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At least 343 records · Page 19
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NASA NTRS1983

A deficiency of O III in the Io plasma torus

Evidence for a deficiency of O III ions in the Io plasma torus is reported and implications of this deficiency for the physical processes controlling the plasma are considered. Observations of the O III 5007-A as well as Cl III and S III emissions from the Io plasma torus were made by a ground-based echelle spectrograph and intensified Reticon detector in February and May, 1981. The O III observations allow an upper limit of 4/cu cm to be placed on torus O III abundance, which is inconsistent with expectations for a low density plasma controlled by electron collisions. The inclusion of ion-ion and ion-atom charge exchange reactions and a depleted high energy electron component in the model is found to suppress O III levels, however observed limiting values are only achieved if it is assumed that the O III is kinetically hot. In addition, the charge-exchange model developed is inconsistent with previous observations of the kinetics of the S II-S III system. The present observations also establish upper limits of 2 R on 5518-A and 5538-A Cl III emission, and an emission rate of 58 + or - 40 R for the S III 6312-A line in the hot torus.

Brown, R. A.↗
NASA NTRS1983

The problem of cooling the cold Io torus

Two models are developed for the transport of ions inward from Io's orbit, the first using the assumption that radial diffusion is the dominant transport mechanism while the second uses a combination of diffusive and convective transport. The models include thermal and number density transport, radiation, ionization and pickup of local neutrals, recombination, charge exchange, and Coulomb interactions. It is found that pure diffusive transport can account for the dramatic inward depletion of the torus only by invoking recombination or by postulating a massive increase in the production rate of torus ions sometime prior to the Voyager mission. Recombination is determined to be far too slow to be the cause of the observed density decrease inside of Io. The model combining convection and diffusion is shown to reasonably match the data, but only if a diffusion coefficient 100 times less than that derived from Pioneer observations is used. In addition, it is shown that the Pioneer derived diffusion rate combined with Voyager temperature and density measurements indicate a large nonradiative sink of energy in the inner torus.

Richardson, J. D.↗
NASA NTRS1983

Observed departure of the Io plasma torus from rigid corotation with Jupiter

A study of forbidden S II red doublet spectra indicates that the Io plasma torus at 5.9 Jupiter radii does not corotate rigidly with Jupiter. The lag is found to be 6 percent + or 4 percent, where the variability range, not the uncertainty, is indicated. Comparison with existing models indicates the lag may be due primarily to ion creation in the Io torus.

Brown, R. A.↗
NASA NTRS1983

Io: Escape and ionization of atmospheric gases

The partitioning of the major ion densities throughout the torus is described. This description is required as input information for the Io oxygen and sulfur cloud models. In the radial interval from 4.9 to 5.4 Rj ion partitioning information is used to initially explore that impact of charge exchange reactions between the neutral gas clouds and the plasma torus ions. Because of the spatial distribution of torus ions, these reactions may be able to introduce a magnetic longitudinal effect in the plasma torus properties. Modeling efforts for the Io oxygen and sulfur clouds and for the sodium cloud data are also discussed.

Smyth, W. H.↗
NASA NTRS1984

Modeling Io volcanism: Maximum volcanic temperatures, depths of melting and magma composition

Interim results of thermal and structural modeling of volcanism on Io were presented. The final results of the modeling are summarized. The basic analysis is an evaluation of the magma trigger mechanism for initiating and maintaining eruptions. Secondary aspects include models of the mechanical mode of magma emplacement, interactions with a sulphur-rich upper crust, and more speculative implications for Io's volcanism.

Crumpler, L. S.↗
NASA NTRS1984

Short time-scale surface changes on Io

Imaging data from the Voyager 1 and 2 encounters with the Jupiter system provide a data set for the examination of short time-scale variations of surface features on Io. Clear evidence exists for variations near the known eruption sites and for other areas which appeared to have erupted between the encounters. Regions outside the known active eruption sites were examined in order to look for variations in the surface scattering properties which is due to undetected small-scale volcanic activity. The phase functions of many areas are intercompared in order to look for regions with phase functions outside the normal range for satellite surface properties. Areas with unusual scattering properties are related to small-scale eruptions of gas or particles. Determination of the distribution of these areas has strong implications for the resurfacing rates for Io.

Terrile, R. J.↗
NASA NTRS1983

Coulomb thermal properties and stability of the Io plasma torus

Coulomb collisional energy exchange rates are computed for a model of the Io plasma torus consisting of newly created pickup ions, a background of thermally degraded intermediary ions, and a population of cooler electrons. The electrons are collisionally heated by both the pickup ions and background ions and are cooled by electron impact excitation of plasma ions which radiate in the EUV. It is found that a relative concentration of S III pickup ions forbidden S III/electrons = 0.1 with a temperature of 340 eV can deliver energy to the electrons at a rate of 3 x 10 to the -13th erg/cu cm per sec, sufficient to power the EUV emissions in the Io torus. The model predicts a background ion temperature Ti of about 53 eV and an electron temperature Te of about 5.5 eV on the basis of steady-state energy balance relations at Coulomb rates. The model also predicts electron temperature fluctuations at the 30 percent level on a time scale of less than 11 hours, consistent with recent observations of this phenomenon.

Barbosa, D. D.↗
NASA NTRS1983

The reflection spectrum of liquid sulfur - Implications for Io

The spectral reflectance from 0.38 to 0.75 micron of a column of liquid sulfur has been measured at several temperatures between the melting point (approximately 118 C) and 173 C. Below 160 C the spectral reflectance was observed to vary reversibly as a function of temperature, indepenent of the previous thermal history of the column. Once the temperature exceeded 160 C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10-19 percent). Combining this information with Voyager spectrophotometry of Jupiter's satellite Io, it is concluded that liquid sulfur at any temperature on Io's surface would be classified as a 'black area' according to the standards used by the Voyager imaging team in their spectrophotometric analysis (Soderblom, Johnson, Morrison, Danielson, Smith, Veverka, Cook,Sagan, Kupferman, Pieri, Mosher, Avis, Gradie, and Clancy /1980/.

Nelson, R. M.↗
NASA NTRS1984

Escape of sulfur and oxygen from Io

Charged particle sputtering is reexamined as a mechanism for escape of sulfur and oxygen from Io. Corotating ions and energetic ions are considered, using the best available determinations of ion fluxes and sputtering yields. It is unclear whether molecular SO2 is the dominant escaping neutral from Io or whether atomic O and S are dominant, but the Voyager detection of SO2(+) in the cold torus implies the presence of molecular SO2 there. Thin atmosphere models encounter serious difficulty in achieving adequate sulfur and oxygen escape rates, but they are not yet ruled out. Ionization and dissociation of SO2 in a sputter corona can dominate direct escape. In thick atmosphere models the neutral escape rates can be much higher, but the oxygen escape rate tends to be too high relative to the sulfur escape rate unless diffusive separation in the atmosphere is limited, perhaps by rapid mixing.

Cheng, A. F.↗
NASA NTRS1984

No sulfur flows on Io

Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.

Young, A. T.↗
NASA NTRS1984

Studies of proton-irradiated SO2 at low temperatures Implications for Io

The infrared absorption spectrum from 3.3 to 27 microns of SO2 ice films has been measured at 20 and 88 K before and after 1-MeV proton irradiation. The radiation flux was chosen to simulate the estimated flux of Jovian magnetospheric 1-MeV protons incident on Io. After irradiation, SO3 is identified as the dominant molecule synthesized in the SO2 ice. This is also the case after irradiation of composite samples of SO2 with sulfur or disulfites. Darkening was observed in irradiated SO2 ice and in irradiated S8 pellets. Photometric and spectral measurements of the thermoluminescence of irradiated SO2 have been made during warming. The spectrum appears as a broad band with a maximum at 4450 A. Analysis of the luminescence data suggests that at Ionian temperatures irradiated SO2 ice would not be a dominant contributor to posteclipse brightening phenomena. After warming to room temperature, a form of SO3 remains along with a sulfate and S8. Based on these experiments, it is reasonable to propose that small amounts of SO3 may exist on the surface of Io as a result of irradiation synthesis in SO2 frosts.

Moore, M. H.↗
NASA NTRS1984

Sulfur volcanoes on Io?

The unusual rheological properties of sulfur are discussed in order to determine the distinctive volcanic flow morphologies which indicate the presence of sulfur volcanoes on the Saturnian satellite Io. An analysis of high resolution Voyager imagery reveals three features which are considered to be possible sulfur volcanoes: Atar Patera, Daedalus Patera, and Kibero Patera. All three features are distinguished by circular-to-oval central masses surrounded by irregular widespread flows. The central zones of the features are interpreted to be domes formed of high temperature sulfur. To confirm the interpretations of the satellite data, molten sulfur was extruded in the laboratory at a temperature of 210 C on a flat surface sloping 0.5 deg to the left. At this temperature, the sulfur formed a viscous domelike mass over the event. As parts of the mass cooled to 170 C the viscosity decreased to a runny stage, forming breakout flows. It is concluded that a case can be made for sulfur volcanoes on Io sufficient to warrant further study, and it is recommended that the upcoming Galileo mission examine these phenomena.

Greeley, R.↗
NASA NTRS1984

Voyager disk-integrated photometry of Io

The results of a comprehensive analysis of disk-integrated photometry of Io derived from Voyager images are presented. Phase curves, rotation curves, and geometric albedos consistent with earth-based observations of Io are obtained, except for a previously noted tendency for the Voyager data to yield somewhat redder colors. It is demonstrated that the phase coefficient decreases with increasing wavelength. The phase integral ranges from about 0.7 in the ultraviolet to about 0.8 in the orange. The Bond albedo is estimated at 0.50 + or -0.10, a value consistent with, though slightly below, previous earth-based estimates.

Simonelli, D. P.↗
NASA NTRS1985

Hot Spots on Io: Correlation of Infrared Emission and Visible Reflectance

The Voyager 1 infrared spectrometer (IRIS) data and two recently compiled data sets (Voyager imaging mosaics and measurements of Io's thermal emission from the NASA Infrared Telescope Facility) are correlated. These data were used to refine the correlation between dark spot optical properties (albedo and color) and thermal emission, to examine this correspondence on a satellite-wide scale, and to identify additional hot spots not included in the IRIS inventory. The results suggest the hot spots are liquid sulfur lava lakes, for the following reasons: (1) the melting point of sulfur is 390 K, and the model hot spot temperatures range from approximately 200 to 450 K; (2) the albedos and color of the dark spots, measured from the global mosaics, are consistent with laboratory measurements for liquid sulfur; (3) high resolution images of the dark features show morphologies suggestive of lava lakes; and (4) this hypothesis provides a simple and direct explanation for why dark spots are hot on Io.

Mcewen, A. S.↗
NASA NTRS1985

Topographic Evidence for Shield Volcanism on Io

A volcano was identified whose morphology, especially topography, supports and extends current evidence that shield volcanoes occur on Io. The feature is centered at 30 deg. S, 246 deg w and lies in a region near the terminator. The presence of shield volcanism on Io was inferred from the planimetric configuration of flaws on such features as Ra Patera. A central edifice roughly 40 to 50 km in diameter is situated on intervent plains material, and a broad, slightly elliptical base 75 to 90 km across. The size of this feature is comparable to large shield volcanoes found on Earth. In addition, the topography is similar to some terrestrial shield volcanoes that exhibit a change in slope; the steepening may be explained by shorter and/or thicker flows emplaced during late stage eruptions.

Albin, E. F.↗
NASA NTRS1985

Sulfur Oxygen Processes on Io

The presence of allotropic sulfur as a possible major constituent of Io's surface and the properties of sulfur in combination with various sulfur-oxygen compounds believed to be present on Io's surface and in its atmosphere are studied by a series of laboratory experiments.

Nelson, R. M.↗
NASA NTRS1985

Accurate Albedos of the Brightest Regions on Io

The brightest, coldest areas on Io, the white regions, may act as cold traps for SO2 gas, and thus have an important role in governing the pressure, diurnal variation, and flow of the satellite's tenuous SO2 atmosphere. Therefore, it is essential to derive accurate albedos for the brightest regions, where the necessary albedos are those in the energy balance equation of the surface used to compute temperatures. Forty-one of the brightest of the white areas, each 60 to 120 km on a side were studied. The simplest way to estimate the required energy balance albedo for each region is to determine the Bond slbedo of a planet covered with that type of material. This process is outlined and resulting albedos are given. with the exception of several darker regions on the poorly-resolved post eclipse face of Io, typical albedos are 0.6 to 0.7. The brightest areas studied are located in the cluster of white regions east of Prometheus (longitudes 90 to 40 deg W). It is possible using Voyager data and fits to Hapke's equation to derive albedos for the bright regions without making any assumptions about the phase integrals.

Simonelli, D. P.↗
NASA NTRS1985

Shape of Io

An approach for measuring the triaxial shape of Io from Voyager images is described. Triaxial shape of a satellite is detrmined by comparing the theoretical shape and location of the body's limb and terminator curves with the actual curves observed in the two dimensional images. Limb or terminator points are found by matching a model for the limb profile with the actual appearance of the limb in the digital data. The theoretical model for the limb model depends on the viewing and illumination geometry, the satellite reflection characteristics and the camera 'point spread function'. The method is applied to imaging data taken of Io in the last 25 hours befor Voyager 1's closet approach to the satellite.

Synnott, S. P.↗