On the vertical wind shear of Saturn's Equatorial Jet at cloud level

In this work we analyze and compare the vertical cloud structure of Saturn's Equatorial Zone in two different epochs: the first one close to the Voyagers flybys (1979-1981) and the second one in 2004, when the Cassini spacecraft entered its orbit around the planet. Our goal is to retrieve the altitude of cloud features used as zonal wind tracers in both epochs. We reanalyze three different sets of photometrically calibrated published data: ground-based in 1979, Voyager 2 PPS and ISS observations in 1981, and we analyze a new set of Hubble Space Telescope images for 2004. For all situations we reproduced the observed reflectivity by means of a similar vertical model with three layers. The results indicate the presence of a changing tropospheric haze in 1979-1981 (Ptop∼100 mbar, τ∼10) and in 2004 (Ptop∼50 mbar, τ∼15) where the tracers are embedded. According to this model the Voyager 2 ISS images locate cloud tracers moving with zonal velocities of 455 to 465 (±2) m/s at a pressure level of 360 ± 140 mbar. For HST observations, our previous works had showed cloud tracers moving with zonal wind speeds of 280±10 m/s at a pressure level of about 50±10 mbar. All these values are calculated in the same region (3°±2° N). This speed difference, if interpreted as a vertical wind shear, requires a change of per scale height, two times greater than that estimated from temperature observations. We also perform an initial guess on Cassini ISS vertical sounding levels, retrieving values compatible with HST ones and Cassini CIRS derived vertical wind shear, but not with Voyager wind measurements. We conclude that the wind speed velocity differences measured between 1979-1981 and 2004 cannot be explained as a wind shear effect alone and demand dynamical processes.     

Saturn's cloud structure and temporal evolution from ten years of Hubble Space Telescope images (1994-2003)

We present a study of the vertical structure of clouds and hazes in the upper atmosphere of Saturn's Southern Hemisphere during 1994-2003, about one third of a Saturn year, based on Hubble Space Telescope images. The photometrically calibrated WFPC2 images cover the spectral region between the near-UV (218-255 nm) and the near-IR (953-1042 nm), including the 890 nm methane band. Using a radiative transfer code, we have reproduced the observed center-to-limb variations in absolute reflectivity at selected latitudes which allowed us to characterize the vertical structure of the entire hemisphere during this period. A model atmosphere with two haze layers has been used to study the variation of hazes with latitude and to characterize their temporal changes. Both hazes are located above a thick cloud, putatively composed of ammonia ice. An upper thin haze in the stratosphere (between 1 and 10 mbar) is found to be persistent and formed by small particles (radii ∼0.2 μm). The lower thicker haze close to the tropopause level shows a strong latitudinal dependence in its optical thickness (typically τ∼20-40 at the equator but τ∼5 at the pole, at 814 nm). This tropospheric haze is blue-absorbent and extends from 50 to 100 mbar to about ∼400 mbar. Both hazes show temporal variability, but at different time-scales. First, there is a tendency for the optical thickness of the stratospheric haze to increase at all latitudes as insolation increases. Second, the tropospheric haze shows mid-term changes (over time scales from months to 1-2 years) in its optical thickness (typically by a factor of 2). Such changes always occur within a rather narrow latitude band (width ∼5-10°), affecting almost all latitudes but at different times. Third, we detected a long-term (∼10 year) decrease in the blue single-scattering albedo of the tropospheric haze particles, most intense in the equatorial and polar areas. Long-term changes follow seasonal insolation variations smoothly without any apparent delay, suggesting photochemical processes that affect the particles optical properties as well as their size. In contrast, mid-term changes are sudden and show various time-scales, pointing to a dynamical origin.     

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.     

Study of the vertical structure of Saturn's atmosphere using HST/WFPC2 images

We have studied the vertical structure of hazes at six different latitudes (−60°, −50°, −30°, −10°, +30°, and +50°) on Saturn's atmosphere. For that purpose we have compared the results of our forward radiative transfer model to limb-to-limb reflectivity scans at four different wavelengths (230, 275, 673.2, and 893 nm). The images were obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 in September 1997, during fall on Saturn's northern hemisphere. The spatial distribution of particles appears to be very variable with latitude both in the stratosphere and troposphere. For the latitude range +50° to −50°, an atmospheric structure consisting of a stratospheric haze and a tropospheric haze interspersed by clear gas regions has been found adequate to explain the center to limb reflectivities at the different wavelengths. This atmospheric structure has been previously used by Ortiz et al. (1996, Icarus 119, 53-66) and Stam et al. (2001, Icarus 152, 407-422). In this work the top of the tropospheric haze is found to be higher at the southern latitudes than at northern latitudes. This hemispherical asymmetry seems to be related to seasonal effects. Different latitudes experience different amount of solar insolation that can affect the atmospheric structure as the season varies with time. The haze optical thickness is largest (about 30 at 673.2 nm) at latitudes ±50 and −10 degrees, and smallest (about 18) at ±30 degrees. The stratospheric haze is found to be optically thin at all studied latitudes from −50 to +50 degrees being maximum at −10° (τ=0.033). At −60° latitude, where the UV images show a strong darkening compared to other regions on the planet, the cloud structure is remarkably different when compared to the other latitudes. Here, aerosol and gas are found to be uniformly mixed down to the 400 mbar level.     

A strong vortex in Saturn's South Pole

Saturn's southern pole was observed at high resolution by the Cassini Imaging Science Subsystem (ISS) during the spacecraft insertion orbit in July 2004. Cloud tracking of individual features on images taken at a wavelength of 938 nm reveal the existence of a strong polar vortex enclosed by a jet with maximum speed of relative to System III rotation frame, and peak at 87 °S planetographic latitude. Radiative transfer models of the reflected light, based on the Cassini images complemented by Hubble Space Telescope images from March 2004, indicate that the aerosol particles in the vortex are structured vertically in three detached layers. We find two hazes and one dense cloud distributed in altitude between ∼500 mbar (top of the dense cloud) and few mbar (top of the stratospheric haze), spanning a vertical altitude range of ∼200 km. The vortex area coincides with a thermal hot spot recently reported, indicating that winds decrease with altitude above polar clouds.     

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.     

Vertical structure modeling of Saturn's equatorial region using high spectral resolution imaging

A series of narrow-band images of Saturn was acquired on 7-11 February 2002 with an acousto-optic imaging spectrometer (AImS) at about 160 wavelengths between 500 and 950 nm. Our unique data set with high spectral agility and wide spectral coverage enabled us to extensively study the cloud structure and aerosol properties of Saturn's equatorial region at −10° latitude. Theoretical center-limb profiles based on twelve cloud models were fit to the observations at 23 wavelengths across the 619-, 727-, and 890-nm methane bands. A simultaneous multiwavelength multivariable fitting algorithm was adopted in varying up to 9 free parameters to efficiently explore the vast multidimensional parameter space, and a total of ∼12,000 initial conditions were tested. From the acceptable ranges of the model parameters, we obtained the following major conclusions: (1) the brightening of Saturn's equatorial region observed near 890 nm in February 2002 (I/F∼0.25 at the central meridian) results from high altitudes of a stratospheric haze layer (τ?∼0.05 above ∼0.04-bar level) and an upper tropospheric cloud (τ∼6 above ∼0.25-bar level), (2) if the upper tropospheric cloud is composed of ammonia ice particles and the Mie theory is applied, the mean particle size is larger than about 0.5 μm, (3) an optically thick cloud layer exists at a level of 0.5-2.2 bar below the upper cloud deck in Saturn's equatorial region. The ongoing observations by the Cassini spacecraft over wider spectral range and from various phase angles will further constrain Saturn's cloud structure and aerosol properties.     

Upper thermal boundary layer of planetary atmosphere: An attempt of developing a general model

In any planetary atmosphere there is an uppermost layer in which the molecular thermal conduction is a significant mechanism of forming the thermal structure of the atmosphere. In this paper, the similarity approach is first used to develop the 1-D general model of aforementioned layer. The main concepts of the model are (i) the radiative equilibrium condition at the lower boundary of the layer and (ii) taking into account a single rovibrational band for radiative cooling of the layer. Five dimensionless parameters of the model characterize both “strengths” and altitudinal distributions of heat sources and sinks in the layer, including an effect of the atmosphere under the layer. By fitting the modeled temperature profile to the mean empirical profile, both the magnitudes of the parameters and the relations between them have been determined for the Earth and Mars. Distinctions between these planets in both the parameter magnitudes and relationships can be accounted for by distinction in composition of their atmospheres. For both planets the model shows weak sensitivity of the modeled temperature profile to significant changes in the state of the underlying atmosphere. The model demonstrates some prognostic capabilities. Namely, the fitting reveals presence of O in the martian thermosphere. (However, the fractional O abundance is overestimated.) From drag deceleration of the MGS orbiter the mean temperature profile of the martian thermosphere between 115 and 170 km has been derived for the solar zenith angle of 45°-70°, the solar longitude of 30°-80°, and the latitude range from −10° to 60°at a moderate level of solar activity.     

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.     

Microwave observations of Saturn's rings: anisotropy in directly transmitted and scattered saturnian thermal emission

We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae.     

A reanalysis of Venus winds at two cloud levels from Galileo SSI images

The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531-1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region.     

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.     

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.     

Effects of a large convective storm on Saturn's equatorial jet

Hubble Space Telescope observations revealed that Saturn's equatorial jet at the cloud level blows at ∼275 m s⁻¹ today, approximately half the ∼470 m s⁻¹ wind during the Voyager flybys in 1980-1981. Radiative transfer calculations estimate the clouds to be significantly higher today than in 1980. The higher clouds make it difficult to observationally isolate any true slowdown from the vertical wind shear because Voyager and Cassini observations show that the winds become slower with altitude. Here, we test the hypothesis that the large equatorial storm in 1990 called the Great White Spot (GWS) decelerated the equatorial jet. We first use order of magnitude estimates to show: (1) if the GWS triggers vertical momentum redistribution, a minor speed change in the troposphere can lead to a substantial stratospheric wind speed change; (2) storm-triggered turbulent mixing slows a prograde equatorial jet; and (3) a prograde equatorial jet inhibits turbulent mixing in latitude. To test whether a GWS-like large storm decelerates the equatorial jet, we perform numerical experiments using the Explicit Planetary Isentropic Coordinate (EPIC) atmosphere model. Our simulation results are consistent with our order of magnitude predictions. We show that the storm excites waves, and the waves transport westward momentum from the troposphere to the stratosphere and decelerate the equatorial jet by as much as ∼40 m s⁻¹ at the 10-mbar level. However, our results show that the storm's effect is too weak at the cloud levels to halve the jet's speed from ∼470 m s⁻¹. Our results suggest that a combination of higher clouds and a true slowdown is necessary to explain the apparent equatorial jet slowdown. We also analyze the effect of waves on the apparent cloud motions, and show that waves can influence cloud-tracking wind speed measurements.     

HST observations of spokes in Saturn's B ring

As part of a long-term study of Saturn's rings, we have used the Hubble Space Telescope's (HST) Wide Field and Planetary Camera (WFPC2) to obtain several hundred high resolution images from 1996 to 2004, spanning the full range of ring tilt and solar phase angles accessible from the Earth. Using these multiwavelength observations and HST archival data, we have measured the photometric properties of spokes in the B ring, visible in a substantial number of images. We determined the spoke particle size distribution by fitting the wavelength-dependent extinction efficiency of a prominent, isolated spoke, using a Mie scattering model. Following Doyle and Grün (1990, Icarus 85, 168-190), we assumed that the spoke particles were sub-micron size spheres of pure water ice, with a Hansen-Hovenier size distribution (Hansen and Hovenier, 1974, J. Atmos. Sci. 31, 1137-1160). The WFPC2 wavelength coverage is broader than that of the Voyager data, resulting in tighter constraints on the nature of spoke particles. The effective particle size was reff=0.57±0.05 μm, and the size distribution was quite narrow with a variance of b=0.09±0.03, very similar to the results of Doyle and Grün (1990, Icarus 85, 168-190), and consistent with predictions of plasma cloud models for spoke production from meteoritic impacts (Goertz and Morfill, 1983, Icarus 53, 219-229; Goertz, 1984, Adv. Space Res. 4, 137-141). In all, we identified 36 spokes or spoke complexes, predominantly on the morning (east) ansa. The photometric contrast of the spokes is strongly dependent on effective ring opening angle, Beff. Spokes were clearly visible on the north face of the rings in 1994, just prior to the most recent ring plane crossing (RPX) epoch, and on the south face shortly after RPX. However, spokes were both less abundant and fainter as the rings opened up, and no spokes were detected after 18 October 1998 (Beff=−15.43°), when a single faint spoke was seen on the morning ansa. The high resolution and photometric quality of the WFPC2 images enabled us to set a detection limit of ?1% in fractional brightness contrast for spokes for the post-1998 observations. We compare the observed trend of spoke contrast with Beff to radiative transfer calculations based on three models of the distribution of spoke material. In the first, the spoke “haze” is uniformly mixed with macroscopic B ring particles. No variation in spoke contrast is predicted for single-scattering, in this case, and only a modest decrease in contrast with Beff is predicted when multiple scattering is taken into account. In the second model, the spoke dust occupies an extended layer that is thicker than the B ring, which gives virtually identical results to a third case, when the haze layer lies exclusively above the ring. Multiple-scattering Monte Carlo calculations for these two extended haze models match the trend of spoke contrast exceptionally well. We compute the predicted spoke contrast for a wide variety of viewing geometries, including forward- and backscattering. Based on these results, spokes should be easily detectable during the Cassini mission when the rings are viewed at relatively small (|B|?10°) ring opening angles.     

HST observations of azimuthal asymmetry in Saturn's rings

From 378 Hubble Space Telescope WFPC2 images obtained between 1996-2004, we have measured the detailed nature of azimuthal brightness variations in Saturn's rings. The extensive geometric coverage, high spatial resolution (), and photometric precision of the UBVRI images have enabled us to determine the dependence of the asymmetry amplitude and longitude of minimum brightness on orbital radius, ring elevation, wavelength, solar phase angle, and solar longitude. We explore a suite of dynamical models of self-gravity wakes for two particle size distributions: a single size and a power law distribution spanning a decade in particle radius. From these N-body simulations, we calculate the resultant wake-driven brightness asymmetry for any given illumination and viewing geometry. The models reproduce many of the observed properties of the asymmetry, including the shape and location of the brightness minimum and the trends with ring elevation and solar longitude. They also account for the “tilt effect” in the A and B rings: the change in mean ring brightness with effective ring opening angle, |Beff|. The predicted asymmetry depends sensitively on dynamical ring particle properties such as the coefficient of restitution and internal mass density, and relatively weakly on photometric parameters such as albedo and scattering phase function. The asymmetry is strongest in the A ring, reaching a maximum amplitude A∼25% near a=128,000 km. Here, the observations are well-matched by an internal particle density near 450 kg m⁻³ and a narrow particle size distribution. The B ring shows significant asymmetry (∼5%) in regions of relatively low optical depth (τ∼0.7). In the middle and outer B ring, where τ?1, the asymmetry is much weaker (∼1%), and in the C ring, A<0.5%. The asymmetry diminishes near opposition and at shorter wavelengths, where the albedo of the ring particles is lower and multiple-scattering effects are diminished. The asymmetry amplitude varies strongly with ring elevation angle, reaching a peak near |Beff|=10° in the A ring and at |Beff|=15-20° in the B ring. These trends provide an estimate of the thickness of the self-gravity wakes responsible for the asymmetry. Local radial variations in the amplitude of the asymmetry within both the A and B rings are probably caused by regional differences in the particle size distribution.     

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.     

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 ().     

Jupiter's polar clouds and waves from Cassini and HST images: 1993-2006

For a variety of reasons, Jupiter's polar areas are probably the less observed regions of the planet. To study the dynamics and cloud vertical structure in the polar regions of the planet (latitudes 50° to 80° in both hemispheres) we have used images of Jupiter obtained from the ultraviolet to near infrared (258 to 939 nm) by the Cassini Imagining Science Subsystem (ISS) in December 2000. The temporal coverage was complemented with archived images from the Hubble Space Telescope (1993-2006) in a similar spectral range. The zonal wind velocities have been measured at three Cassini ISS wavelengths (CB2, MT3 and UV1, corresponding to 750, 890 and 258 nm) sounding different altitude levels. The three eastward jets detected in CB2 images (lower cloud) go to zero velocity when measured in the UV1 filter (upper haze). A radiative transfer analysis has been performed to characterize the vertical structure of cloud and hazes distribution at the poles. We also present a characterization (phase speed, amplitude and zonal wavenumber) of the previously detected circumpolar waves at 67° N and S at 890 nm and at about 50° N and −57° S at 258 nm that are a permanent phenomenon in Jupiter with some variability in its structure during the analyzed period. From the ensemble of data analyzed we propose the waves are Rossby waves whose dynamic behavior constrains plausible values for their meridional and vertical wavenumbers. This work demonstrates the long-term nature of Jupiter's polar waves, providing a dynamical and vertical characterization which supports a detailed analysis of these phenomena in terms of a Rossby wave model.     

The cloud structure of the jovian atmosphere as seen by the Cassini/CIRS experiment

We analyze the thermal infrared spectra of Jupiter obtained by the Cassini-CIRS instrument during the 2000 flyby to infer temperature and cloud density in the jovian stratosphere and upper troposphere. We use an inversion technique to derive zonal mean vertical profiles of cloud absorption coefficient and optical thickness from a narrow spectral window centered at 1392 cm⁻¹ (7.18 μm). At this wavenumber atmospheric absorption due to ammonia gas is very weak and uncertainties in the ammonia abundance do not impact the cloud retrieval results. For cloud-free conditions the atmospheric transmission is limited by the absorption of molecular hydrogen and methane. The gaseous optical depth of the atmosphere is of order unity at about 1200 mbar. This allows us to probe the structure of the atmosphere through a layer where ammonia cloud formation is expected. The results are presented as height vs latitude cross-sections of the zonal mean cloud optical depth and cloud absorption coefficient. The cloud optical depth and the cloud base pressure exhibit a significant variability with latitude. In regions with thin cloud cover (cloud optical depth less than 2), the cloud absorption coefficient peaks at 1.1±0.05 bar, whereas in regions with thick clouds the peak cloud absorption coefficient occurs in the vicinity of 900±50 mbar. If the cloud optical depth is too large the location of the cloud peak cannot be identified. Based on theoretical expectations for the ammonia condensation pressure we conclude that the detected clouds are probably a system of two different cloud layers: a top ammonia ice layer at about 900 mbar covering only limited latitudes and a second, deeper layer at 1100 mbar, possibly made of ammonium hydrosulfide.