Molecular Origin of Io's Fast Sodium

Neutral sodium emissions encircling Jupiter exhibit an intricate and variable structure that is well matched by a simple loss process from Io's atmosphere. These observations imply that fast neutral sodium is created locally in the Io plasma torus, both near Io and as much as 8 hours downstream. Sodium-bearing molecules may be present in Io's upper atmosphere, where they are ionized by the plasma torus and swept downstream. The molecular ions dissociate and dissociatively recombine on a short time scale, releasing neutral fragments into escape trajectories from Jupiter. This theory explains a diverse set of sodium observations, and it implies that molecular reactions (particularly electron impact ionization and dissociation) are important at the top of Io's atmosphere.     

The jupiter-io connection: an alfven engine in space

Much has been learned about the electromagnetic interaction between Jupiter and its satellite Io from in situ observations. Io, in its motion through the Io plasma torus at Jupiter, continuously generates an Alfvén wing that carries two billion kilowatts of power into the jovian ionosphere. Concurrently, Io is acted upon by a J x B force tending to propel it out of the jovian system. The energy source for these processes is the rotation of Jupiter. This unusual planet-satellite coupling serves as an archetype for the interaction of a large moving conductor with a magnetized plasma, a problem of general space and astrophysical interest.     

Pioneer 10 observations of the ultraviolet glow in the vicinity of jupiter

A two-channel ultraviolet photometer aboard Pioneer 10 has made several observations of the ultraviolet glow in the wavelength range from 170 to 1400 angstroms in the vicinity of Jupiter. Preliminary results indicate a Jovian hydrogen (1216 angstrom) glow with a brightness of about 1000 rayleighs and a helium (584 angstrom) glow with a brightness of about 10 to 20 rayleighs. In addition, Jupiter appears to have an extensive hydrogen torus surrounding it in the orbital plane of Io. The mean diameter of the torus is about equal to the diameter of the orbit of Io. Several observations of the Galilean satellites have also occurred but only a rather striking Io observation has been analyzed to date. If the observed Io glow is predominantly that of Lyman-alpha, the surface brightness is about 10,000 rayleighs.     

Extreme ultraviolet observations from voyager 1 encounter with jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 10(5) K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman alpha by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (>/= 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.     

Detection of SO in Io's exosphere

The Galileo orbiter's close pass by Io in 1995 produced evidence for extensive mass loading of the plasma torus through the ionization of SO2. On 11 October 1999, Galileo passed even closer to Io, this time across the upstream side relative to the flow of magnetospheric plasma that corotates with Jupiter. On the first flyby, ion cyclotron waves gave direct evidence for the production of SO2+ ions. On the second flyby, ion cyclotron waves associated with SO+ were stronger and more persistent. Moreover, SO+ emissions were seen closer to Io than SO2+ emissions, suggesting that the exosphere was spatially inhomogeneous. The location of the waves suggests a fan-shaped region of ion pickup extending in the anti-Jupiter direction. Because the wave spectra were different even where the 1995 and 1999 trajectories crossed, we infer that Io's exosphere is temporally variable.     

Plasma observations near jupiter: initial results from voyager 1

Extensive measurements of low-energy positive ions and electrons were made throughout the Jupiter encounter of Voyager 1. The bow shock and magneto-pause were crossed several times at distances consistent with variations in the upstream solar wind pressure measured on Voyager 2. During the inbound pass, the number density increased by six orders of magnitude between the innermost magnetopause crossing at approximately 47 Jupiter radii and near closest approach at approximately 5 Jupiter radii; the plasma flow during this period was predominately in the direction of corotation. Marked increases in number density were observed twice per planetary rotation, near the magnetic equator. Jupiterward of the Io plasma torus, a cold, corotating plasma was observed and the energylcharge spectra show well-resolved, heavy-ion peaks at mass-to-charge ratios A/Z* = 8, 16, 32, and 64.     

Io: evidence for silicate volcanism in 1986

Infrared observations of Io during the 1986 apparition of Jupiter indicate that a large eruptive event occurred on the leading side of Io on 7 August 1986, Universal Time. Measurements made at 4.8, 8.7, and 20 micrometers suggest that the source of the event was about 15 kilometers in radius with a model temperature of approximately 900 Kelvin. Together with previously reported events, these measurements indicate that high-temperature volcanic activity on the leading side of Io may be more frequent than previously thought. The inferred temperature is significantly above the boiling point of sulfur in a vacuum(715 Kelvin) and thus constitutes strong evidence for active silicate volcanism on the surface of Io.     

Extreme ultraviolet observations from voyager 1 encounter with saturn

The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.     

Radio science with voyager at jupiter: initial voyager 2 results and a voyager 1 measure of the io torus

Voyager 2 radio signals were observed essentially continuously during a grazing occultation of the spacecraft by the southern limb of Jupiter. Intensity data show a classic atmospheric occultation profile and the effects of turbulence and ionospheric focusing and defocusing. No reliable profile of the neutral atmosphere has yet been obtained, primarily because of a combination of large trajectory uncertainties and error multiplication effects associated with the grazing geometry of the Voyager 2 occultation. Analysis of the dispersive ionospheric refraction data yields preliminary profiles for the topside ionosphere at 66.7 degrees S (entry in the evening) and 50.1 degrees S (exit in the morning) that are reversed with respect to corresponding Voyager 1 profiles in terms of plasma concentration at a fixed altitude. Plasma scale heights and temperatures of 880 kilometers, 1200 K and 1040 kilometers, 1600 K were obtained for morning and evening conditions, respectively. Preliminary reduction of the pre-encounter occultation of Voyager 1 by the Io torus yields an average plasma density of about 1000 electrons per cubic centimeter.     

Planetary radio astronomy observations from voyager 2 near jupiter

The Voyager 2 Planetary Radio Astronomy experiment to Jupiter has confirmed and extended to higher zenomagnetic latitudes results from the identical experiment carried by Voyager 1. The kilometric emissions discovered by Voyager 1 often extended to 1 megahertz or higher on Voyager 2 and often consisted of negatively or, less frequently, positively drifting narrowband bursts. On the basis of tentative identification of plasma wave emissions similar to those detected by Voyager 1, the plasma torus associated with Io appeared somewhat denser to Voyager 2 than it did to Voyager 1. We report here on quasiperiodic sinusoidal or impulsive bursts in the broadcast band range of wavelengths (800 to 1800 kilohertz). A Faraday effect appears at decametric frequencies, which probably results from propagation of the radiation near its sources on Jupiter. Finally, we discuss the occurrence of decametric emission in homologous arc families.     

The jupiter system through the eyes of voyager 1

The cameras aboard Voyager 1 have provided a closeup view of the Jupiter system, revealing heretofore unknown characteristics and phenomena associated with the planet's atmosphere and the surfaces of its five major satellites. On Jupiter itself, atmospheric motions-the interaction of cloud systems-display complex vorticity. On its dark side, lightning and auroras are observed. A ring was discovered surrounding Jupiter. The satellite surfaces display dramatic differences including extensive active volcanismn on Io, complex tectonism on Ganymnede and possibly Europa, and flattened remnants of enormous impact features on Callisto.     

The hot plasma environment at jupiter: ulysses results

Measurements of the hot plasma environment during the Ulysses flyby of Jupiter have revealed several new discoveries related to this large rotating astrophysical system. The Jovian magnetosphere was found by Ulysses to be very extended, with the day-side magnetopause located at approximately 105 Jupiter radii. The heavy ion (sulfur, oxygen, and sodium) population in the day-side magnetosphere increased sharply at approximately 86 Jupiter radii. This is somewhat more extended than the "inner" magnetosphere boundary region identified by the Voyager hot plasma measurements. In the day-side magnetosphere, the ion fluxes have the anisotropy direction expected for corotation with the planet, with the magnitude of the anisotropy increasing when the spacecraft becomes more immersed in the hot plasma sheet. The relative abundances of sulfur, oxygen, and sodium to helium decreased somewhat with decreasing radial distance from the planet on the day-side, which suggests that the abundances of the Jupiter-derived species are dependent on latitude. In the dusk-side, high-latitude region, intense fluxes of counter-streaming ions and electrons were discovered from the edge of the plasma sheet to the dusk-side magnetopause. These beams of electrons and ions were found to be very tightly aligned with the magnetic field and to be superimposed on a time- and space-variable isotropic hot plasma background. The currents carried by the measured hot plasma particles are typically approximately 1.6 x 10(-4) microamperes per square meter or approximately 8 x 10(5) amperes per squared Jupiter radius throughout the high-latitude magnetosphere volume. It is likely that the intense particle beams discovered at high Jovian latitudes produce auroras in the polar caps of the planet.     

Sulfur volcanoes on io

Widespread volcanism on Jupiter's satellite Io, if it occurred over the age of the solar system, would quickly reduce the inventory of most common volatiles needed to drive such volcanism. One exception is the volatile element sulfur. It is therefore postulated that sulfur is the driving volatile for Ionian volcanism. Its presence is consistent with a carbonaceous-chondrite-like bulk composition for the original material that formed Io 4.5 billion years ago. The ubiquity of sulfur on Io today demonstrates the importance of this element in the processes that formed its surface.     

A primordial origin of the Laplace relation among the Galilean satellites

Understanding the origin of the orbital resonances of the Galilean satellites of Jupiter will constrain the longevity of the extensive volcanism on Io, may explain a liquid ocean on Europa, and may guide studies of the dissipative properties of stars and Jupiter-like planets. The differential migration of the newly formed Galilean satellites due to interactions with a circumjovian disk can lead to the primordial formation of the Laplace relation n(1) - 3n(2) + 2n(3) = 0, where the n(i) are the mean orbital angular velocities of Io, Europa, and Ganymede, respectively. This contrasts with the formation of the resonances by differential expansion of the orbits from tidal torques from Jupiter.     

Io: longitudinal distribution of sulfur dioxide frost

Twenty spectra of Io (0.26 to 0.33 micrometer), acquired with the International Ultraviolet Explorer spacecraft, have been studied. There is a strong ultraviolet absorption shortward of 0.33 micrometer that is consistent with earlier ground-based spectrophotometry; its strength is strongly dependent on Io's rotational phase angle at the time of observation. This spectral feature and its variation are interpreted as indicative of a longitudinal variation in the distribution of sulfur dioxide frost on Io. The frost is most abundant at orbital longitudes 72 degrees to 137 degrees and least abundant at longitudes 250 degrees to 323 degrees . Variations in spectral reflectivity between 0.4 and 0.5 micrometer, reported in earlier ground-based spectral studies, correlate inversely with variations in reflectivity between 0.26 and 0.33 micrometer. It is concluded that this is because the Io surface component with the highest visible reflectivity (sulfur dioxide frost) has the lowest ultraviolet reflectivity. At least one other component is present and may be sulfur allotropes or alkali sulfides. This model is consistent with ground-based ultraviolet, visible, and infrared spectrophotometry. Comparison with Voyager color photographs indicates that the sulfur dioxide frost is in greatest concentration in the "white" areas on Io and the other sulfurous components are in greatest concentration in the "red" areas.     

Voyager 1: energetic ions and electrons in the jovian magnetosphere

The observations of the cosmic-ray subsystem have added significantly to our knowledge of Jupiter's magnetosphere. The most surprising result is the existence of energetic sulfur, sodium, and oxygen nuclei with energies above 7 megaelectron volts per nucleon which were found inside of Io's orbit. Also, significant fluxes of similarly energetic ions reflecting solar cosmic-ray composition were observed throughout the magnetosphere beyond 11 times the radius of Jupiter. It was also found that energetic protons are enhanced by 30 to 70 percent in the active hemisphere. Finally, the first observations were made of the magnetospheric tail in the dawn direction out to 160 Jupiter radii.     

Gravitational parameters of the jupiter system from the Doppler tracking of pioneer 10

Preliminary analyses of Doppler data from Pioneer 10 during its encounter with Jupiter indicate that the mass of Io is about 20 percent greater than previously thought and that Io's mean density is about 3.5 grams per cubic centimeter. A determination of the dynamical flattening of Jupiter (a - b)/a (where a is the semimajor axis and b is the semiminor axis) is found to lie in the neighborhood of 0.065, which agrees with the value determined from satellite perturbations.     

Extreme ultraviolet observations from voyager 2 encounter with jupiter

Extreme ultraviolet spectral observations of the Jovian planetary system made during the Voyager 2 encounter have extended our knowledge of many of the phenomena and physical processes discovered by the Voyager 1 ultraviolet spectrometer. In the 4 months between encounters, the radiation from Io's plasma torus has increased in intensity by a factor of about 2. This change was accompanied by a decrease in plasma temperature of about 30 percent. The high-latitude auroral zones have been positively associated with the magnetic projection of the plasma torus onto the planet. Emission in molecular hydrogen bands has been detected from the equatorial regions of Jupiter, indicating planetwide electron precipitation. Hydrogen Lyman alpha from the dark side of the planet has been measured at an intensity of about 1 kilorayleigh. An observation of the occultation of alpha Leonis by Jupiter was carried out successfully and the data are being analyzed in detail.     

Ulysses radio and plasma wave observations in the jupiter environment

The Unified Radio and Plasma Wave (URAP) experiment has produced new observations of the Jupiter environment, owing to the unique capabilities of the instrument and the traversal of high Jovian latitudes. Broad-band continuum radio emission from Jupiter and in situ plasma waves have proved valuable in delineating the magnetospheric boundaries. Simultaneous measurements of electric and magnetic wave fields have yielded new evidence of whistler-mode radiation within the magnetosphere. Observations of aurorallike hiss provided evidence of a Jovian cusp. The source direction and polarization capabilities of URAP have demonstrated that the outer region of the lo plasma torus supported at least five separate radio sources that reoccurred during successive rotations with a measurable corotation lag. Thermal noise measurements of the lo torus densities yielded values in the densest portion that are similar to models suggested on the basis of Voyager observations of 13 years ago. The URAP measurements also suggest complex beaming and polarization characteristics of Jovian radio components. In addition, a new class of kilometer-wavelength striated Jovian bursts has been observed.