Ring Plane Crossings Research Articles

Cassini’s CDA observes a variety of dust populations just outside Saturn’s main rings

ABSTRACT Before the end of its mission, the Cassini spacecraft orbited Saturn in a series of highly inclined elliptical ‘Ring-Grazing’ orbits (RGO). During the RGO, the spacecraft passed repeatedly through the ring plane outside the F ring, near the orbits of Janus and Epimetheus, at an average relative speed of ∼20 km s–1. For the first time, Cassini’s Cosmic Dust Analyser (CDA) directly sampled dust particles from this region. Here, we analyse the compositions of dust grains sampled within ±15 min relative to nine ring plane crossings of the RGO. The compositions of most analysed RGO grains are similar to those of E ring ice grains, implying that the E ring extends to within at least 2.45 Saturn radii (RS) of Saturn. The compositional distribution of these grains point at a similar average period (decades) since ejection from Enceladus as of particles in the outer E ring (beyond 8 RS). Higher fractions of larger grains are found near the orbits of Janus and Epimetheus, which probably represent ejecta from these moons. Most of these grains have compositions similar to the background E ring grains, indicating that E ring material is coating the surfaces of Janus and Epimetheus. We also report the detection of several types of mineral grains on prograde orbits, one of which, a water ice/silicate mixture, has never been observed by CDA elsewhere. These mineral grains appear to have a different origin from the E ring, and may arise from nearby moons, the F ring, or main rings.

Risk Assessment of Hypervelocity Impact of Saturn Dust on Cassini Sun Sensors

Cassini is one of the most sophisticated interplanetary spacecraft that humans have ever built and launched. Since achieving orbit at Saturn in 2004, Cassini has collected science data throughout its four-year prime mission (2004–2008), and it has since been approved for first and second extended missions through September 2017. In April 2017, the Cassini spacecraft will begin a daring set of ballistic orbits that will hop the rings and dive between the upper atmosphere of Saturn and its innermost D ring. The “dusty” environment of the inner D-ring region the spacecraft must fly through is hazardous because of the possible damage that dust particles, travelling at speeds as high as (relative to the spacecraft), can do to spacecraft hardware. During proximal ring-plane crossings, the mission operation team plans to point the high-gain antenna to the spacecraft velocity relative to dust particles in order to protect most of spacecraft instruments from the incoming dust particles. However, this attitude exposes two sun sensors (that are mounted on the antenna) to the incoming particles. High-velocity impacts on the sun sensor coverglass might penetrate it. Moreover, craters created by these impacts on the coverglass will degrade the transmissibility of light through it. Analyses performed to quantify the risks that the sun sensors must contend with during these hazardous ring-plane crossings are given in this paper.

The vertical structure of the F ring of Saturn from ring-plane crossings

We present a photometric model of the rings of Saturn which includes the main rings and an F ring, inclined to the main rings, with a Gaussian vertical profile of optical depth. This model reproduces the asymmetry in brightness between the east and west ansae of the rings of Saturn that was observed by the Hubble Space Telescope (HST) within a few hours after the Earth ring-plane crossing (RPX) of 10 August 1995. The model shows that during this observation the inclined F ring unevenly blocked the east and west ansae of the main rings. The brightness asymmetry produced by the model is highly sensitive to the vertical thickness and radial optical depth of the F ring. The F-ring model that best matches the observations has a vertical full width at half maximum of 13±7km and an equivalent depth of 10±4km. The model also reproduces the shape of the HST profiles of ring brightness vs. distance from Saturn, both before and after the time of ring-plane crossing. Smaller asymmetries observed before the RPX, when the Earth was on the dark side of the rings, cannot be explained by blocking of the main rings by the F ring or vice versa and are probably instead due to the intrinsic longitudinal variation exhibited by the F ring.

Keck and VLT AO observations and models of the uranian rings during the 2007 ring plane crossings

We present observations of the uranian ring system at a wavelength of 2.2μm, taken between 2003 and 2008 with NIRC2 on the W.M. Keck telescope in Hawaii, and on 15–17 August 2007 with NaCo on the Very Large Telescope (VLT) in Chile. Of particular interest are the data taken around the time of the uranian ring plane crossing with Earth on 16 August 2007, and with the Sun (equinox) on 7 December 2007. We model the data at the different viewing aspects with a Monte Carlo model to determine: (1) the normal optical depth τ0, the location, and the radial extent of the main rings, and (2) the parameter Aτ0 (A is the particle geometric albedo), the location, and the radial plus vertical extent of the dusty rings. Our main conclusions are: (i) The brightness of the ∊ ring is significantly enhanced at small phase and ring inclination angles; we suggest this extreme opposition effect to probably be dominated by a reduction in interparticle shadowing. (ii) A broad sheet of dust particles extends inwards from the λ ring almost to the planet itself. This dust sheet has a vertical extent of ∼140km, and Aτ0=2.2×10−6. (iii) The dusty rings between ring 4 and the α ring and between the α and β rings are vertically extended with a thickness of ∼300km. (iv) The ζ ring extends from ∼41,350km almost all the way inwards to the planet. The main ζ ring, centered at ∼39,500km from the planet, is characterized by Aτ0=3.7×10−6; this parameter decreases closer to the planet. The ζ ring has a full vertical extent of order 800–900km, with a pronounced density enhancement in the mid-plane. (v) The ηc ring is optically thin and less than several tens of km in the vertical direction. This ring may be composed of macroscopic material, surrounded by clumps of dust.

Plasma conditions and the structure of the Jovian ring

We explore the dynamics of small charged dust particles in Jupiter's innermost magnetosphere and show that the systematic charge variation of the grains results in surprisingly short lifetimes. Assuming a constant production of small dust particles via continual micrometeoroid bombardment of the larger parent bodies of the main ring, this model reproduces remote sensing observations of the ring/halo region at Jupiter made by Voyager, Galileo, Cassini, and New Horizons spacecraft and observations from the ground by the Keck telescope during ring plane crossings. We use this model to make predictions for the dust impact rates for the JUNO mission, which is expected to traverse this region multiple times starting in 2016.

The E ring in the vicinity of Enceladus: I. Spatial distribution and properties of the ring particles

Saturn's diffuse E ring is the largest ring of the Solar System and extends from about 3.1 R S (Saturn radius R S = 60 , 330 km ) to at least 8 R S encompassing the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea. After Cassini's insertion into her saturnian orbit in July 2004, the spacecraft performed a number of equatorial as well as steep traversals through the E ring inside the orbit of the icy moon Dione. Here, we report about dust impact data we obtained during 2 shallow and 6 steep crossings of the orbit of the dominant ring source—the ice moon Enceladus. Based on impact data of grains exceeding 0.9 μm we conclude that Enceladus feeds a torus populated by grains of at least this size along its orbit. The vertical ring structure at 3.95 R S agrees well with a Gaussian with a full-width–half-maximum (FWHM) of ∼ 4200 km . We show that the FWHM at 3.95 R S is due to three-body interactions of dust grains ejected by Enceladus' recently discovered ice volcanoes with the moon during their first orbit. We find that particles with initial speeds between 225 and 235 m s −1 relative to the moon's surface dominate the vertical distribution of dust. Particles with initial velocities exceeding the moon's escape speed of 207 m s −1 but slower than 225 m s −1 re-collide with Enceladus and do not contribute to the ring particle population. We find the peak number density to range between 16 × 10 −2 m −3 and 21 × 10 −2 m −3 for grains larger 0.9 μm, and 2.1 × 10 −2 m −3 and 7.6 × 10 −2 m −3 for grains larger than 1.6 μm. Our data imply that the densest point is displaced outwards by at least 0.05 R S with respect of the Enceladus orbit. This finding provides direct evidence for plume particles dragged outwards by the ambient plasma. The differential size distribution n ( s d ) d s d ∼ s d − q s d s d for grains > 0.9 μm is described best by a power law with slopes between 4 and 5. We also obtained dust data during ring plane crossings in the vicinity of the orbits of Mimas and Tethys. The vertical distribution of grains > 0.8 μm at Mimas orbit is also well described by Gaussian with a FWHM of ∼ 5400 km and displaced southwards by ∼ 1200 km with respect to the geometrical equator. The vertical distribution of ring particles in the vicinity of Tethys, however, does not match a Gaussian. We use the FWHM values obtained from the vertical crossings to establish a 2-dimensional model for the ring particle distribution which matches our observations during vertical and equatorial traversals through the E ring.

Characteristics of dust particles detected near Saturn's ring plane with the Cassini Radio and Plasma Wave instrument

During the inbound and outbound passes of the Cassini spacecraft through Saturn's ring plane on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many small particles striking the spacecraft. When a small particle strikes the spacecraft at a high velocity, it is instantly vaporized and produces a small cloud of plasma that expands radially outward from the impact site. As the plasma cloud expands away from the spacecraft it produces a voltage pulse on the RPWS electric field antennas, the amplitude of which is proportional to the mass of the impacting particle. Two types of measurements are made: waveform measurements from the x-axis dipole antenna, and spectrum measurements from the w-axis monopole antenna. The waveform measurements from the dipole antenna provide a determination of the impact rate and the relative mass distribution, and the spectrum measurements from the monopole antenna provide a determination of the root-mean-square particle mass. The impact rate at both ring plane crossings provides a good fit to the sum of two Gaussians, with an average impact rate of about 1200 per second (the exact value depends on the voltage threshold used), and a north–south thickness of about 300 km. The mass distribution depends on the distance from the ring plane, varying from about m −2 near the ring plane at z = 0 ± 100 km , where z is the north–south distance from the ring plane, to as steep as m −4 well away from the ring plane at z = 500 ± 100 km . The mechanisms involved in the impact detection are discussed and a formula relating the root-mean-square particle mass to the root-mean-square voltage on the w-axis monopole is derived. Using this formula, the root-mean-square mass is estimated to be 7.7×10 −11 g, which for water ice particles with a density of 0.92 g cm −3 gives a root-mean-square radius of about 2.6 μm.

In situ dust measurements in the inner Saturnian system

In July 2004 the Cassini–Huygens mission reached the Saturnian system and started its orbital tour. A total of 75 orbits will be carried out during the primary mission until August 2008. In these four years Cassini crosses the ring plane 150 times and spends approx. 400 h within Titan's orbit. The Cosmic Dust Analyser (CDA) onboard Cassini characterises the dust environment with its extended E ring and embedded moons. Here, we focus on the CDA results of the first year and we present the Dust Analyser (DA) data within Titan's orbit. This paper does investigate High Rate Detector data and dust composition measurements. The authors focus on the analysis of impact rates, which were strongly variable primarily due to changes of the spacecraft pointing. An overview is given about the ring plane crossings and the DA counter measurements. The DA dust impact rates are compared with the DA boresight configuration around all ring plane crossings between June 2004 and July 2005. Dust impacts were registered at altitudes as high as 100 000 km above the ring plane at distances from Saturn between 4 and 10 Saturn radii. In those regions the dust density of particles bigger than 0.5 μ m can reach values of 0.001 m - 3 .

The size distribution of Jupiter's main ring from Galileo imaging and spectroscopy

Galileo's Solid State Imaging experiment (SSI) obtained 36 visible wavelength images of Jupiter's ring system during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188–213) and another 21 during the extended mission. The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter's main ring during orbit C3 at wavelengths from 0.7 to 5.2 μm; a second observation was attempted during orbit E4. We analyze the high phase angle NIMS and SSI observations to constrain the size distribution of the main ring's micron-sized dust population. This portion of the population is best constrained at high phase angles, as the light scattering behavior of small dust grains dominates at these geometries and contributions from larger ring particles are negligible. High phase angle images of the main ring obtained by the Voyager spacecraft covered phase angles between 173.8° and 176.9° (Showalter et al., 1987, Icarus 69, 458–498). Galileo images extend this range up to 178.6°. We model the Galileo phase curve and the ring spectra from the C3 NIMS ring observation as the combination of two power law distributions. Our analysis of the main ring phase curve and the NIMS spectra suggests the size distribution of the smallest ring particles is a power law with an index of 2.0±0.3 below a size of ∼15 μm that transitions to a power law with an index of 5.0±1.5 at larger sizes. This combined power law distribution, or “broken power law” distribution, yields a better fit to the NIMS data than do the power law distributions that have previously been fit to the Voyager imaging data (Showalter et al., 1987, Icarus 69, 458–498). The broken power law distribution reconciles the results of Showalter et al. (1987, Icarus 69, 458–498) and McMuldroch et al. (2000, Icarus 146, 1–11), who also analyzed the NIMS data, and can be considered as an obvious extension of a simple power law. This more complex size distribution could indicate that ring particle production rates and/or lifetimes vary with size and may relate to the physical processes that control their evolution. The significant near arm/far arm asymmetry reported elsewhere (see Showalter et al., 1987, Icarus 69, 458–498; Ockert-Bell et al., 1999, Icarus 138, 188–213) persists in the data even after the main ring is isolated in the SSI images. However, the sense of the asymmetry seen in Galileo images differs from that seen in Voyager images. We interpret this asymmetry as a broad-scale, azimuthal brightness variation. No consistent association with the magnetic field of Jupiter has been observed. It is possible that these longitudinal variations may be similar to the random brightness fluctuations observed in Saturn's F ring by Voyager (Smith et al., 1982, Science 215, 504–537) and during the 1995 ring plane crossings (Nicholson et al., 1996, Science 272, 509–515; Bosh and Rivkin, 1996, Science 272, 518–521; Poulet et al., 2000, Icarus 144, 135–148). Stochastic events may thus play a significant role in the evolution of the jovian main ring.

Saturn’s wayward shepherds: the peregrinations of Prometheus and Pandora

Saturn’s narrow F ring is flanked by two nearby small satellites, Prometheus and Pandora, discovered in Voyager images taken in 1980 and 1981 (Synnott et al., 1983, Icarus 53, 156–158). Observations with the Hubble Space Telescope (HST) during the ring plane crossings (RPX) of 1995 led to the unexpected finding that Prometheus was ∼19° behind its predicted orbital longitude, based on the Synnott et al. (1983) Voyager ephemeris (Bosh and Rivkin, 1996 Science 272, 518–521; Nicholson et al., 1996, Science 272, 509–515). Whereas Pandora was at its predicted location in August 1995, McGhee (2000, Ph.D. thesis, Cornell University) found from the May and November 1995 RPX data that Pandora also deviates from the Synnott et al. (1983) Voyager ephemeris. Using archival HST data from 1994, previously unexamined RPX images, and a large series of targeted WFPC2 observations between 1996 and 2002, we have determined highly accurate sky-plane positions for Prometheus, Pandora, and nine other satellites found in our images. We compare the Prometheus and Pandora measurements to the predictions of substantially revised and improved ephemerides for the two satellites based on an extensive analysis of a large set of Voyager images (Murray et al., 2000, Bull. Am. Astron. Soc. 32, 1090; Evans, 2001 Ph.D. thesis, Queen Mary College). From December 1994 to December 2000, Prometheus’ orbital longitude lag was changing by −0.71° year −1 relative to the new Voyager ephemeris. In contrast, Pandora is ahead of the revised Voyager prediction. From 1994 to 2000, its longitude offset changed by +0.44° year −1, showing in addition an ∼585 day oscillatory component with amplitude Δλ CR 0 = 0.65 ± 0.07° whose phase matches the expected perturbation due to the nearby 3:2 corotation resonance with Mimas, modulated by the 71-year libration in the longitude of Mimas due to its 4:2 resonance with Tethys. We determine orbital elements for freely precessing equatorial orbits from fits to the 1994–2000 HST observations, from which we conclude that Prometheus’ semimajor axis was 0.31 km larger, and Pandora’s was 0.20 km smaller, than during the Voyager epoch. Subsequent observations in 2001–2002 reveal a new twist in the meanderings of these satellites: Prometheus’ mean motion changed suddenly by an additional −0.77° year −1, equivalent to a further increase in semimajor axis of 0.33 km, at the same time that Pandora’s mean motion changed by +0.92° year −1, corresponding to a change of −0.42 km in its semimajor axis. There is an apparent anticorrelation of the motions of these two moons seen in the 2001–2002 observations, as well as over the 20-year interval since the Voyager epoch. This suggests a common origin for their wanderings, perhaps through direct exchange of energy between the satellites as the result of resonances, possibly involving the F ring.

HST Observations of Saturnian Satellites during the 1995 Ring Plane Crossings

In May, August, and November 1995, Hubble Space Telescope (HST) observations during Saturn's ring-plane crossings allowed us to view saturnian satellites normally hidden to Earth-based observers in the glare of the rings. New measurements of Janus, Epimetheus, Prometheus, and Pandora have been combined to form revised orbital solutions using all three HST data sets. These measurements and orbit fits are presented, as well as similar fits for the brighter satellites Mimas, Tethys, Enceladus, Dione, and Rhea. Observations of the Lagrangian satellites Telesto, Calypso, and Helene are also reported. While most satellites were found to be close to their expected positions based on previous orbital solutions (Nicholson et al. 1992, Icarus 100, 464–484; Jacobson 1996, Bull. Amer. Astron. Soc. 28, 1185; and Harper and Taylor 1993, Astron. Astrophys. 268, 326–349). Prometheus lagged behind its predicted longitude by 18.85°±0.04°. A systematic drift in Pandora's longitude of −1.85° relative to the Voyager ephemeris was observed between May and November. The new data on the coorbital satellites Janus and Epimetheus have resulted in a revised mass for Janus, ∼6% smaller than the previous value (Jacobson 1995, Bull. Amer. Astron. Soc. 27, 1202). Subtraction of light from the edge-on rings has led to additional detections of objects S/1995-S1 and S3 (Bosh and Rivkin 1995, Science 272, 518–521) in the May data, and S/1995-S5, S6, S7 (Nicholson et al. 1995, Bull. Amer. Astron. Soc. 27, 1202) and S/1995-S9 (Roddier et al. 1996) in the August images. S1 is identified with Atlas but leads its predicted position by ∼25°. S3 has an orbit consistent with that of the narrow F ring, but S5, S6, S7, and S9 now appear to orbit ∼530–950 km interior to this ring. S7 and S9 may even be coorbital with Prometheus. An object in the May images, possibly corresponding to S7, is also found to lie very close to Prometheus' orbit (∼800 km interior to the F ring), but no convincing detections of an analogous object in this orbit at this longitude were made in the November data set. In addition to these previously reported objects, eight new objects were identified during multiple HST visits in May and August; all have orbits consistent with that of the F ring and are likely to have been short-lived clumps within this ring, similar to those seen in Voyager images (Showalter 1997, Bull. Amer. Astron. Soc. 29, 999).

Saturn's Ring-Plane Crossings of August and November 1995: A Model for the New F-Ring Objects

We analyze observations made in August and November 1995 during the Earth and Sun crossings of Saturn's ring plane, respectively. The August 1995 observations combine data taken with the Adonis adaptive optics system at the European Southern Observatory (ESO) and images from the Hubble Space Telescope (HST). The November 1995 data are based on HST images only. We report here the detections of four new objects (three in August, one in November) orbiting near, or within, the F ring of Saturn. Two of the objects observed at ESO in August 1995 are most probably S/1995 S5 and S/1995 S6, reported by P. D. Nicholson et al. (1996, Science 272, 509–515) from the HST observations on August 10, 1995. The third object, S/1995 S20 cannot be clearly linked with any other objects reported by other observers. An elongated object, or arc, is tracked in November 1995, and can be connected to one of the arcs also reported by Nicholson et al. Our combined measurements improve the determination of the orbital parameters of S/1995 S5 and the arc, indicating that these objects orbit, within the error bars (≲±140 km), in the F ring. We discuss the nature and origin of these F-ring features. We propose that they are clouds of regolith ejecta resulting from collisions between large particles, or “parent bodies,” within the F ring. From the available constraints (brightness and lifetime of the objects), we show that the observations are consistent with the presence of several hundred 1-km-sized (and/or several thousand 100-m-sized) unseen parent bodies embedded in the F ring, each of which is covered by a regolith layer tens of centimeters to ∼1 m in thickness.

Mass spectrometer calibration of high velocity impact ionization based cosmic dust analyzer

We are calibrating the time of flight mass spectrometer of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft. The CDA measures the flux of particles in the 10 −15 to 10 −9 g range at intersection velocities of up to 100 km/s. Of special interest are the chemical composition of the particles in orbit about Saturn and/or its satellites that are expected to be captured by CDA during ring plane crossings and upon close encounter with the satellites. Upon impacting a rhodium plate, particles are expected to partially ionize and their chemical composition is expected to be determined from mass analysis of the positive ions. In order to optimize impact ionization calibration experiments using a light gas-gun launched microspheric particles, we have done initial testing with a short duration pulsed laser (4 ns duration nitrogen laser (337 nm)). The beam is focused to deliver the 300μJ energy per laser pulse onto a 33 μm 2. The laser power density (≈10 10 W/cm 2) simulates the impact of particles with various combinations of density and velocities, e.g., 8 g/cm 3 (Fe) projectile at 23 km/s or 1 g/cm 3 projectile at 65 km/s. The CDA spectrometer will operate in the near vacuum of Saturnian zone environment is housed in a laboratory chamber at 10 −6 mbar. The ions and electrons are separated by 680 V between target and grid. The laser ionization produces charge of 4.6pC (mostly Al +1) in aluminum and 2.8pC (Fe +1) in stainless steel. Estimating that each Al +1 and Fe +1 ion requires an energy of 5.98 and 7.90 eV/ion implies that ∼10 −5 % of the laser pulse energy produces ions and the present system has a 10% detection efficiency. Using multi-channel plate detector to detect ions from aluminum alloy and kamacite yields well defined peaks at 24(Mg +1), 27(Al +1) and 64 (Cu +1), and, 56(Fe +1), 58(Ni +1) and 60(Ni +1) amu, respectively. Also contaminant ions at 23 (Na +1) and 39(K +1) amu are detected.

Spectrophotometry of Saturn's small satellites and rings from Hubble Space Telescope images

We investigate the photometric properties of four of Saturn's inner satellites (Prometheus, Pandora, Janus, and Epimetheus) and five ring regions (the A, B, and C rings; the Cassini division; and the F ring). We have used Hubble Space Telescope images taken in July 1991 and during the Saturn's ring plane crossings by the Earth (August 1995) and the Sun (November 1995). The spectral range of this study is ∼0.30–0.95 μm. We confirm qualitatively but not quantitatively several interesting color properties of the main rings already derived by Estrada and Cuzzi [1996]. The derived albedos of Prometheus and Pandora are consistent with the lower limits based on Voyager images [Thomas et al., 1983]. The albedos of the coorbital satellites Janus and Epimetheus are smaller than previously determined. The colors of Janus and Pandora are similar to those of the main rings, indicating the presence of ices altered by red contaminants. Epimetheus is more neutral, with spectral shape similar to that of the G ring. Our data for Prometheus are fragmentary in terms of wavelength coverage; otherwise, the satellite looks like the F ring, which is neutral or slightly blue. This unusual color could be explained if there is an inward migration of the F ring particles through Poynting‐Robertson drag, which subsequently sandblast the surface of the satellite. We find a correlation of color with the distance from Saturn. This indicates that the inner satellites are spectrally closer to the main rings than to the major Saturnian satellites. The implications for the origin of the inner satellites are discussed.

G Ring Particle Sizes Derived from Ring Plane Crossing Observations

The Saturn ring plane crossings in 1995–1996 allowed observers using the Hubble Space Telescope and the W. M. Keck telescope to image the planet's diffuse rings from 0.3 to 2.2 μm at a scattering angle θ ∼ 175°. We calculate the G ring reflectance for size distributions of dust to kilometer-sized bodies derived from a physical, evolutionary model. The model tracks the evolution of the G ring from its initial formation following the disruption of a progenitor satellite (R. M. Canup and L. W. Esposito, 1997,Icarus126,28–41) until a steady state distribution is reached. We calculate the total particle scattering from contributions due to Mie scattering, isotropic scattering, and Lambert scattering and compare the spectra, phase curves, and RMS particle mass from our physical model to that observed by HST, Keck, and Voyager. A range of particle size distributions from the models are consistent with the observations. These distributions have a dust component that can be described by the differential power law exponentqdust, in the range 1.5–3.5. A quasi-Gaussian size distribution centered at 15 μm also matches the observations, although is not predicted by the evolutionary model. Distributions withqdust≳ 4, such as that proposed by M. R. Showalter and J. N. Cuzzi (1993),Icarus103,124–143) based on Voyager G ring photometry, are too blue to match the spectrum. In order to fit the visible optical depth, many of the models require longer particle lifetimes against plasma drag than Voyager plasma measurements imply. This may suggest that plasma densities are overestimated, that the ring has unaccounted-for dust sources, or that the ring is not in steady-state and we are seeing it at a particularly bright moment.

Erratum: Observations of Saturn's Ring-Plane Crossings in August and November 1995

Figure 8 (p. 514) of the Research Article "Observations of Saturn's ring-plane crossings in August and November 1995" by P. D. Nicholson et al. (26 Apr., p. 509) was printed upside down.

Keck Infrared Observations of Saturn's E and G Rings during Earth's 1995 Ring Plane Crossings

We report the first ground-based detection of Saturn's G ring, together with the first infrared images of the E ring. The data were taken during the May and August 1995 ring plane crossings using the W. M. Keck telescope at a wavelength of 2.26 μm. The E ring's radial and vertical structure is found to be generally consistent with previous results from Voyager and Earth-based observatories; it has a brightness peak near the orbit of Enceladus and extends outward to ∼6RS(Saturn radii). The E ring is ∼40% fainter at 2.26 μm than in the visual, confirming and extending previous reports that it is very blue in color. The G ring is located at 2.8RSand is unresolved in vertical thickness. Its reflected intensity is similar to that found in the comparable Voyager image, implying that the ring is neutral in color from the visual to the infrared.

Observations of Saturn's Ring-Plane Crossings in August and November 1995

Observations of Saturn's ring system with the Hubble Space Telescope during the 10 August 1995 Earth crossing and the 17 to 21 November 1995 solar crossing indicate that the F ring dominates their apparent edge-on thickness of 1.2 to 1.5 kilometers. The F ring is slightly inclined with respect to the A ring, which may explain the approximately 50-minute difference in apparent crossing times for the east and west ring ansae in August. Prometheus lags its predicted position by about 19 degrees in longitude. The faint G ring is neutral or reddish in color and is confined to a radial range of 2.72 to 2.85 Saturn radii. The broad, distinctly blue E ring flares outward to a maximum thickness of about 15,000 kilometers at 7.5 Saturn radii and appears to have a spatially uniform particle size distribution.

Saturn on edge

Many of the world's telescopes, including NASA's Hubble Space Telescope, saw a once in 15‐year event last week:a Saturn ring plane crossing. The rare event occurs when the alignment of the Sun, Saturn, and Earth cause the rings to seemingly disappear for about 24 minutes, allowing astronomers to look for new moons and other features that are normally difficult to see when the rings are illuminated. It's somewhat like rotating a phonograph record 90° and viewing it edge‐on, explains Ellis Miner, NASA science manager for the spacescraft Cassini. In the next year, on August 10, 1995, and February 11, 1996, the planes will cross again to complete the cycle. Thirteen of Saturn's 18 known moons were discovered during past ring plane crossings. Learning more about Saturn's moons and rings will help astronomers guide Cassini through its observations when it flies in October of 1997. Particles larger than a millimeter would be able to penetrate the spacecraft at the speed it will be flying, Miner says.

Dust distribution around Neptune: Grain impacts near the ring plane measured by the Voyager Planetary Radio Astronomy Experiment

During the Voyager 2 flyby of Neptune the planetary radio astronomy (PRA) experiment recorded an intense noise near the equatorial plane around 3.4 and 4.2 RN, as already observed during previous Voyager ring plane crossings at Saturn and Uranus. This noise is interpreted as being due to impact ionization of dust grains striking the spacecraft. We deduce a power law index of the grain mass distribution of about 2. The PRA system is sensitive to particles with radii larger than ∼1.6 μm, and the largest particles, detected near the ring plane, are evaluated to have a radius of ∼10 μm. The spatial dust distribution along the spacecraft trajectory around the two equatorial crossings is found not to be symmetrical with respect to the ring plane and spread over wide regions: over ∼2 RN perpendicularly to the equatorial plane with the densest part concentrated within ∼700 km. The vertical optical depth τ of this dense region is found to be 10−6 – 10−8.