Saturn Radii Research Articles

Fluorescent hydroxyl emissions from Saturn's ring atmosphere.

Just before earth passed through Saturn's ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (Rs). A spatial scan 0.28Rs above the A and B rings indicates OH column densities of about 10(13) cm(-2) and number densities of up to 700 cm(-3). Saturn's rings must produce roughly 10(25) to 10(29) OH molecules per second to maintain the observed OH distribution.

Origin of Saturn's E Ring: Self-Sustained, Naturally

Saturn's diffuse E ring spans the region between 3 and 8 saturnian radii (R(s)), has its peak brightness near the orbit of the satellite Enceladus (3.95 R(s)), and is thought to be composed primarily of icy particles 1.0 +/- 0.3 micrometers in radius. Such particles are shown to move periodically along highly elliptical paths that cross the orbits of several saturnian satellites; the resulting energetic collisions of E ring particles with embedded satellites are capable of sustaining the E ring at its current optical depth. With several reasonable assumptions, this model naturally selects Enceladus as the primary source of ring material and may also provide mechanisms that explain the generation of the unusual amount of submicrometer dust in the neighboring F and G rings, the excess of OH molecules observed within the E ring, and the orbital brightness variations of nearby satellites.

The dynamics of Saturn's E ring particles

Saturn's tenuous E ring, located between 3 and 8 Saturnian radii (RS), peaks sharply near Enceladus' orbit (3.95 RS) and has recently been found to be composed predominantly of grains 1 micrometer in radius. We study analytically and numerically the motion of such grains launched from Enceladus as they evolve under the action of Saturn's oblate gravity field, solar radiation pressure, and electromagnetic forces. The latter arise because grains are charged (usually to negative values) and also orbit through a dipolar magnetic field. In the absence of planetary shadowing, solar radiation pressure cannot change an orbit's semimajor axis, but it can produce periodic changes in orbital eccentricity that vary at the orbital precession rate. The orbital precession rates caused by the planetary oblateness and the Lorentz force on grains of 1 μm radius are shown to be approximately equal in magnitude but opposite in sign at Enceladus' distance. The near-equality of these precessions for micrometer-sized grains introduced at Enceladus allows very large orbital eccentricities and correspondingly large radial excursions to develop in just a few years. Although particles on eccentric orbits are preferentially found at apocenter, the area covered by an annulus of width Δr is smallest at pericenter; these two effects combine such that the normal optical depth distribution is radially symmetric about the source. Owing to the long time spent at small eccentricities, however, particles injected at Enceladus are most commonly located near its orbit. In addition, solar radiation has a time-dependent component out of the ring plane arising from Saturn's obliquity and motion about the Sun. This force will cause orbital inclinations to develop and is most effective when particles are on highly eccentric orbits. Furthermore, the out-of-plane component of radiation pressure causes the orbital nodes to lock at radial distances similar to that of the source, hence the greatest ring thickness occurs furthest from the planet while the ring is thinnest near the source. By plotting the position of a single particle over time, we show the distribution of 1-μm grains that are injected at Enceladus and move swiftly under the above forces; this distribution has many of the characteristics of the observed E ring. Finally, we note that particles with slightly different sizes attain much smaller eccentricities since the gravitational and electromagnetic contributions to the pericenter precession rate do not cancel nearly as well.

A model of Saturn's magnetic field based on all available data

The SPV (Saturn Pioneer Voyager) model of Saturn's magnetic field presented here uses the most accurate magnetometer data transmitted from Pioneer 11, Voyager 1, and Voyager 2 during their Saturn encounters. It fits the data observed between 1.3 and 8.0 Saturn radii from the planet's center with a 1.13% weighted root‐mean‐square average of the percent differences between the observed and modeled fields. The coefficients for the interior source terms for the SPV model are g10 = 21,160. nT, g20 = 1560. nT, and g30 = 2320. nT for a Saturn radius of 60,330 km. They are the same for all three encounters and should be valid for a decade and perhaps very much longer. The distance unit (the radius of Saturn) affects the values of the Gaussian coefficients. We have adjusted the coefficients of all the models to correspond to 60,330 km so that they can be compared. On the basis of our weights and the use of percent residuals, rather than absolute residuals, the SPV model fits the observed fields better than does the previously published P1184 model or the Z3 model. The part of the field due to currents in the outer regions is not axisymmetric and varies by approximately 5 nT from one encounter to another and from the inbound to outbound legs. The best available, but crude, models for the part of the field due to these external sources are six uniform fields, two for each encounter.

4885549 Method of phase and amplitude correction of NMR signals using a reference marker

We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn's inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1 RS=60,268 km) from the planet, with a north-southward extension of ±2RS. The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125 cm−3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, |z|<0.5RS, shows a slower decrease away from the planet than towards. The outward decrease, from 5 RS, is well described by the relation neq=2.2×104(1/R)3.63. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm−3, only present at the south side of the ring plane. A less prominent density peak, of 115 cm−3, is also detected at the orbit of Tethys, at ∼4.9RS. No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity vi,θ shows a clear general trend in the region between 3 and 7 RS, described by vi,θ=1.5R2−8.7R+39. The average vi,θ starts to deviate from corotation at around 3 RS, reaching ∼68% of corotation close to 5 RS.

Limits on the extent of Saturn's hydrogen cloud

The 30‐day reports from the Voyager Ultraviolet Spectrometer team characterize the atomic hydrogen observed near Saturn as a torus with a half width of 7 Rs extending from about 8 to 25 Rs. The source of this torus was hypothesized to be escape of hydrogen from Titan's atmosphere. Several recent papers propose that the source is actually Saturn's exosphere and that the hydrogen cloud extends inwards to the planet. We model the atomic processes occurring in the inner magnetosphere, including sputtering, ionization, charge exchange, ion‐atom interchange, recombination, and transport, and add a neutral hydrogen source to test the proposal that a hydrogen cloud exists in this region. Plasma observations in the inner magnetosphere are found to be inconsistent with the presence of a dense (20 cm−3) atomic hydrogen cloud. The presence of such a cloud in the observed plasma environment would result in a much larger proton density than observed and would remove nearly all heavy ions from the magnetosphere. Thus we conclude that the hydrogen cloud must be a torus as originally reported, and that the cloud does not extend inward to the orbits of the inner satellites of Saturn.

Titan's atomic nitrogen torus: Inferred properties and consequences for the Saturnian aurora

Nitrogen atoms escaping from the sunlit trailing hemisphere of Titan with a radial velocity of 1–2 km/sec fill a volume extending from 8 to 25 Saturnian radii with a thickness of 8 R s–16 R s about the equatorial plane. For a nitrogen escape rate of Q N ⋍ 6 × 10 26 sec −1 the estimated mean nitrogen density is 6 cm −3 at Voyager epoch. Ionization of the nitrogen torus by both solar EUV photon and magnetospheric electron impact produces a population of warm, approximately kiloelectronvolt N − ions which supply the bulk of the mass and energy input to the magnetosphere at a power level of 2 × 10 11 W. Pickup ions also accelerate magnetospheric electrons to kiloelectronvolt energies by means of lower hybrid emissions on a time scale comparable to the plasma residence time. Magnetic field lines threading the N + pickup region in the outer magnetosphere map to magnetic latitudes at low altitudes where the UV aurora is observed. It is proposed that the aurora is excited by ion and electron precipitation from the outer magnetosphere and that the energetics can be accounted for solely by internal (titanogenic) plasma supply in a corotation-dominated magnetosphere.

Does Saturn have rings outside 10 Rs?

Vasundhara et al.1 and Mahra et al.2 have recently reported detections of new saturnian ring systems near 12.5 and 19.5 Saturn radii (Rs), respectively. Lazarus et al.3 have previously suggested that one or more tenuous rings may exist between 14 and 19 RS, based on observations of unexplained drops in low-energy plasma ion densities in this region of the magnetosphere by Pioneer 11 and Voyagers 1 and 2. However, Baron and Elliot4 have placed a stringent upper limit on the I-band brightness of any material in the region 10–36 Rs west of Saturn. Here we review the Voyagerion and electron data in the energy range 30–1,000 keV as measured by the Low Energy Charged Particle (LECP) experiment. There is no convincing evidence for ring matter outside 10 Rs in these data. Features in the charged particle fluxes in these regions are more readily explained by temporal variations and/or spatial structure unrelated to ring matter, such as the mantle on the dayside5 and/or detached plasma sheets6.

Dynamics of Saturn's E ring

Photographic evidence for a wide outer ring of Saturn, the E ring, was first introduced in 1967 and detailed observations were obtained in 1980. The brightness profile of the E ring indicates that the ring extends from three Saturn radii, the orbit of Mimas, to at least eight Saturn radii, just inside the orbit of Rhea. The brightness profile has a peak at the orbit of Enceladus, but to the limit of the observations there is no evidence of variations in the brightness profile at the orbital distances of Tethys or Dione. It is known that there are satellites at the Lagrangian points of Tethys and Dione within the E ring. Thus the E ring is not constrained by satellites, but rather coexists at the same orbital distances as satellites which have other satellites at their Lagrangian points. Preliminary numerical and analytical investigations of the motions of the particles indicate that the majority of the particles are strictly influenced by Saturn. Particles in narrow rings at the orbital distance of a satellite move with revolution or librational motion and sometimes switch between these modes. The librational motion can be tadpole orbits at the L 4 and L 5 points, horseshoes including L 4. L 3, and L 5 points and segment librations between the L 4 or L 5 points and the satellite.

Micron-size particles detected near Saturn by the Voyager plasma wave instrument

During the Voyager 2 Saturn encounter the plasma wave instrument detected a region of intense impulsive noise centered on the ring plane at a distance of 2.88 Saturn radii, slightly outside of the G ring. The noise has been attributed to small micron-size particles hitting the spacecraft. Investigation of various coupling mechanisms suggests that the noise was produced by impact ionization of particles striking the spacecraft body, thereby releasing a cloud of charge particles, some of which are collected by the plasma wave antenna. Reasonably reliable estimates of the charge yield per unit mass are available from laboratory impact ionization measurements. Based on the assumption that the voltage induced on the antenna is proportional to the mass of the colliding particle, a method is developed to determine the mass and size distribution of the particles from the rms voltage of the induced noise and the impulse rate. The results obtained show that the mass distribution varies as m-3, and that most of the particles detected had radii in the range from 0.3 to 3 µ meters. The effective north-south thickness of the particle distribution is 106 km. The mass distribution function derived from these data is shown to be in reasonable agreement with optical depth estimates obtained from Imaging measurements and absorption effects detected by energetic charged particle measurements.

Source of saturnian myriametric radiation

The Voyager 1 and 2 flybys of Saturn revealed the presence of a variety of plasma waves associated with Saturn's magnetosphere1,2, some electrostatic in nature, others apparently electromagnetic. Possibly the most unambiguous identification of the latter in the low-frequency range (< 10 kHz) is of narrow-banded emissions observed over a 3-day period when Voyager 1 was outbound3. Persistent bands of emission near 5 kHz were seen from 19 to 58 Rs (where Rs is Saturn's radius = 60,330 km), and, as the plasma parameters varied widely along the trajectory over this range, it is difficult to envisage how these emissions could be anything but freely-propagating electromagnetic waves. Hitherto their source was unknown, but similar emissions have been observed within the magnetospheres of Jupiter4 and Earth5–7 and, based on our knowledge of the latter in particular, it is proposed here that the source of the saturnian narrow-band emissions lies in electrostatic upper-hybrid waves near the equatorial plane just beyond the orbit of the moon Rhea.

Eccentric Ringlet in the Maxwell Gap at 1.45 Saturn Radii: Multi-Instrument Voyager Observations

The Voyager spacecraft observed a narrow, eccentric ringlet in the Maxwell gap (1.45 Saturn radii) in Saturn's rings. Intercomparison of the Voyager imaging, photopolarimeter, ultraviolet spectrometer, and radio science observations yields results not available from individual observations. The width of the ringlet varies from about 30 to about 100 kilometers, its edges are sharp on a radial scale < 1 kilometer, and its opacity exhibits a double peak near the center. The shape and width of the ringlet are consistent with a set of uniformly precessing, confocal ellipses with foci at Saturn's center of mass. The ringlet precesses as a unit at a rate consistent with the known dynamical oblateness of Saturn; the lack of differential precession across the ringlet yields a ringlet mass of about 5 x 10(18) grams. The ratio of surface mass density to particle cross-sectional area is about five times smaller than values obtained elsewhere in the Saturn ring system, indicating a relatively larger fraction of small particles. Also, comparison of the measured transmission of the ringlet at radio, visible, and ultraviolet wavelengths indicates that about half of the total extinction is due to particles smaller than 1 centimeter in radius, in contrast even with nearby regions of the C ring. However, the color and brightness of the ringlet material are not measurably different from those of nearby C ring particles. We find this ringlet is similar to several of the rings of Uranus.

Particle size distributions in Saturn's rings from voyager 1 radio occultation

Observations of microwave opacity τ[λ] and near forward scatter from Saturn's rings at wavelengths λ of 3.6 and 13 cm from the Voyager 1 ring occultation experiment contain information regarding ring particle sizes in the range of about a = 0.01 to 15 m radius. The opacity measurements τ[3.6] and τ[13] are sufficient to constrain the scale factor n( a 0) and index q of a power law incremental size distribution n( a) = n( a 0)[ a 0/ a] q , assuming known minimum and maximum sizes and a many-particle-thick model. The families of such distributions are highly convergent in the centimeter-size range. Forward scatter at 3.6 cm can be used to solve for a general distribution over the radius range 1 ⪷ a ⪷ 15 m by integral inversion and inverse scattering methods, again assuming a many-particle-thick slab-type radiative transfer model. Distributions n( a) valid over 0.01 ⪷ a ⪷ 15 m are obtained by combining the results from the two types of measurements above. Mass distributions may be computed directly from n( a). Such distributions, partly measured and partly synthesized, have been obtained for four features in the ring system centered at 1.35, 1.51, 2.01, and 2.12 Saturn radii ( R s ). The size and mass distributions both cut off sharply at a ≅ 4–5 m; the mass distribution peaks over the narrow size range 3 ⪷ a ⪷ 4 m for all four locations. No single power law distribution is consistent with the data over the entire interval 0.01 ⪷ a ⪷ 5 m , although a power law-type model is consistent with the data over a limited size range of 0.01 ⪷ a ⪷ 1 m , where the indices q = 3.4 and 3.3 are obtained from the slab model for the features located at 1.51 and 2.01 R s . The fractional contribution of the suprameter particles to the microwave opacity in each feature appears to be about 1 3 , 1 3 , 2 3 , and 1 , respectively, with the fraction at 2.12 R s being the least certain. The cumulative surface mass per unit area obtained for the classical slab model is approximately 11, 16, 41, and 132 g/cm 2 for the four features, respectively, if the particles are solid H 2O ice. Both the fractional opacity and the mass density estimates represent upper bounds implied by the assumption of a uniformly mixed set of particles in a many-particle-thick vertical profile; lower estimates would result if the rings were assumed to be nearly a monolayer or if the vertical distribution of particles were size dependent.

A CCD imager search for possible material orbiting Saturn between 10 and 36 Saturnian radii

Optical imaging of the area west of Saturn from 10 to 36 Saturnian radii was used to search for possible orbiting material to a limit of I = +22/sq arcsec for material distributed over a region larger than the seeing disk of about 3 arcsec. For sources smaller than the seeing disk, a limit of I = +19.5 was reached.

The magnetosphere of Saturn

The Pioneer 11, Voyagers 1 and 2 encounters with Saturn provided a wealth of information about its complex magnetosphere. The magnetic dipole moment of Saturn is rotationally aligned and only one‐fifth of that expected from pre‐encounter modeling. The bow shock stand‐off distance is about 22 RS and varies with solar wind pressure proportional to p−1/6. The satellites Titan, Dione, and Tethys are probably the primary sources of magnetospheric plasma. For &lt;10 keV thermal plasma, β is less than one in most of the magnetosphere, but contributions from ∼20 keV plasma may increase this value significantly. Outside of ∼4 RS, energetic particles are energized by diffusing inward while conserving their first and second adiabatic invariants. Particles are lost by satellite sweep‐out, absorption by the E ring and probably also by plasma interactions, which produce a slot region between 4 and 9 RS. The inner magnetosphere is characterized by a cold plasma population (&lt;10 eV/charge), which probably consists primarily of 0+ and has a scale height from the Equator of only 0.2 RS. Intense penetrating radiation exists in the inner magnetosphere from 4 RS to 2.265 RS, which coincides with the outer edge of the A ring. The energetic protons ⩾50 MeV have approximately the spectrum expected from a cosmic ray albedo neutron decay source. A proton component below 0.5 MeV was also found, as well as an electron flux with energies primarily above 1.5 MeV. Submicron charged dust gains constitute a new type of magnetospheric particle and their properties may explain the evolution of the B ring spokes and other B ring features.

Long-lived particulate or gaseous structure in Saturn's outer magnetosphere?

During the encounters with Saturn by the Voyager 1, Voyager 2 and the Pioneer 11 spacecraft, large variations in the number density of low-energy plasma ions were observed in the outer magnetosphere. Those density variations were thought to be due to changing external conditions, remnants of gas from Titan, or detached portions of Saturn's main plasma sheet. We have found evidence which suggests that at least some of the density variations (which appear as dips in a curve which generally shows rising density as Saturn is approached) are due to absorbing material near Saturn's equatorial plane in the region between 10 and 20 Saturn radii (Rs). Although the entire picture is still speculative, we present our evidence to encourage further optical examination of that region near Saturn. Notably, in 1978 Colombo suggested that a deep survey be made of the region between Tethys and Titan (10–20 Rs) because of its similarity to the asteroid belt in the Solar System1.

Micron-sized particles detected near Saturn by the Voyager plasma wave instrument

During the Voyager 2 Saturn encounter the plasma wave instrument detected a region of intense impulsive noise centered on the ring plane at a distance of 2.88 Saturn radii, slightly outside of the G ring. The noise has been attributed to small micron-sized particles hitting the spacecraft. Investigation of various coupling mechanisms suggested that the noise was produced by impact ionization of particles striking the spacecraft body, thereby releasing a cloud of charged particles, some of which were collected by the plasma wave antenna. Reasonably reliable estimates of the charge yield per unit mass are available from laboratory impact ionization measurements. Based on the assumption that the voltage induced on the antenna is proportional to the mass of the colliding particle, a method was developed to determine the mass and size distribution of the particles from the rms voltage of the induced noise and the impulse rate. The results obtained show that the mass distribution varies as m −3, and that most of the particles detected had radii in the range from 0.3 to 3 μm. The effective north-south thickness of the particle distribution is 106 km. The mass distribution function derived from these data is shown to be in reasonable agreement with optical depth estimates obtained from imaging measurements and absorption effects detected by energetic charged particle measurements.

Electrostatic discharges in Saturn's B-ring

The Voyager observations of electrical discharges in Saturn's rings strongly support earlier speculations on the role played by electrostatics, magnetic fields, and lightning phenomena in the primitive solar system. They also suggest conditions then by direct analogy rather than by extrapolating backwards through time from conditions now. The observed discharges show a pronounced 10 h periodicity, which suggests a source in Keplerian orbit at 1.80 ± 0.01 Saturn radii (1 R S = 60,330 km). In that region, the B ring is thicker than optical depth 1.8 for about 5,000 km. At 1.805 ± 0.001 Saturn radii, however, the ring is virtually transparent for a gap of width 200 m. We conclude that a small satellite orbits Saturn at that radius and clears the gap. The gap edges must prevent diffusive filling of the gap by fine material which is especially abundant at this position in the rings and would otherwise destroy the gap in minutes. The discharges represent the satellite's interaction with the outer edge of the gap. Spoke formation may involve the interaction of ring material in the vicinity of the gap.

Resonances in Saturn's rings

The locations and strengths of the major resonances of Saturn's known moons with particles orbiting within Saturn's rings are calculated. The resonant effects of an outer satellite on Saturn's rings is analyzed by Fourier expanding the satellite's potential, and it is found that the forcing at an l:(m-1) resonance depends on the moon's eccentricity to the (l-m) power. As most of Saturn's inner moons are in very nearly circular orbits, only their strongest resonances, with l = m and l = m+1, are calculated. For Mimas, which has a somewhat larger eccentricity, resonances with l = m+2 are also computed. All of the resonances of these forms which are located between Saturn's cloud tops and 2.267 Saturn radii are tabulated, except some of those due to tiny 1980 S28.

Observation and photometry of an outer ring of saturn

A faint outer ring (E ring), which lies outside the classical rings A, B, C, and F, has been detected out to eight Saturn radii. We first observed it on November 1, 1979, and thereby confirmed the 1966 observation by Feibelman. Our plates were taken with a coronographic design and are specially intended for photometry. They are directly scaled in reflectance by reference to the Saturn disk which is properly attenuated. Photometry of the edge-on ring E lineament shows a strong brightness increase at small phase angles, which is compatible with scattering by particles of several microns in radius. The excess reflectivity in blue compared to the B ring implies a significant contribution of small particles in the scattering process. The E ring shows brightness and radial gradient changes, with condensations, which differ between east and west limbs and are not always the same from night to night. The E ring is probably a flat structure with a condensation centered at a distance of 4 R s, but without a simple axial symmetry. It is probably shaped by segments or lumps and may have streamerlike structures.