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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Io's wobbling flux tube and nonuniform surface conductivity - Longitude control of decametric emission and other magnetospheric interactions

Study of systematic relations between Io's flux tube orientation, decametric emission control, and areal surface properties suggest a model that can account for longitude control of principal Io-associated decametric emissions and other observed Io/magnetosphere interactions. The model is based on the fact that Jupiter's magnetic field structure is dominated by a tilted dipole rotating at a different angular velocity than Io's orbital motion. This caused Io's flux tube near Io to wobble (precess) with respect to Io's rotational axis. Discrete contact junctions are invoked between the active current-sheet regions in the flux tube and Io's surface.

Nash, D. B.↗
NASA NTRS1983

Io's 4-micron band and the role of adsorbed SO2

The role of adsorbed SO2 on Io's surface particles in producing the observed spectral absorption band near 4 microns in Io's reflectance spectrum is explored. Calculations show that a modest 50 percent monolayer coating of adsorbed So2 molecules on submicron grains of sulfur or alkali sulfide, assumed to make up Io's uppermost optical surface ('radialith'), will result in a nu 1 + nu 3 absorption band near 4 microns with depth about 30 percent below the adjacent continuum, consistent with the observed strength of the Io band. The precise wavelength position of the nu 1 + nu 3 band of SO2 in different phase states such as frost, ice, adsorbate, and gas are summarized from the experimental literature and compared with the available telescopic measurements of the Io band position. The results suggest that the 4-micron band in Io's full disk spectrum can best be explained by the presence on Io's surface of widespread SO2 in the form of adsorbate rather than ice or frost.

Nash, D. B.↗
NASA NTRS1985

Hot-spot tectonics on Io

The thesis is that extensional tectonics and low-angle detachment faults probably occur on Io in association with the hot spots. These processes may occur on a much shorter timescale on Ion than on Earth, so that Io could be a natural laboratory for the study of thermotectonics. Furthermore, studies of heat and detachment in crustal extension on Earth and the other terresrial planets (especially Venus and Mars) may provide analogs to processes on Io. The geology of Io is dominated by volcanism and hot spots, most likely the result of tidal heating. Hot spots cover 1 to 2% of Io's surface, radiating at temperatures typically from 200 to 400 K, and occasionally up to 700K. Heat loss from the largest hot spots on Io, such as Loki Patera, is about 300 times the heat loss from Yellowstone, so a tremendous quantity of energy is available for volcanic and tectonic work. Active volcanism on Io results in a resurfacing rate as high as 10 cm per year, yet many structural features are apparent on the surface. Therefore, the tectonics must be highly active.

Mcewen, A. S.↗
NASA NTRS1986

Io

The present work reviews the history of Io studies and describes the current level of understanding of Io's physics, chemistry, geology, orbital dynamics, and geophysics. Consideration is given to the satellite's internal, superficial, atmospheric, plasma, and magnetospheric properties and how they interrelate. A pictorial map of Io's surface based on Voyager 1 and 2 images is presented. It is found that Io's surface color and spectra are dominated by sulfur compounds which may include various sulfur allotropes. Volcanic processes yielding three kinds of surface features (vent regions, plains, and mountains) dominate Io's surface geology. The Io plasma torus corotates with Jupiter's magnetic field in the plane of Jupiter's centrifugal equator centered at Io's orbital radius.

Nash, Douglas B.↗
NASA NTRS1999

Studies for the Loss of Atomic and Molecular Species from Io

A summary discussion of research undertaken in this project is presented and is related to six published papers attached in the appendix. The discussion is divided into six sections describing a variety of studies for the loss of atomic and molecular species from Io. They include studies for: (1) atomic sodium, (2) SO2, (3) O and S, (4) spectacular high-spatial resolution ultraviolet image observations of O, S, and possibly H in Io's atmosphere and/or corona acquired by the Space Telescope Imaging Spectrometer (STIS) of the Hubble Space Telescope (HST), (5) spectacular high-spatial resolution visible Io eclipse image observations acquired by the Solid State Imager (SSI) of Galileo spacecraft, (6) ground-based observations acquired by the Solid State Imager (SSI) of Galileo spacecraft, and (7) ground-based observations of Io's neutral cloud in [OI] 6300 angstrom emission. New source rates at Io's exobase for SO2, O, and H are given and a variety of interesting implications for Io's atmosphere and for the Io plasma torus are discussed. Appendices that are comprised of articles published during the contract are also presented.

Smyth, William H.↗
NASA NTRS1972

Io's triaxial figure.

Timings from photoelectric observations of four immersions and two emersions of the recent occultation of Beta Scorpii C by Io have made it possible to derive Io's apparent equatorial radius to an accuracy of 2 km. However, when the distortion of Io attributable to rotation and tides raised by Jupiter is considered, the derived mean radius becomes several kilometers less than the observed equatorial radius. If Io were a homogeneous fluid body in hydrostatic equilibrium and in synchronous rotation, it would show a bulge along a line to Jupiter about 20 km in radius greater than the polar radius. The mean radius thus derived is (1818 plus or minus 5) km and mean density (2.88 plus or minus 0.34) g/cu cm, the largest uncertainty being attributable to the value of Io's mass. The new value for Io's radius is higher than those previously determined and suggests about 30% lower values for mean densities of all Galilean satellites.

O'Leary, B.↗
NASA NTRS1977

Io's surface and the histories of the Galilean satellites

Similarities and differences among the Galilean satellites are discussed. A hypothesis is offered that the surface of Io is largely covered by 'evaporite' salts produced by defluidization of Io's interior, migration of salt-saturated solutions to Io's surface, and subsequent H2O loss to space. Laboratory reflectance studies show that evaporites constitute a good match to Io's spectrum in the infrared, in contrast to ices or frosts, the presence of which is not considered likely in view of the absence of near IR ice bands in Io's surface spectrum. Likely coloring agents in the blue include elemental sulfur, which may be produced from sulfates by proton irradiation or other processes, and F-centers produced by irradiation with magnetospheric protons. Preferential irradiation of material in the polar regions may account for Io's peculiar dark polar caps.

Fanale, F. P.↗
NASA NTRS1980

Standing Alfven wave current system at Io: Voyager 1 observations

The enigmatic control of the occurrence frequency of Jupiter's decametric emissions by the satellite Io is explained theoretically on the basis of its strong electrodynamic interaction with the corotating Jovian magnetosphere leading to field aligned currents connecting Io with the Jovian ionosphere. Direct measurements of the perturbation magnetic fields due to this current system were obtained by the magnetic field experiment on Voyager 1 on 5 March 1979 when it passed within 20,500 km south of Io. An interpretation in the framework of Alfven waves radiated by Io leads to current estimates of 2.8 million amps. A mass density of 7400 to 13600 proton mass units per Cu cm is derived which compares very favorably with independent observations of the torus composition characterized by 7-9 proton mass units per electron for a local electron density of 1050 to 1500 per cu cm. The power dissipated in the current system may be important for heating the Io heavy ion torus, inner magnetosphere, Jovian ionosphere, and possibly the ionosphere or even the interior of Io.

Acuna, M. H.↗
NASA NTRS1980

Mass-loading and diffusion-loss rates of the Io plasma torus

Limits to the mass-loading and diffusion-loss rates of ions in the Io plasma torus have been calculated on the assumption that observed optical emissions are controlled by electron-ion collisions. Calculations of the yield of emission from the vicinity of Io limit the mass-loading rate to the order of 10 to the 27th per s for S II or O II, on the grounds that electron-excited emissions associated with the location of Io have not been observed in the optical spectrum. This mass-loading limit is dependent on the assumptions that Io is the source of torus particles and that most of the neutral atoms are converted to ions within 1 R(J) of Io. According to the calculations presented below, the observed partitioning of sulfur ion species in the hot torus at the time of Voyager 1 encounter indicates that the diffusion-loss time of the ions is of the order of 1/D = 100 days. The two results limiting the mass-loading and diffusion-loss rates are compatible and suggest that the energy required to maintain the observed radiated power cannot be supplied by acceleration of ions formed at Io in Jupiter's rotating magnetic field.

Shemansky, D. E.↗
NASA NTRS1981

A survey of Io's potassium cloud

Io's potassium cloud exhibits spatial and temporal variations similar to those observed for Io's sodium cloud. Spectra from five apparitions show that the potassium cloud is elongated so that it extends forward from Io's leading, inner hemisphere and makes an angle with Io's orbit of 10-30 deg, slightly less than the angle for the sodium cloud. The potassium cloud is a long-lived phenomenon which undergoes periodic fluctuations in response to solar radiation pressure and the ionizing influence of Jupiter's plasma torus. These give rise to east-west and north-south asymmetry variations similar to those observed for the sodium cloud. Evidence for temporary jets of potassium streaming from Io have also been observed. These similarities with the sodium cloud suggest that both sodium and potassium are ejected from nearly the same regions of Io by the same physical mechanism.

Trafton, L.↗
NASA NTRS1981

Standing Alfven wave current system at Io - Voyager 1 observations

The enigmatic control of the occurrence frequency of Jupiter's decametric emissions by the satellite Io has been explained theoretically on the basis of its strong electrodynamic interaction with the corotating Jovian magnetosphere leading to field-aligned currents connecting Io with the Jovian ionosphere. Direct measurements of the perturbation magnetic fields due to this current system were obtained by the Goddard Space Flight Center magnetic field experiment on Voyager 1 on March 5, 1979, when it passed within 20,500 km south of Io. An interpretation in the framework of Alfven waves radiated by Io leads to current estimates of 2.8 x 10 to the 6th A. A mass density of 7400-13,600 proton mass units/cu cm is derived, which compares very favorably with independent observations of the torus composition characterized by 7-9 proton mass units per electron for a local electron density of 1050-1500/cu cm. The power dissipated in the current system may be important for heating the Io heavy ion torus, inner magnetosphere, Jovian ionosphere, and possibly the ionosphere or even the interior of Io.

Acuna, M. H.↗
NASA NTRS1982

Radio emission from Io

Radio observations of Io taken with the VLA at 2, 6, and 21 cm show flux densities of 11.6 + or - 0.2 mJy, and less than 0.5 mJy, respectively, where the latter is a 2 sigma upper limit. These flux densities correspond to Io disk brightness temperatures of T(b) (2 cm) = 98 + or - 17 K, T(b) (6 cm) = 85 + or - 16 K, and T(b) (21 cm) less than 400 K, respectively. These radio brightness temperatures are consistent with thermal emission expected from Io's surface on the basis of its infrared brightness temperature and its radio emissivity derived from radar studies. No evidence for nonthermal radio emission from Io is found, such as has been reported by Mingaliev et al (1979). By using a model for the generation of synchrotron emission at 21 cm by energetic electrons in a hypothetical Io magnetosphere, an upper limit of Io's dipole magnetic moment less than 2 x 10 to the 25th gauss/cu cm is found.

De Pater, I.↗
NASA NTRS1982

Io: Escape and ionization of atmospheric gases

The model for the Io oxygen cloud was improved and is now capable of calculating the two dimensional sky plane intensity for the 6300A, 1304A and 880A lines, where volume excitation and ionization rates are determined by impact collisions with Io plasma torus electrons. These three emission lines are those for which observations were performed by ground based, rocket, Earth orbiting satellites and Voyager spacecraft nstruments. Comparison of model results with observations at 6300A suggests an isotropic oxygen flux from Io of about (1.5 to 3.0) x 10 to the 9th power atoms cm/sec or an overall source rate of (0.6 to 1.2) x 10 to the 27th power atoms/sec. A model for the expected but yet undetected Io sulfur cloud was also developed and very preliminary results are discussed. Quantitative analysis of the Io sodium cloud focused upon the initial task of acquiring and preliminary evaluation of sodium cloud and Io plasma torus data.

Smyth, W. H.↗
NASA NTRS1983

Spectrophotometric studies of the Io Torus

A toroidal volume near Io's orbit is made luminous by multiple optical and ultraviolet line emissions excited by resonant scattering of sunlight and by electron collisions. These emitting atoms and ions have been lost from Io. The spectrophotometric measurements of these emissions and their physical interpretation are considered. It is now known that the flow of material from Io dominates the particle and energy budgets of the Jovian magnetosphere. The observed emitting species in the Io torus are examined, and the atomic clouds are discussed, taking into account morphology and kinematics, atomic cloud supply rates, ion-atom collisions, and charge-exchange collisions. Observations and studies concerning the plasma torus are reported, giving attention to the forbidden lines, the extreme ultraviolet lines, and aspects of ion temperature and spatial distribution. Two types of radial transport in the Io torus include the ballistic motion of neutrals escaping from Io and the cross-L transport of ions.

Brown, R. A.↗
NASA NTRS1983

Io's atmosphere - Pressure control by regolith cold trapping and surface venting

A new model for the basal pressure of Io's atmosphere is developed. This model takes into account the previously ignored fact that much of Io's surface has very high porosity, typically about 90 pct. Such porosity allows efficient subsurface cold trapping of atmospheric gases which tends to keep ambient surface pressures very low. SO2 is the only gas identified on Io, and the basal pressures for atmospheric models are usually pegged to local surface temperature via the SO2 vapor pressure equilibrium curve. Near Io's subsolar point the pressure in equilibrium with a surface SO2 frost deposit is about 1/10,000,000th bar. Porous surface models of the type developed invoke equilibrium with the colder, subsurface permafrost (at about 3-cm depth) and yield pressures of about 1/10 to the 12th bar. The subsurface cold trapping model explains many but not all observations relevant to Io's atmosphere. The new subsurface cold trapping model and the earlier surface frost equilibrium model, when taken together, provide lower and upper limits, respectively, on the basal SO2 pressure of Io's atmosphere.

Matson, D. L.↗
NASA NTRS1983

Magnetic drifts at Io - Depletion of 10-MeV electrons at Voyager 1 encounter due to a forbidden zone

A model for magnetic field lines is used to track the Voyager spacecraft trajectory past Io back to the equatorial plane in order to determine the cause of an observed depletion of 10 MeV electrons in Io's orbit. The field lines are in the vicinity of an Alfven wing, a noncompressive field perturbation. It is suggested that the Jovian magnetospheric drift features influence the behavior of particles around Io, which also has a zone of reduced convection velocity. An analytical model is developed that demonstrates that a forbidden zone may exist around Io due to the gradient drifts and reduced convection velocity, and projections of particles convecting to the Io surface agree well with the Voyager data for particle concentrations. Discrepancies in the measured 9 MeV fluxes with respect to Pioneer data indicate that a possible change in the Io conductance in the interval between the passages of the two spacecraft.

Goldstein, B. E.↗
NASA NTRS1986

On Io's control of Jovian decametric radio emissions

Io's control of Jovian decametric radio emission (DAM) has been attributed to Io distorting the electron distribution in the inner Jovian magnetosphere. Observations of Faraday rotation in DAM are used to determine the properties of the electron distribution before and after its interaction with Io. It is shown that there is an enhancement in the density of the energetic component in the Io plasma torus correlated with certain Jovian longitude. Io's interaction with this energetic component can produce heating of this component. The Io-controlled emission is attributed to enhanced emission from the heated electrons moving down the field lines to Jupiter.

Winglee, R. M.↗
NASA NTRS1986

In situ measurements of the plasma bulk velocity near the Io flux tube

Data obtained with the Voyager Plasma Science Experiment (PSE) during a flyby of the Jovian moon Io are analyzed to quantify the bulk plasma parameters near the Io flux tube. The PSE contains four modulated grid Faraday cups, three pentagonally shaped and the other circular. The path of the spacecraft around Jupiter and past Io ensured that the maximum flux was passing perpendicular to the spacecraft instruments when Voyager was closest to Io (20,000 km). A total of 11 high resolution (M mode) plasma spectra were obtained, revealing 6 species of ions: H(+), O(+), O(2+), S(+), S(2+) and S(3+). All species had the same bulk velocity and convected maxwellian distributions; the heavy ions species were all the same temperature. The mass density of the plasma near Io was 20,000-25,000 amu/cu cm and the electron density was 1300-1700/cu cm. Calculations of the Alfven speed yielded a value ranging from 250-350 km/sec and an Alfven Mach no. of 0.17-0.21. The data were equivalent to that expected from the flow of an incompressible fluid around a long cylinder with a radius about 1.25 that of Io's.

Barnett, A.↗