drift velocity, longevity and merging behavior are very sensitive to these two atmospheric properties. The best results at the BS latitude occur for static stability conditions that use a Brunt-Väisäla frequency constant in the upper troposphere (from 0.5 to 10 bar) above 3.2×¹⁰⁻³ s⁻¹ and suggest that the wind speed slightly decays below the visible cloud deck from ∼0.5 to 10 bar at a rate per scale height. Changing the vortex latitude within the band domain introduces latitude oscillations in the vortex but not a significant meridional migration. Simulated mergers always showed orbiting movements with a typical merging time of about three days, very close to the time-span observed in the interaction of real vortices. Although these results are not unique in view of the unknowns of Saturn's deep atmosphere, they serve to constrain realistically its structure for ongoing Cassini observations.     

The three-dimensional structure of Saturn's equatorial jet at cloud level

The three-dimensional structure of Saturn's intense equatorial jet from latitudes 8° N to 20° S is revealed from detailed measurements of the motions and spectral reflectivity of clouds at visible wavelengths on high resolution images obtained by the Cassini Imaging Science Subsystem (ISS) in 2004 and early 2005. Cloud speeds at two altitude levels are measured in the near infrared filters CB2 and CB3 matching the continuum (effective wavelengths 750 and 939 nm) and in the MT2 and MT3 filters matching two methane absorption bands (effective wavelengths 727 and 889 nm). Radiative transfer models in selective filters covering an ample spectral range (250-950 nm) require the existence of two detached aerosol layers in the equator: an uppermost thin stratospheric haze extending between the pressure levels ∼20 and 40 mbar (tropopause level) and below it, a dense tropospheric haze-cloud layer extending between 50 mbar and the base of the ammonia cloud (between ∼1 and 1.4 bar). Individual cloud elements are detected and tracked in the tropospheric dense haze at 50 and 700 mbar (altitude levels separated by 142 km). Between latitudes 5° N and 12° S the winds increase their velocity with depth from 265 m s⁻¹ at the 50 mbar pressure level to 365 m s⁻¹ at 700 mbar. These values are below the high wind speeds of 475 m s⁻¹ measured at these latitudes during the Voyager era in 1980-1981, indicating that the equatorial jet has suffered a significant intensity change between that period and 1996-2005 or that the tracers of the flow used in the Voyager images were rooted at deeper levels than those in Cassini images.     

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.     

Production, ionization and redistribution of O2 in Saturn's ring atmosphere

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturn's magnetosphere. A model is described for the toroidal O₂ atmosphere indicated by the detection of and O⁺ over the main rings. The O₂ ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O₂ has a long lifetime and interacts frequently with the ring particles, equivalent columns of O₂ exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O₂ column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O⁺ is produced with a significant excess energy, is not. Therefore, should mirror well below those altitudes at which ions were detected. However, scattering by ion-molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturn's toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.     

Structure of self-gravity wakes in Saturn's A ring as measured by Cassini CIRS

The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturn's A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624-2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H/λ=0.1607±0.0002, a width-over-spacing W/λ=0.3833±0.0008. Gaps between wakes, which are filled with particles, have an optical depth τG=0.1231±0.0005. The wakes mean pitch angle ΦW is 70.70°±0.07°, relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm⁻² [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14-34], the expected spacing of wakes is λ≈60 m. Their height and width would then be H≈10 m and W≈24 m, values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502-523].     

Investigation of energetic proton penetration in Titan's atmosphere using the Cassini INCA instrument

Saturn's largest moon, Titan, provides an interesting opportunity to study how dense atmospheres interact with the surrounding plasma environment. Without an intrinsic magnetic field, this satellite's nitrogen-rich atmosphere is relatively unprotected from plasma interactions. Therefore, the energy-deposition rate is important for understanding chemistry and dynamics in Titan's atmosphere. Since the plasma environment can vary significantly we focus here on the T18 Titan encounter using in-situ data from instruments on board the Cassini spacecraft. These instruments cannot provide in-situ information below the spacecraft closest approach altitude (∼>960 km) so we use the Cassini magnetospheric imaging instrument (MIMI) ion-neutral camera (INCA) to remotely image energetic hydrogen particle fluxes (20-80 keV) at altitudes below Titan closest approach. We also use the MIMI low-energy magnetosphere measurements system (LEMMS) to measure the incident ion fluxes as the spacecraft approaches Titan and combine these data sets with an atmospheric model to first reproduce INCA images. We then use this model to calculate the energy-deposition profiles for the observed incident proton flux. Our model is able to reproduce the INCA observations and give the energy density deposited vs. altitude in Titan's atmosphere; however, we find that the incident fluxes and energy-deposition profiles vary significantly during the encounter.     

Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system

Cassini VIMS has obtained spatially resolved imaging spectroscopy data on numerous satellites of Saturn. A very close fly-by of Dione provided key information for solving the riddle of the origin of the dark material in the Saturn system. The Dione VIMS data show a pattern of bombardment of fine, sub-0.5-μm diameter particles impacting the satellite from the trailing side direction. Multiple lines of evidence point to an external origin for the dark material on Dione, including the global spatial pattern of dark material, local patterns including crater and cliff walls shielding implantation on slopes facing away from the trailing side, exposing clean ice, and slopes facing the trailing direction which show higher abundances of dark material. Multiple spectral features of the dark material match those seen on Phoebe, Iapetus, Hyperion, Epimetheus and the F-ring, implying the material has a common composition throughout the Saturn system. However, the exact composition of the dark material remains a mystery, except that bound water and, tentatively, ammonia are detected, and there is evidence both for and against cyanide compounds. Exact identification of composition requires additional laboratory work. A blue scattering peak with a strong UV-visible absorption is observed in spectra of all satellites which contain dark material, and the cause is Rayleigh scattering, again pointing to a common origin. The Rayleigh scattering effect is confirmed with laboratory experiments using ice and 0.2-μm diameter carbon grains when the carbon abundance is less than about 2% by weight. Rayleigh scattering in solids is also confirmed in naturally occurring terrestrial rocks, and in previously published reflectance studies. The spatial pattern, Rayleigh scattering effect, and spectral properties argue that the dark material is only a thin coating on Dione's surface, and by extension is only a thin coating on Phoebe, Hyperion, and Iapetus, although the dark material abundance appears higher on Iapetus, and may be locally thick. As previously concluded for Phoebe, the dark material appears to be external to the Saturn system and may be cometary in origin. We also report a possible detection of material around Dione which may indicate Dione is active and contributes material to the E-ring, but this observation must be confirmed.     

Infrared observations of Saturn's rings by Cassini CIRS : Phase angle and local time dependence

Since the Saturn orbit insertion (SOI) of the Cassini spacecraft, in July 2004, the Cassini Composite Infrared Spectrometer (CIRS) has obtained a large number of thermal infrared spectra of Saturn's rings. Over the two and a half years of observations to date, ring temperatures were retrieved for a large range of unique geometries, inaccessible from Earth. Understanding their dependencies with phase angle and local time is a clue to understanding the thermal properties and dynamics of Saturn's ring particles.Azimuthal scans of rings, which have been obtained by CIRS at constant radial distance from the planet, have been planned to measure ring temperature variations with local hour angle. Over 47 azimuthal scans for Saturn's main rings (A, B, C and Cassini Division) have been retrieved to date, on both lit and unlit sides, at different phase angles and spacecraft elevations. The first measurements of the transient thermal episode of eclipse cooling in the planetary shadow have also been obtained for all three rings.In this paper, we present an overview of all azimuthal scans obtained by the Cassini/CIRS instrument so far and the dependencies of the temperature and the filling factor with the phase angle and the local hour angle. The ring temperature varies with longitude as the input heating flux coming from Saturn and the Sun changes. The decrease in temperature with the increasing phase angle on both the lit and the unlit sides and for most of the local time also suggests the presence of slowly rotating particles. The crossing of the planet's shadow generates drastic azimuthal variations in temperature, up to 20 K in the C ring. The strong anisotropy of emission observed outside the shadow between low and high phase angles decreases when ring particles cross the shadow, suggesting that particles are almost isothermal in the shadow. This suggests a thermal inertia associated with a rotating rate of particles low enough to have a thermal contrast on their surface.The temperature in the B ring is less sensitive to the phase angle effect on the lit side, suggesting that particles are close enough to form a flat layer at a scale larger than the particle's radius. On the unlit side, particles in the B ring are less sensitive to the lack of solar input than in the C ring or in the A ring. Azimuthal variations of the filling factor in the A ring are also detected with changing ring local time. This effect might be created by the presence of gravitational instabilities (wakes).     

Disentangling Saturn's F Ring. I. Clump orbits and lifetimes

A comprehensive analysis of the Voyager images reveals the kinematics and lifetimes of clumps in the F Ring. At any given time, the ring has 2-3 major clumps, each several times brighter than the typical ring, plus numerous smaller features. A total of 34 individual clumps have been tracked over periods of 1-7 weeks. The clumps orbited at measurably different rates, implying a 100-km range of semimajor axes centered on 140,220 km. Most are centered around the nominal mean motion of the ring's core, but a few outliers may be associated with a different strand, or with no strand at all. Most clumps change very little over the ∼30 days that they can be detected; however, no clump persisted for the nine-month interval between the two Voyager encounters. The brightest Voyager 2 clump is unusual in that it travels at a rapid mean motion and seems to be associated with the formation of several other clumps.     

Saturn's latitudinal C2H2 and C2H6 abundance profiles from Cassini/CIRS and ground-based observations

Hydrocarbons in the upper atmosphere of Saturn are known, from Voyager, ground-based, and early Cassini results, to vary in emission intensity with latitude. Of particular interest is the marked increase in hydrocarbon line intensity near the south pole during southern summer, as the increased line intensity cannot be simply explained by the increased temperatures observed in that region since the variations between C₂H₂ and C₂H₆ emission in the south pole region are different. In order to measure the latitudinal variations of hydrocarbons in Saturn's southern hemisphere we have used 3 cm⁻¹ resolution Cassini CIRS data from 2006 and combined this with measurements from the ground in October 2006 at NASA's IRTF using Celeste, an infrared high-resolution cryogenic grating spectrometer. These two data sets have been used to infer the molecular abundances of C₂H₂ and C₂H₆ across the southern hemisphere in the 1-10 mbar altitude region. We find that the latitudinal acetylene profile follows the yearly average mean daily insolation except at the southern pole where it peaks in abundance. Near the equator (5° S) the C₂H₂ abundance at the 1.2 mbar level is (1.6±0.19)×¹⁰−7 and it decreases by a factor of 2.7 from the equator toward the pole. However, at the pole (∼87° S) the C₂H₂ abundance jumps to (1.8±0.3)×¹⁰−7, approximately the equatorial value. The C₂H₆ abundance near the equator at the 2 mbar level is (0.7±0.1)×¹⁰−5 and stays approximately constant until mid-latitudes where it increases gradually toward the pole, attaining a value of (1.4±0.4)×¹⁰−5 there. The increase in ethane toward the pole with the corresponding decrease in acetylene is consistent with southern hemisphere meridional winds [Greathouse, T.K., Lacy, J.H., Bézard, B., Moses, J.I., Griffith, C.A., Richter, M.J., 2005. Icarus 177, 18-31]. The localized increase in acetylene at the pole provides evidence that there is dynamical transport of hydrocarbons from the equator to the southern pole.     

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results

Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197-1210], one can estimate the ion total flux tube content, N_IONL², for protons, H⁺, and water group ions, W⁺, as a function of radial distance or dipole L shell. In Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion-electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with (T_⊥/T_∥)_W⁺∼5 and (T_⊥/T_∥)_H⁺∼2. The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N_IONL² for H⁺ and W⁺ peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S_ION∼10²⁷ ions/s as function of dipole L using N_H⁺, N_W⁺ and the time scale for ion loss due to radial transport τ_D and ion-electron recombination τ_REC. The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S_W, from our ion production rate, S_ION, and the time scale for loss of neutrals by ionization, τ_ION, and charge exchange, τ_CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S_W∼2×10²⁸ mol/s.     

Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H₂ have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200-1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between 1.4×¹⁰¹⁵ and . Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244-298] atmospheric model of Saturn and an electron energy-H₂ column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10-18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900-1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v^″<2 vibrational levels of the H₂ electronic ground state are partially self-absorbed by H₂ molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H₂ columns. On the other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H₂ rotational lines and makes it possible to determine the H₂ temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H₂ vertical column of . When converted to energy of precipitating electrons, this H₂ column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H₂ temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H₂ columns exceeding , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10-18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H₂ emission is thus likely located above but close to the methane homopause. The H₂ auroral brightness in the 800-1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.