Ring Optical Depth Research Articles

Cassini microwave observations provide clues to the origin of Saturn's C ring

Despite considerable study, Saturn's rings continue to challenge current theories for their provenance. Water ice comprises the bulk of Saturn's rings, yet it is the small fraction of non-icy material that is arguably more valuable in revealing clues about the system's origin and age. Herein, we present new measurements of the non-icy material fraction in Saturn's C ring, determined from microwave radiometry observations acquired by the Cassini spacecraft. Our observations show an exceptionally high brightness at near-zero azimuthal angles, suggesting a high porosity of 70–75% for the C ring particles. Furthermore, our results show that most regions in the C ring contain about 1–2% silicates. These results are consistent with an initially nearly pure-ice ring system that has been continuously contaminated by in-falling micrometeoroids over ∼15–90 million years, using the currently accepted value of the micrometeoroid flux at infinity of ∼4.5×10−17g cm−2 s−1, and assuming that the C ring optical depth and surface density has not changed significantly during that time. This absolute time scale is inversely proportional not only to the flux at infinity, but also to the amount of gravitational focusing by Saturn the micrometeoroids experience before encountering the rings. We also find an enhanced abundance of non-icy material concentrated in the middle C ring. When assumed to be mixed volumetrically (“intramixed”) with water ice, this enhanced contamination reaches a maximum concentration of 6–11% silicates by volume around a ring radius of 83,000km, depending on the volume mixing model used. This is significantly higher than the inner and outer C ring. As opposed to an intramixing model, we also consider a silicate-core, icy-mantle model to address the fact that silicates may be present in chunks instead of fine powder in the ring particles. Such a model naturally helps to account for the observed opacity distribution. We propose several models to explain the radially varied non-icy material contamination. Our preferred model is that the C ring has been continuously polluted by meteoroid bombardment since it first formed, while the middle C ring was further contaminated by an incoming Centaur, a rocky object torn apart by tides and ultimately broken into pieces that currently reside in the middle C ring. If correct, the spatial extent of the enhanced non-icy material fraction suggests that the Centaur was likely to be captured and integrated into the rings perhaps as recently as ∼10–20 million years ago.

Morphology and variability of the Titan ringlet and Huygens ringlet edges

We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges. More than 100 stellar occultations of the Huygens ringlet and Titan ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two-parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness ( h > 1 km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model. Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ∼300 m in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ. Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern.

Saturn’s Rings at True Opposition

ABSTRACTOn 2005 January 14, the Saturn system was observed at true opposition with the planetary camera of the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. This was the culmination of nearly a decade of similar UBVRI observations, yielding a uniform set of over 400 high spatial resolution and photometrically accurate radial profiles of the rings that spans the full range of ring inclinations and solar phase angles accessible from the Earth. Using a subset of these images at similar effective ring opening angles Beff∼ -23°, we measured the normalized ring reflectivity I/F over broad regions of the A, B, and C rings as a function of solar phase angle and wavelength. There is a strong surge in brightness near opposition. To measure the width and amplitude of the opposition effect, we fitted two models to the observations: a simple four‐parameter linear‐exponential model and a more complex model (B. Hapke) that incorporates a wavelength‐dependent description of the coherent backscatter opposition effect, as well as the shadow‐hiding opposition effect by a particulate surface. From fits to the linear‐exponential model, the half‐width at half‐maximum for the rings is ∼0.1° at BVRI wavelengths, increasing to ∼0.16° and ∼0.19° for the A and B rings, respectively, in the U filter (338 nm). To assess the contribution of the shadowing of ring particles by each other to the opposition surge, we used Monte Carlo simulations of dynamical models of the rings for a variety of optical depths and particle size distributions, both with and without self‐gravity. Multiple scattering is very weak at low phase angles, and thus these simulations are nearly wavelength independent. The interparticle shadow‐hiding opposition surge increases in strength with ring optical depth and with broadened size distributions. Self‐gravity produces wakes that somewhat complicate the picture, because mutual shadowing by wakes is strongly dependent on illumination and viewing geometry. The observed opposition surge in the rings is much stronger and narrower than that caused by interparticle shadowing. We examined regional variations in the opposition surge across the A, B, and C rings using linear‐exponential model fits. Some of these are most easily explained on the basis of optical depth, volume filling factor, and the relative width of the particle size distribution. The opposition effect for the A, B, and C rings is substantially narrower than for two nearby icy satellites, Mimas and Enceladus, indicating that they have distinctly different surface properties.

Signatures of Enceladus in Saturn's E ring

Dust production from Enceladus has long been suggested as the main source of dust in Saturn's E ring. This was supported by the observation that the ring's optical depth peaks close to the orbit of this moon. However, both HST and Keck observations indicated that the peak of the optical depth distribution in fact lies outside the orbit of Enceladus, with a displacement of approximately 104 km. Though this outward shift has been suspected to be a result of electromagnetic forces, it could not be reproduced in earlier studies. Here we show that it is intimately related to the initial inclinations of the grains produced in the recently discovered plumes. For grains with radii rg > 0.5 μm, the small initial inclination greatly reduces their re‐collision probability, allowing for sufficiently long lifetimes for plasma drag to transport them outwards. Our numerical results can also be used in the simultaneous interpretation of the measurements by the Cassini CDA, RPWS instruments and imaging.

Self-gravity wakes and radial structure of Saturn's B ring

We analyze stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph and find large variations in the apparent normal optical depth of the B ring with viewing angle. The line-of-sight optical depth is roughly independent of the viewing angle out of the ring plane so that optical depth is independent of the path length of the line-of-sight. This suggests the ring is composed of virtually opaque clumps separated by nearly transparent gaps, with the relative abundance of clumps and gaps controlling the observed optical depth. The observations can be explained with a model of self-gravity wakes like those observed in the A ring. These trailing spiral density enhancements are due to the competing processes of self-gravitational accretion of ring particles and Kepler shear. The B ring wakes are flatter and more closely packed than their neighbors in the A ring, with height-to-width ratios <0.1 for most of the ring. The self-gravity wakes are seen in all regions of the B ring that are not opaque. The observed variation in total B ring optical depth is explained by the amount of relatively empty space between the self-gravity wakes. Wakes are more tightly packed in regions where the apparent normal optical depth is high, and the wakes are more widely spaced in lower optical depth regions. The normal optical depth of the gaps between the wakes is typically less than 0.5 and shows no correlation with position or overall optical depth in the ring. The wake height-to-width ratio varies with the overall optical depth, with flatter, more tightly packed wakes as the overall optical depth increases. The highly flattened profile of the wakes suggests that the self-gravity wakes in Saturn's B ring correspond to a monolayer of the largest particles in the ring. The wakes are canted to the orbital direction in the trailing sense, with a trend of decreasing cant angle with increasing orbital radius in the B ring. We present self-gravity wake properties across the B ring that can be used in radiative transfer modeling of the ring. A high radial resolution ( ∼ 10 m ) scan of one part of the B ring during a grazing occultation shows a dominant wavelength of 160 m due to structures that have zero cant angle. These structures are seen at the same radial wavelength on both ingress and egress, but the individual peaks and troughs in optical depth do not match between ingress and egress. The structures are therefore not continuous ringlets and may be a manifestation of viscous overstability.

Plasma electrons above Saturn's main rings: CAPS observations

We present observations of thermal (∼0.6–100eV) electrons observed near Saturn's main rings during Cassini's Saturn Orbit Insertion (SOI) on 1 July 2004. We find that the intensity of electrons is broadly anticorrelated with the ring optical depth at the magnetic footprint of the field line joining the spacecraft to the rings. We see enhancements corresponding to the Cassini division and Encke gap. We suggest that some of the electrons are generated by photoemission from ring particle surfaces on the illuminated side of the rings, the far side from the spacecraft. Structure in the energy spectrum over the Cassini division and A‐ring may be related to photoelectron emission followed by acceleration, or, more likely, due to photoelectron production in the ring atmosphere or ionosphere.

Apse Alignment of the Uranian Rings

An explanation of the dynamical mechanism for apse alignment of the eccentric uranian rings is necessary before observations can be used to determine properties such as ring masses, particle sizes, and elasticities. The leading model (P. Goldreich and S. Tremaine 1979, Astron J. 84, 1638–1641) relies on the ring self-gravity to accomplish this task, yet it yields equilibrium masses which are not in accord with Voyager radio measurements. We explore possible solutions such that the self-gravity and the collisional terms are both involved in the process of apse alignment. We consider limits that correspond to a hot and a cold ring, and we show that pressure terms may play a significant role in the equilibrium conditions for the narrow uranian rings. In the cold ring case, where the scale height of the ring near periapse is comparable to the ring particle size, we introduce a new pressure correction pertaining to a region of the ring where the particles are locked in their relative positions and jammed against their neighbors and the velocity dispersion is so low that the collisions are nearly elastic. In this case, we find a solution such that the ring self-gravity maintains apse alignment against both differential precession ( m=1 mode) and the fluid pressure. We apply this model to the uranian α ring and show that, compared to the previous self-gravity model, the mass estimate for this ring increases by an order of magnitude. In the case of a hot ring, where the scale height can reach a value as much as 50 times the particle size, we find velocity dispersion profiles that result in pressure forces which act in such a way as to alter the ring equilibrium conditions, again leading to a ring mass increase of an order of magnitude. We find that such a velocity dispersion profile would require a different mechanism than is currently envisioned for establishing a heating/cooling balance in a finite-sized, inelastic particle ring. Finally, we introduce an important correction to the model of E. I. Chiang and P. Goldreich (2000, Astrophys. J. 540, 1084–1090.). These authors relied on collisional forces in the last ∼100 m of an ∼10 km wide ring to increase ring equilibrium masses by up to a factor of ∼100. However, their treatment of ring edges as one-sided surface density drops leads to a strong dependence of the ring mass on the adjustable parameter λ (the length scale over which the ring's optical depth drops from order unity to zero at the edge). A treatment of the ring edges that takes into account their ridgelike structure retains the increase of ring mass of the order of ∼100 for a 10 km wide ring, while exhibiting weak dependence on λ. We conclude that a modified Chiang–Goldreich model can likely account for the masses of narrow, eccentric planetary rings; however, the role of shepherd satellites both in forming ring edges and in altering the streamline precession conditions near them needs to be explored further. It is also unclear whether a fully self-consistent ring model allows for the possibility of rings with negative eccentricity gradients.

An Instability in Planetary Rings Due to Ballistic Transport

Ballistic transport in planetary rings is the net radial transport of mass and angular momentum due to exchanges of meteoroid impact ejecta between neighboring ring regions. The detailed linear stability analysis in this paper demonstrates that ballistic transport causes wavelike disturbances to grow and propagate in an otherwise uniform ring. The growth is strongest for intermediate values of the normal ring optical depth τ = 0.1 to 1.0 and goes to zero as τ → 0 and τ → ∞. For nominal values of various physical parameters, the minimum e-folding time is ∼ 105 years for τ ∼ 0.4. The direction of propagation is opposite to the sense of any asymmetry that may exist in the ejecta direction distribution (inward for prograde ejecta and outward for retrograde ejecta). The additional effect of viscous transport tends to damp wavelike perturbations strongly at short wavelengths and at high values of τ. The quantitative agreement between this analytic work and numerical simulations reported elsewhere is generally quite good.As applied to Saturn's rings, the results in this paper strengthen the earlier conclusion from numerical calculations that the 100-km structure in the inner B Ring is caused by ballistic transport. However, it is also clear that ballistic transport cannot produce the complex structure seen in the outer two-thirds of the B Ring where τ ≤ 1.5. Wavelike structures in the C Ring might also be attributed to ballistic transport; but this requires further study.

Optical Depths Profiles and Streamlines of the Uranian (epsilon) Ring

view Abstract Citations (16) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Depths Profiles and Streamlines of the Uranian (epsilon) Ring Graps, Amara L. ; Showalter, Mark R. ; Lissauer, Jack J. ; Kary, David M. Abstract We present a new method for obtaining ring optical depth profiles and equivalent depth measurements from stellar occultation data possessing a large statistical noise. Our procedure is to model the ring by the smoothest possible profile that is consistent with the data and its noise properties. This method has the advantage of a vastly increased signal-to-noise ratio, but without as severe a loss in radial resolution as is caused by simple averaging. We show the resulting optical depth profiles and calculated equivalent depths for the Uranian ɛ Ring in several Voyager 2 Photopolarimeter (PPS) occultation cuts. By combining our profiles with the Voyager radio occultation data, we are able to calculate streamlines and to measure the local gradient in eccentricity within the ring. Our measurements indicate that the streamlines are well modeled by Keplerian ellipses in the inner portion of Ring ɛ but not near the outer edge. The eccentricity gradients that we compute appear to be incompatible with the self-gravity model for the maintenance of ring eccentricity against the differential precession induced by planetary oblateness. Publication: The Astronomical Journal Pub Date: May 1995 DOI: 10.1086/117451 Bibcode: 1995AJ....109.2262G Keywords: PLANETS AND SATELLITES: INDIVIDUAL: URANUS; OCCULTATIONS full text sources ADS |

Saturn's Central Flash from the 3 July 1989 Occultation of 28 Sgr

We present observations of Saturn's central flash obtained from Palomar and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shadow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in infrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 μm (McDonald). These are the first reported observations of a central flash due to Saturn, and the first for any planet in which the signal from each stellar image could be determined separately, permitting a comparison of both intensity and position for each image with model predictions. The starlight forming the central flash was strongly modulated by passage through Saturn's rings. Four separate flashes were observed from each observatory, corresponding to the points on the limb where the starlight passed through the Cassini Division and the relatively transparent C Ring, with maximum brightnesses reaching 1-2% of the unocculted stellar intensity. The more usual brightening seen during previous central flashes as the observer crossed the caustic formed by the planet's oblate limb was not detectable in this event due to strong attenuation by the B Ring. The timing of the flashes is quite sensitive to the shape of Saturn's limb, which depends in turn on the planet's zonal gravity harmonics and on the zonal wind profile in the lower stratosphere, near the 2.5-mbar pressure level. The locations of the images along the limb, as well as the timing, shapes, and amplitudes of the individual flash light curves, are well matched by a smoothed model based on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile obtained from the Voyager Photopolarimeter experiment. The smoothing required to give the best match to the data exceeds that attributable to the finite angular extent of the occulted star and may be due to refractive scattering by turbulence or wave structure in Saturn's atmosphere. The detailed agreement of the models and observations is improved by assuming a uniform zonal wind speed of 40 msec -1 at 25-70° north latitude in the stratosphere, suggesting that midlatitude zonal winds on Saturn decay with height above the troposphere to the nonzero mean of local tropospheric winds. There is no evidence of significant atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption.

Structure and particle properties of Saturn's E Ring

We have systematically reanalyzed a large subset of the available photometric data on Saturn's E Ring, including Voyager images and star tracker data, Earth-based CCD images, photometry, and spectrophotometry. In order to compare data sets with vastly different vi viewing geometries on this broad, vertically extended ring, we have calibrated every observation based on a common three-dimensional model. The ring shows a density peak at a location indistinguishable from the Enceladus orbit, with a radial offset of <3000 km. We have found that a very simple power-law model describes the ring's normal optical depth profile with orbital radius. The ring shows a general increase in vertical thickness with distance from the planet, ranging from 6000 km FWHM at its inner bound to nearly 40, 000 km at its outer. However, this trend is violated near the density peak, where a localized decrease in thickness by ∼30% is observed. The ring is also displaced northward from the planet's equatorial plane by 310 ± 25 km in one set of Voyager images, although the overall nature of the ring's vertical asymmetry is unclear. No power-law size distribution is compatible with the available photometry. A narrow distribution of slightly nonspherical particles of radius 1.0 ± 0.3 μm provides the best fit to the data. This highly peculiar size distribution clearly indicates that the ring does not originate from collisional or disruptive processes, and is therefore unlike any other known ring. Hence, we can give some credence to the possibility that the E Ring originates in “geyser-like” eruptions from the surface of Enceladus. Depending on the precise particle size chosen, the ring's peak normal optical depth is found to be 1.5(±0.4) × 10 −5, corresponding to a geometric cross-section per unit area of 5.3(±1.3) × 10 −5. Macroscopic bodies comprise less than 1% of the ring's optical depth.

A model of dust production in the Neptune ring system

Voyager 2 images of the Neptunian ring system show it to be relatively dusty with the fraction of micron‐sized dust particles in the rings between about 0.2 and 0.7 (Smith et al 1989). We apply a numerical model developed for the dust bands (Colwell and Esposito, Icarus (in press)) to the Neptune system. Our results show that 1) the suggested distribution of dust fractions in the rings is not related to ring optical depth and may be due to different ring particle size distributions. 2) Collision velocities between the macroscopic ring particles must be ∼ 1 m/sec if they are to produce im fractions above 1/2. 3) The small moons of Neptune have regoliths for meteoroid excavation to produce the dust population at Voyager ring plane crossing. 4) Dust optical depths interior to 1989N3R are negligible unless there are macroscopic source bodies in that region.

The structure of Saturn's rings: Implications from the Voyager stellar occultation

The Voyager 2 photopolarimeter observed the star δ Scorpii as it was occulted by Saturn's rings, giving ∼ 100 m resolution in the radial direction across the entire ring system. Radial structure can be seen down to the resolution limit and considerable new structure is seen. In the outer B ring none of this structure is due to imbedded large particles (moonlets) in the ring system. An automatic search with finite Fourier transforms located 13 density waves exicted by resonances with Saturn's satellites. More waves were found in searches by eye of predicted resonance locations. Although strong density waves can be located quite easily, no waves have been identified having predicted torques per surface mass density less than 4 × 10 16 cm 4/sec 2. This puts a limit of less than 100 waves likely to be found in Saturn's rings, mostly in the A ring. Unresolved or overlapping waves do not play a major role in creating the most obvious radial structure. The total mass of Saturn's rings is estimated as 5 × 10 −8 of the mass of Saturn. Measurements of wave damping imply a ring thickness of ∼ 30 m in the outer A ring. The power spectrum for the rings shows no dominant individual wavelengths: A preferred size for radial structure in the rings is not seen. At wavelengths less than ∼ 15 km the ring power spectrum drops below the power of noise in the data. The majority of the variance in the ring system is characterized by distance scales greater than 20 km. It is concluded that the majority of ring structures and the majority of variance in ring optical depth are not explained by currently proposed physical mechanisms.

Saturn's rings: Infrared brightness variation with solar elevation

The variation in infrared equilibrium brightness temperature of Saturn's A, B, and C rings is modeled as a function of solar elevation B′ with respect to the ring plane. The basic model includes estimates of minimum and maximum interparticle shadowing in a monolayer approximation. Simple laboratory observations of random particle distributions at various illumination angles provide more realistic shadowing functions. Radiation balance calculations yield the physical (kinetic) temperature of particles in equilibrium with radiation from the Sun, Saturn, and neighboring particles. Infrared brightness temperatures as a function of B′ are then computed and compared to the available 20-μm data (Pioneer results are also briefly discussed). The A and B rings are well modeled by an optically thick monolayer, or equivalently, a flat sheet, radiating from one side only. This points to a temperature contrast between the two sides, possibly due to particles with low thermal inertia. Other existing models for the B ring are discussed. The good fit for the monolayer model does not rule out the possibility that the A and B rings are many particles thick. It could well be that a multilayer ring produces an infrared behavior (as a function of tilt angle) similar to that of a monolayer. The C ring brightness increases as B′ decreases. This contrast in behavior can be understood simply in terms of the low C ring optical depth and small amount of interparticle shadowing. High-albedo particles ( A≳0.5) can fit the C ring infrared data if they radiate mostly from one hemisphere due to slow rotation or low thermal inertia (or both). Alternatively, particles isothermal over their surface (owing to a rapid spin, high inertia, or small size), and significantly darker ( A≲0.3) than the A and B ring particles, can produce a similar brightness variation with ring inclination. In any case, the C ring particles have significantly hotter physical temperatures than the particles in the A and B rings, whether or not the rings form a monolayer.

Saturn's rings: Particle composition and size distribution as constrained by observations at microwave wavelengths: II. Radio interferometric observations

The sizes, composition, and number of particles comprising the rings of Saturn may be meaningfully constrained by a combination of radar- and radio-astronomical observations. In a previous paper, we have discussed constraints obtained from radar observations. In this paper, we discuss the constraints imposed by complementary “passive” radio observations at similar wavelengths. First, we present theoretical models of the brightness of Saturn's rings at microwave wavelengths (0.34–21.0 cm), including both intrinsic ring emission and diffuse scattering by the rings of the planetary emission. The models are accurate simulations of the behavior of realistic ring particles and are parameterized only by particle composition and size distribution, and ring optical depth. Second, we have reanalyzed several previously existing sets of interferometric observations of the Saturn system at 0.83-, 3.71-, 6.0-, 11.1-, and 21.0-cm wavelengths. These observations all have spatial resolution sufficient to resolve the rings and planetary disk, and most have resolution sufficient to resolve the ring-occulted region of the disk as well. Using our ring models and a realistic model of the planetary brightness distribution, we are able to establish improved constraints on the properties of the rings. In particular, we find that: (a) the maximum optical depth in the rings is ∼ 1.5 ± 0.3 referred to visible wavelengths; (b) a significant decrease in ring optical depth from λ3.7 to λ21.0 cm allows us to rule out the possibility that more than ∼30% of the cross section of the rings is composed of particles larger than a meter or so; this assertion is essentially independent of uncertainties in particle adsorption coefficient; and (c) the ring particles cannot be primarily of silicate composition, independently of particle size, and the particles cannot be primarily smaller than ∼0.1 cm, independently of composition.

Temperatures and optical depths of Saturn's rings and a brightness temperature for Titan

The Pioneer Saturn infrared radiometer viewed Saturn's rings at 20‐ and 45‐µm wavelength under several conditions of illumination. The data are analyzed to infer radial locations of major ring boundaries, temperatures and temperature gradients, and normal optical depths. Error bounds on the above inferred quantities are given. Most ring boundaries are defined to ±0.01 Rs(1 Rs≡6 × 104 km) and are in good agreement with those inferred from the imaging photopolarimeter experiment. Temperatures generally decrease with radial distance from the planet. A significant temperature gradient exists from the colder north (unilluminated) side of the rings to the warmer south side. The gradient appears to be steepest on the south side. Ring optical depths are greater than some previously published values and are approximately 0.1 for the Cassini division and the C ring. In addition, the C ring optical depth decreases towards the planet. The temperature drop during eclipse is ≳10 K, implying low thermal inertia for the ring particles. Titan's 45‐µm brightness temperature is 75±5 K, in good agreement with earth‐based observations.

Interferometry of Saturn and its rings at 1.30-cm wavelength

We present interferometric observations of Saturn and its ring system made at the Hat Creek Radio Astronomy Observatory at a wavelength of 1.30 cm. The data have been analyzed by both model-fitting and aperture synthesis techniques to determine the brightness temperature and optical thickness of the ring system and estimate the amount of planetary limb darkening. We find that the ring optical depth is close to that observed at visible wavelenghts, while the ring brightness temperature is only 7 ± 1°K. These observational constraints require the ring particles to be nearly conservative scatterers at this wavelength. A conservative lower limit to the single-scattering albedo of the particles at 1.30-cm wavelength is 0.95, and if their composition is assumed to be water ice, then this lower limit implies an upper limit of 2.4 m for the radius of a typical ring particle. The aperture synthesis maps show evidence for a small offset in the position of Saturn from that given in the American Ephemeris and Nautical Almanac. The direction and magnitude of this offset are consistent with that found from a similar analysis of 3.71-cm interferometric data which we have previously presented (F.P. Schloerb, D.O. Muhleman, and G.L. Berge, 1979b, Icarus 39, 232–250). Limb darkening of the planetary disk has been estimated by solving for the best-fitting disk radius in the models. The best-fitting radius is 0.998 ± 0.004 times the nominal Saturn radius and indicates that the planet is not appreciably limb dark at 1.30 cm. Since our previous 3.71-cm data also indicated that the planet was not strongly limb dark (F.P. Schloerb, D. O. Muhleman, and G.L. Berge, 1979a, Icarus 39, 214–230), we feel that the limb darkening is not strongly wavelength dependent between 1.30 and 3.71 cm. The difference between the best-fitting disk radii at 3.71 and 1.30 cm is +0.007 ± 0.007 times the nominal Saturn radius and suggests that the planet is more limb dark at 1.30 cm than at 3.71 cm. Models of the atmosphere which have NH 3 as the principal source of microwave opacity predict that the planet will be less limb dark at 1.30 cm. However, the magnitude of the effect predicted by the NH 3 models is −0.009 and only marginally different from the observed value.