Jovian Ring Research Articles

A new analysis of Galileo dust data near Jupiter

The Galileo Dust Detection System (DDS) detected a population of micron-sized grains in and amongst the orbits of Io, Europa, Ganymede and Callisto. Previous studies, using roughly 50% of the data now available, concluded that the dominant sources for the impacts were magnetospherically captured interplanetary particles largely on retrograde orbits (Colwell et al., 1998b; Thiessenhusen et al., 2000) and impact-generated ejecta from the Galilean satellites (Krüger et al., 1999b; Krivov et al., 2002a). Here we revisit the problem with the full data set and broaden our consideration to include four additional source populations: debris from the outer satellites, interplanetary and interstellar grains and particles accelerated outwards from Io and the jovian rings. We develop a model of detectable orbits at each Galileo position and we find that about 10% of the impact data require non-circular orbits with eccentricities greater than 0.1. In addition, ~3% of impacts require orbital solutions with eccentricities in excess of 0.7. Using the spatial distribution of particles, we are able to exclude, as dominant sources, all the additional source populations except for outer satellite particles. A study of DDS directional information demonstrates that none of the six standard sources fit the data well and thus a combination of sources is necessary. There are insufficient data to uniquely identify the relative strengths of the various contributions. However, we find an excess of large particles that is consistent with retrograde trajectories.

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 Jovian Rings

AbstractA comparison of the Jovian and Saturnian rings is made by reviewing the recent advances in planetary spacecraft exploration and theoretical study. Two main issues are addressed, namely, the different structures of these two planetary ring systems and the water ice composition of the Saturnian rings. It is suggested that answers might be found by invoking tidal capture of Trans-Neptunian Objects with highly differentiated structures even though catastrophic breakup of pre-existing satellites in the ring regions remains a real possibility. Erosion mechanisms such as meteoroid impact, photo-sputtering, orbital instability of charged dust particles and thermal evaporation acting at different time scales could lead to the preservation of the Saturnian ring system but not the Jovian ring system of large mass originally.

Evidence for short-term variability of Jupiter's decimetric emission from VLA observations

Aims. We present the first evidence of short-term variations of Jupiter’s radiation-belt emission obtained with interferometric measurements. Over a two-month period of observational time in 2002, the Jovian synchrotron emission was observed with the Very Large Array (VLA). Methods. The images constructed at the wavelength of 6 cm demonstrate significant changes in the spatial structure of the brightness distribution. The comparisons of the two-dimensional maps with another campaign of VLA observations made in May 1997 confirm our discovery of changes in Jupiter’s synchrotron emission on a time-scale of days to weeks. Results. For a series of central meridian longitudes (CMLs), the radiation peak near 1.4 RJ was observed to shift back and forth from one side of the planet to the other between October and December 2002. The change in the location of the emission peak was found to be the result of fluctuations in the peak brightness distribution by 10% up to 40%. These fluctuations are too significant to be associated with the small variation of the geometric parameter DE (the declination of the Earth as seen from Jupiter) during the campaign of observations. We have demonstrated that the variability of the synchrotron emission peak was observed when the angular sectors covering the Jupiter SIII longitudes λIII where the field strength along the equatorial magnetic surface is maximum or minimum were monitored. Conclusions. Short-term variations of Jupiter’s synchrotron emission are expected to be reported when specific CMLs are observed and changes in the distributions of the 30–40 MeV electron population occur simultaneously. Our discussions led to the conclusion that the short-term variations of Jupiter’s Decimetric brightness distribution may be the result of submicrometre charged dust particles undergoing significant electromagnetic perturbations while interacting with the radiation-belt electrons in the Jovian ring’s innermost component.

Keck observations of the 2002–2003 jovian ring plane crossing

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼ 5000 km extension on the inside. The normal optical depth is 8 × 10 −6 , 15% of which is provided by bodies with radii a ≳ 5 cm . These bodies are as red as Metis. Half the optical depth, τ ≈ 4 × 10 −6 , is attributed to micron-sized dust, and the remaining τ ≈ 3 × 10 −6 to grains tens to hundreds of μm in size. The inward extension consists of micron-sized ( a ≲ 10 μm ) dust, which probably migrates inward under Poynting–Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r = 122 , 400 km ), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000–20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼ 5 × 10 −7 , up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.

Clump Detections and Limits on Moons in Jupiter's Ring System

The dusty jovian ring system must be replenished continuously from embedded source bodies. The New Horizons spacecraft has performed a comprehensive search for kilometer-sized moons within the system, which might have revealed the larger members of this population. No new moons were found, however, indicating a sharp cutoff in the population of jovian bodies smaller than 8-kilometer-radius Adrastea. However, the search revealed two families of clumps in the main ring: one close pair and one cluster of three to five. All orbit within a brighter ringlet just interior to Adrastea. Their properties are very different from those of the few other clumpy rings known; the origin and nonrandom distribution of these features remain unexplained, but resonant confinement by Metis may play a role.

The jovian rings: new results derived from Cassini, Galileo, Voyager, and Earth-based observations

Cassini's Imaging Science Subsystem (ISS) instrument took nearly 1200 images of the Jupiter ring system during the spacecraft's 6-month encounter with Jupiter (Porco et al., 2003, Science 299, 1541–1547). These observations constitute the most complete data set of the ring taken by a single instrument, both in phase angle (0.5°–120° at seven angles) and wavelength (0.45–0.93 μm through eight filters). The main ring was detected in all targeted exposures; the halo and gossamer rings were too faint to be detected above the planet's stray light. The optical depth and radial profile of the main ring are consistent with previous observations. No broad asymmetries within the ring were seen; we did identify possible hints of 1000 km-scale azimuthal clumps within the ring. Cassini observations taken within 0.02° of the ring plane place an upper limit on the ring's full thickness of 80 km at a phase angle of 64°. We have combined the Cassini ISS and VIMS (Visible and Infrared Mapping Spectrometer) observations with those from Voyager, HST (Hubble Space Telescope), Keck, Galileo, Palomar, and IRTF (Infrared Telescope Facility). We have fit the entire suite of data using a photometric model that includes microscopic silicate dust grains as well as larger, long-lived ‘parent bodies’ that engender this dust. Our best-fit model to all the data indicates an optical depth of small particles of τ s =4.7×10 −6 and large bodies τ l =1.3×10 −6. The dust's cross-sectional area peaks near 15 μm. The data are fit significantly better using non-spherical rather than spherical dust grains. The parent bodies themselves must be very red from 0.4–2.5 μm, and may have absorption features near 0.8 and 2.2 μm.

Cassini imaging of Jupiter's atmosphere, satellites, and rings.

The Cassini Imaging Science Subsystem acquired about 26,000 images of the Jupiter system as the spacecraft encountered the giant planet en route to Saturn. We report findings on Jupiter's zonal winds, convective storms, low-latitude upper troposphere, polar stratosphere, and northern aurora. We also describe previously unseen emissions arising from Io and Europa in eclipse, a giant volcanic plume over Io's north pole, disk-resolved images of the satellite Himalia, circumstantial evidence for a causal relation between the satellites Metis and Adrastea and the main jovian ring, and information on the nature of the ring particles.

Galileo NIMS Near-Infrared Observations of Jupiter's Ring System

We present the first observations of the jovian ring system viewed at near-infrared wavelengths in forward-scattered light. The eclipse of the Sun by Jupiter during the “C3” orbit of the Galileo spacecraft afforded favorable conditions for the Near Infrared Mapping Spectrometer (NIMS) to observe the ring system between 0.7 and 5.2 μm at a spectral resolution of ≈0.04 μm. NIMS images show the nearly edge-on main ring from just inside the ansa at 1.80 R J down to 1.05 R J (where 1 R J=71398 km), at a spatial resolution of 450 km along the ring and 1130 km out of the ring plane. At these spatial resolutions, no fine structure is apparent in the ring. The large wavelength coverage of the NIMS observations permits an analysis of the scattering properties of the ring system even though the ring subtends only a small range of scattering angles. Our retrieved particle size distribution shows a decrease in the number of particles at larger radii although there is a reversal of this trend between 0.6 and 18 μm. This distribution can be modeled as the combination of a power law with index p=3.9±0.2 and a log-normal distribution of mean radius 4.5 μm. Analysis shows that Voyager data lacked the required wavelength coverage and viewing geometry to determine the particle distribution uniquely. We conclude that the NIMS data set is a better determinant of the particle size distribution, especially for particles larger than a micrometer. We suggest that the power law distribution is the result of collisional processes that form grains less than 1 micrometer, in agreement with previous theory. We suggest that the log-normal distribution is characteristic of another, yet undetermined, process that dominates grain formation and evolution for particles several micrometers in size.

Near Infrared Photometry of the Jovian Ring and Adrastea

The near IR spectral reflectance of the Jupiter dust ring is poorly known because of problems with scattered light from the planet. Here we report colors for the jovian ring and one of the two ring satellites, Adrastea, using observations from the near-IR camera NICMOS on the Hubble Space Telescope. Near the time when the Earth crossed the jovian ring plane in the fall of 1997, we recorded broad-band images at ∼1.1 (F110W), ∼1.6 (F160W), and ∼2.05 μm (F205W) and derived a single-pass, in radial direction measured ring brightness of 19.19±0.07, 18.76±0.06, and 18.49±0.04 mag linear arcsec −1, respectively. These single-pass radial ring brightnesses were derived from the observable part of the ring at a projected distance of >1.2 R J using a model to remove projection effects. The corresponding apparent magnitudes for Adrastea are 18.30±0.10 (F110W), 17.73±0.09 (F160W), and 17.57±0.07 mag (F205W), obtained at a phase angle of φ=11.3°. The relative spectral reflectance of the ring and that of Adrastea turn out to be nearly identical, slightly reddish with a slope of about 15–20% between 1 and 2 μm. No evidence for transient ice crystals to be present in the main ring is seen. Our data are also in reasonable agreement with earlier ground-based measurements by Neugebauer et al. (1981), if we take their relatively large errors into account. The similarities of the colors of all inner satellites, including Io, are striking. The measured ring color provides evidence that the backscattered light from the ring is due to grains with mean particle sizes in excess of several micrometers. We were also able to infer a spatial particle distribution for the main ring. Its radial surface-density profile peaks sharply near the outer edge of the ring at the orbit of Adrastea, suggesting a strong dynamical relationship between the satellite and the ring particles. Our radial profile of the main ring is in excellent agreement with the results from Voyager images in backscattered light at visible wavelengths, except that we could not resolve any fine structures. The halo above and below the ring plane with a peak brightness near the inner edge of the ring appears to have a blue color compared to the main ring, but due to the low surface brightness of the halo the statistical significance of this color trend is only marginal. Such a color trend would be consistent with a dust population dominated by particles smaller than those in the main ring.

Keck Infrared Observations of Jupiter's Ring System near Earth's 1997 Ring Plane Crossing

We imaged the jovian ring system at a wavelength of 2.27 μm with the 10-m W. M. Keck telescope on August 14 and 15, 1997, when the ring plane was almost edge-on (opening angle β=0.17°) and near opposition (phase angle α≈1.1°). The resolution in the images is 0.6″=0.025RJ. We obtained the first images of the jovian halo and gossamer ring in back-scattered light, and the best ground-based images to date of Jupiter's main ring. The main ring is radially confined between 1.70 and 1.82RJ(where 1RJ=71398 km), with a maximum (after inversion) at 1.79RJ, in agreement with the Voyager findings. The halo extends inward from the main ring (at 1.71RJ) down to 1.40RJ, apparently bounded by the locations of Lorentz resonances. Roughly 50% of the halo's intensity originates from a region within ∼700 km from the equatorial plane, although it is visible up to ∼10,000 km above and below the plane. Although the vertical extent agrees with Voyager findings, the halo's intensity relative to that of the main ring in the Keck images is much less than in forward-scattered Voyager images, which we attribute to a predominance of micrometer-sized particles, which scatter visible light preferentially in the forward direction. The gossamer ring is found to have two components, with steep dropoffs in brightness at the orbits of Amalthea and Thebe. The first, Amalthea's gossamer ring, is visible between the main ring's periphery and ∼2.55RJ; it is relatively uniform in brightness and has a vertical thickness (FWHM) of 0.06RJ, clearly broader than the FWHM of the main ring (0.045RJ) and the image resolution. The other component, Thebe's gossamer ring, is a factor of five fainter than Amalthea's ring and about twice as broad vertically (FWHM≈0.12RJ). This ring extends outward to 3.11RJ, but additional material is visible, albeit barely, out to ∼3.6RJ, near the edge of our images. The vertical extent of both the Thebe and Amalthea rings decreases with decreasing distance to the planet.

The Structure of Jupiter's Ring System as Revealed by the Galileo Imaging Experiment

The jovian ring system was observed during four orbits of Galileo's nominal mission, when 25 clear-filter images of the rings were taken at spatial resolutions of 23 to 134 km/pixel; the ring appeared fortuitously in an additional 11 images. The tenuous jovian ring system (normal optical depths<10−5) has three components: the halo, main ring, and gossamer ring.The innermost component, atoroidal halo, extends radially from approximately 92,000 to about 122,500 km (near the 3 : 2 Lorentz resonance) and has a full-width, half-maximum thickness of 12,500 km; its brightness decreases with height off the equatorial plane and decreases as the planet is approached. Themain ringreaches from the halo's outer boundary across 6440 to 128,940 km, just interior to Adrastea's orbit (128,980 km); at its outer edge, the main ring takes nearly 1000 km to develop its full brightness. The ring's brightness noticeably decreases around 127,850 km in the vicinity of Metis' semimajor axis (127,980 km). The precise location and nature of the main ring's outer periphery may shift slightly from image to image; the same is true for the “notch” near Metis. The main ring has a faint, vertically extended component that thickens as the halo region is approached. Brightness variations of ±10% are visible in the central main ring and may be due to vertical corrugations, density clumps, or “spokes” in the ring. Unexplained differences between the near- and far-arm brightnesses are visible. We have discovered that thegossamer ring, lying exterior to the main ring, has two primary components, each of which is fairly uniform: one originates just interior to Amalthea's orbit (181,366 km) while the other is situated radially interior to Thebe's orbit (221,888 km). Very faint material continues past Thebe, blending into the background at 250,000 km. The gossamer rings have thicknesses that are comparable to the maximum elevations of these satellites off Jupiter's equatorial plane; from Galileo's nearly equatorial view, the gossamer rings present rectangular end-profiles with greater intensities along their top and bottom surfaces. The rings seem to be derived from the satellites.

Jupiter's exogenic dust ring

Interplanetary and interstellar dust particles stream through the Jovian magnetosphere where their orbits are perturbed by the Lorentz force. Energy and angular momentum exchange with the magnetosphere can lead to capture of some submicron dust particles into dynamically stable orbits around Jupiter. The captured grains can be on prograde or retrograde orbits, and their final orbits are usually in the semimajor axis range of 2 to 20 Jupiter radii, with eccentricities ∼0.1–0.8 and inclinations ≲20°. Captured grains have radii between about 0.5 and 1.5 μm. These captured dust particles form a diffuse band of material around Jupiter, outside the main Jovian ring, with a peak normal optical depth of τ≲10−9 While this is too low for optical detection, the Galileo dust detector has detected particles on prograde and retrograde orbits around Jupiter whose sizes, orbital properties, and number density are consistent with captured interplanetary and interstellar dust.

Time variations of the Jovian synchrotron radiation following the collisional impacts of comet Shoemaker-Levy 9

Abstract The collisional interaction of the dust cloud associated with the nuclei- fragment chain of Comet Shoemakep-Levy 9 with the Jovian ring complex might lead to an increase of the ring mass. We estimate that the ring mass increase as a result of surface bombardment of the two inner satellites, Adrastea and Metis, could be relatively small and the upper limit should be 2 × 10 11 −10 12 g which is near the lower limit of the mass of the current Jovian ring. An increase in the ring mass, if significant, could produce observable reduction in the synchrotron radiation from energetic electrons as a result of the enhancement in tbe charged particle absorption effect. The resulting time variations of the radio emissions should also be diagnostic of the radial diffusion process near Jupiter.

New Jovian ring?

We follow the orbital evolution of small dust particles generated from the break up of comet Shoemaker‐Levy 9. In addition to the usual perturbations, we also consider electrostatic charging and the consequent magnetospheric effects. We show, that in about ten years following the break up, a small fraction of the dust will settle into orbits well inside the magnetosphere. The forming ring will be comprised of dust particles with radii in the range of 1.5 &lt; a &lt; 2.5 µm that follow retrograde orbits in the radial range of 4.5 &lt; r &lt; 6 RJ. We estimate the peak optical depth of this new ring in the range of 10−8 &lt; τmax &lt; 2 × 10−6.

Energetic particle absorption by atmosphereless satellites and rings

In outer planet magnetospheres, theoretical estimates of the rate of energetic particle absorption by solid satellites and rings have two primary applications. First, in regions where satellite absorption is the dominant energetic particle loss process, model calculations combined with measured fluxes and derived phase space densities can be employed to constrain the amplitude and L-dependence of the magnetospheric radial diffusion process that is responsible for populating the radiation belts. Since only flyby data are available, this approach provides the only present means for probing the dynamics of outer planet radiation belts. Second, if the rate of radial diffusion is independently constrained, observed ring absorption signatures can be combined with model calculations to solve for certain undetermined parameters such as the mean particulate size in a planetary ring. Examples of applications to radial diffusion in the Uranian radiation belts and to the interpretation of the Jovian ring absorption signature in Pioneer 11 energetic particle data are discussed.

Magnitudes of selected stellar occultation candidates for Pluto and other planets, with new predictions for Mars and Jupiter

Occultation predictions for the planets Mars and Jupiter are presented along with BVRI magnitudes of 45 occultation candidates for Mars, Jupiter, Saturn, Uranus, and Pluto. Observers can use these magnitudes to plan observations of occultation events. The optical depth of the Jovian ring can be probed by a nearly central occultation on 1992 July 8. Mars occults an unusually red star in early 1993, and the occultations for Pluto involving the brightest candidates would possibly occur in the spring of 1992 and the fall of 1993.

Lorentz resonances and the vertical structure of dusty rings: Analytical and numerical results

Dust grains orbiting the giant outer planets carry a weak and nearly constant electrical charge. Through interaction with the planet's irregular magnetic field, the inclinations i and eccentricities e of these charged grains can be pumped up significantly. This occurs especially when a period of the Lorentz force matches the radial or out-of-plane epicyclic periods. Since orbiting particles sample the spatial structure of the planetary magnetic field at differing rates depending on their mean orbital radius a, these Lorentz resonances (LR) occur at specific orbital radii. Several of the strongest LR have been related to structural features of the Jovian dust ring system. In this paper we use a combination of analytical and numerical techniques in order to understand the nature of these resonances. A simple extension of the perturbation theory for the LRs yields the charge-to-mass ratio ( q/ m) corrections to the periods of the in-plane and out-of-plane motion, thus allowing for an accurate determination of the resonance locations. The analysis also indicates that the two modes of oscillation are weakly coupled. We then show that the mean orbital radius a, assumed constant in previous analytical developments, undergoes a small but significant resonant forcing as well, resulting in shifts in a which break the exact linear resonance condition. An analysis of the energy of the grain's orbit - for small values of eccentricity e and inclination i - shows that it can be decomposed into three parts: energies of vertical and horizontal oscillators, and the energy of the circular equatorial orbit from which these grains are perturbed by the Lorentz force. In a reference frame corotating with the planet where no electric field is present, the Lorentz force cannot do work but it can exert torque; hence it can transfer energy between the circular orbit and the horizontal and vertical oscillators. Numerical integrations in the resonance zones support this view of the dynamical interaction, and also confirm the existence of a small range of charge-to-mass ratios where the vertical and horizontal resonances overlap, thereby producing what appears to be chaotic motion. Resonance zones can be traversed as orbits evolve in semimajor axis, e.g., due to plasma drag. Using numerical simulations to study this passage through resonance, we find that the orbital elements a, e and i undergo large jumps, and we determine the size of these jumps as a function of ( q/ m) and drag evolution rates. We also ascertain the distribution of final orbital parameters, after passage, for a distribution of initial launch longitudes. In particular we show that the longitude of nodes, Ω, is uniformly distributed over the interval 0 < Ω < 2π. In this case, a cross-section of the Jovian halo would be predicted to have symmetry about the equatorial plane, as confirmed through analysis of the Voyager data by Showalter et al. (1987, Icarus, 69, 458–498). Using adiabatic invariants of the motion, we show that the large inclinations induced in passing through one resonance survive during the excursion to the next resonance encounter. Thus many features of the Jovian ring halo are explained by the Lorentz resonance mechanism.

Charged dust dynamics: Orbital resonance due to planetary shadows

First, we discuss the dynamics of a weakly charged dust grain orbiting in the equatorial plane of a planet surrounded by a rigidly corotating magnetosphere, where the magnetic field can be represented by a centered aligned dipole field. If the grain has a constant charge, we can introduce an effective one‐dimensional potential, from which we find that the perturbation due to electrostatic forces induces a motion of the pericenter, similar to the effect of the planetary oblateness. However, the electric charge on an orbiting dust grain is generally not constant. Accordingly, we explore the case where the charge varies periodically due to the modulation of the photoelectron current that occurs as the grain enters and leaves the planetary shadow. Thus the electromagnetic perturbation resonates with the orbital period and can modify the orbit's size and eccentricity. We demonstrate this effect both numerically and analytically for small grains comprising the Jovian ring. We show that the resulting changes are periodic, and their amplitude is much bigger than the amplitude of the periodic changes due to light‐pressure perturbation or the secular changes due to resonant charge variations that develop over a comparable time span.

Near-infrared observations of the Jovian ring and small satellites

On 3 December 1988 we obtained 119 images of the Jovian ring, using the Caltech Cassegrain infrared camera at a wavelenght of 2.2 μm. The ring is seen at a near-maximum opening angle of 3.2° and at a phase angle of 2.2°. Our measurements indicate a ring radius at maximum brightness of 42.″9, or 126,100 ± 500 km, compatible with Voyager data at high phase angles. Many of the images also show either Metis or Adrastea, the two small satellites associated with the ring and previously seen only in Voyager images. From a set of 98 images spanning a period of 5.7 hr, we have determined circular orbital elements for both satellites. The derived orbits are consistent with those determined by S.P. Synnott (1984, Icarus 58, 178–181) from Voyager images, but the greatly extemded temporal baseline provided by combining our measurements with those of Synnott permits us to substantially improve the mean motions, to n Metis = 1221.2547 ± 0.0002 deg/day and n Adrastea = 1206.9987 ± 0.0002 deg/day. The postfit residuals from our circular orbit fits indicate upper limits on either satellite's eccentricity of ∼0.005, no evidence of an inclination ≳0.1° for Metis, but a possible inclination for Adrastea of ∼0.25°.