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At least 379 records · Page 21

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NASA NTRS1987

Sputtering of sulfur by kiloelectronvolt ions - Application to the magnetospheric plasma interaction with Io

Accurate measurements of the yields, mass spectra, and energy spectra of ejected sulfur are presented based on vapor deposits of sulfur at temperatures and ion energies relevant to the plasma interaction with the surface of Io. The measured sputtering yields are much lower than previous estimates for room temperature sulfur films, but are comparable to previous measurements of low-temperature keV ion sputtering of SO2. Results suggest that if ions reach the surface of Io its atmosphere will have a nonnegligible sulfur component which is primarily S2. Comparison of injection rates determined for sulfur with those for SO2 indicates that injection from sulfur deposits contributes 13 percent to the total mass injection rate of about 2-3 x 10 to the 29th amu/sec.

Chrisey, D. B.↗

NASA NTRS1987

Subsidence of topography on Io

The underlying roots of Io's topographic features are softened and eroded by contact with the hot mantle, resulting in a subsidence which is analogous to the progress of a butter pat on a frying pan. This process would be offset by crustal thickening due to continuing volcanism if the rate for this phenomenon were more than the observed 1 cm/year or less. Because the crustal thinning would occur at about 50 cm/year if the material underneath were a pure magma ocean, Io cannot have a global magna ocean, and interior viscosities greater than about 10 to the 10th P are implied.

Webb, Erik K.↗

NASA NTRS1987

Volcanic control of the Io atmosphere and neutral and plasma torus

Rate equations for the column density of Io's atmosphere and the number density of neutral atoms and ions in the torus have been formulated in terms of surface sublimation and venting of volcanic gases, sputtering efficiency, thermal escape, and the plasma processes that take place in the torus which included electron impact ionization, charge exchange, and transport. In the limit of low volcanic activity, estimates of the densities can be obtained, and the baseline configuration that would exist in the absence of volcanic activity is found. Solution of the coupled steady state rate equations shows that the densities are sensitive, to a varying degree, to the level of Io volcanism. The steady state densities predicted by the model are consistent with a volcanic flux of no more than 10 to the 12th/sq cm per sec. This is significantly lower than the values obtained on the basis of resurfacing rates and appears to imply that surface lava flows play a major role in the resurfacing process.

Eviatar, Aharon↗

NASA NTRS1988

A modeling analysis program for the JPL table mountain Io sodium cloud data

Research in the third and final year of this project is divided into three main areas: (1) completion of data processing and calibration for 34 of the 1981 Region B/C images, selected from the massive JPL sodium cloud data set; (2) identification and examination of the basic features and observed changes in the morphological characteristics of the sodium cloud images; and (3) successful physical interpretation of these basic features and observed changes using the highly developed numerical sodium cloud model at AER. The modeling analysis has led to a number of definite conclusions regarding the local structure of Io's atmosphere, the gas escape mechanism at Io, and the presence of an east-west electric field and a System III longitudinal asymmetry in the plasma torus. Large scale stability, as well as some smaller scale time variability for both the sodium cloud and the structure of the plasma torus over a several year time period are also discussed.

Smyth, William H.↗

NASA NTRS1987

Eclipse measurements of Io's sodium atmosphere

The satellites of Jupiter eclipsed each other in 1985, and these events allowed an unusual measurement of the sodium in Io's extended atmosphere. Europa was used as a mirror to look back through the Io atmosphere at the sun. The measured column abundances suggest that the atmosphere is collisionally thin above 700 km and may be collisionally thin to the surface. The sodium radial profile above 700 km resembles a 1500 K exosphere with a surface density near 20,000 sodium atoms per cubic centimeter, but a complete explanation of the dynamics requires a more complex nonthermal model: the calculated loss rates suggest that the atmosphere is being replaced on a time scale of hours.

Schneider, Nicholas M.↗

NASA NTRS1988

A comprehensive model of ion diffusion and charge exchange in the cold Io torus

A comprehensive analytic model of radial diffusion in the cold Io torus is developed. The model involves a generalized molecular cloud theory of SO2 and its dissociation fragments SO, O2, S, and O, which are formed at a relatively large rate by solar UV photodissociation of SO2. The key component of the new theory is SO, which can react with S(+) through a near-resonant charge exchange process that is exothermic. This provides a mechanism for the rapid depletion of singly ionized sulfur in the cold torus and can account for the large decrease in the total flux tube content inward of Io's orbit. The model is used to demonstrate quantitatively the effects of radial diffusion in a charge exchange environment that acts as a combined source and sink for ions in various charge states. A detailed quantitative explanation for the O(2+) component of the cold torus is given, and insight is derived into the workings of the so-called plasma 'ribbon'.

Barbosa, D. D.↗

NASA NTRS1988

Tidal dissipation, surface heat flow, and figure of viscoelastic models of Io

The deformation of Io, the tidal dissipation rate, and its interior spatial distribution are investigated by means of numerical simulations based on (1) a three-layer model (with dissipation in the mantle) or (2) a four-layer model (with dissipation in the asthenosphere). The mathematical derivation of the models is outlined; the selection of the input-parameter values is explained; the results are presented in extensive graphs and contour maps; and the constraints imposed on the models by observational data on the hot-spot distribution, tidal deformation, and gravity field are discussed in detail. It is found that both dissipation mechanisms may play a role on Io: model (2) is better able to explain the concentration of hot spots near the equator, while the presence of a large hot spot near the south pole (if confirmed by observations) would favor model (1).

Segatz, M.↗

NASA NTRS1988

Infrared observations of eclipses of Io, its thermophysical parameters, and the thermal radiation of the Loki volcano and environs

Observations of Io during eclipses by Jupiter in 1981-1984 are reported. Data obtained at 3.45-30 microns using bolometer system No. 1 on the 3-m IRTF telescope at Mauna Kea are presented in extensive tables and graphs and analyzed by means of least-squares fitting of thermophysical models to the eclipse cooling and heating curves, thermal-radiation calculations for the Io volcanoes, and comparison with Voyager data. Best fits are obtained for a model comprising (1) a bright region with a vertically inhomogeneous surface and (2) a dark vertically homogeneous region with thermal inertia only about 0.1 times that of (1). Little evidence of volcanic-flux variability during the period is found, and the majority (but not all) of the excess thermal IR radiation in the sub-Jovian hemisphere is attributed to the Loki volcano and its lava lake.

Sinton, William M.↗

NASA NTRS1988

Ejection of sodium from sodium sulfide by the sputtering of the surface of Io

The mechanism by which Na is removed from the surface of Io prior to its injection into the plasma torus is investigated experimentally. Na2S films of thickness 3-8 microns were produced by spray coating an Ni substrate in a dry N2 atmosphere and subjected to sputtering by 34-keV Ar(+), Ne(+), Kr(+), or Xe(+) ions up to total doses of about 5 x 10 to the 18th ions/sq cm. The sputtering yields and mass spectra are found to be consistent with ejection of only small amounts of atomic Na and somewhat larger amounts of Na-containing molecules. It is concluded that the amount of Na ejected by magnetospheric-ion sputtering of Na2S would be insufficient to account for the amounts observed in the Io neutral cloud. A scenario involving sputtering of larger polysulfide molecules is considered.

Chrisey, D. B.↗

NASA NTRS1988

Two classes of models for temporal variability of the Io torus

Two classes of spatially uniform models for the Io torus are analyzed for the stability and temporal variability of the torus. It is shown that stable-equilibrium torus models exist in two classes of models, one with nonlinear radial transport loss of plasma (e.g., from centrifugally driven interchanges) and one with linear transport (e.g., from externally driven interchanges). The two classes of models make substantially different predictions regarding temporal variability of the Io torus. Observational tests are proposed that make it possible to discriminate between the two classes.

Cheng, Andrew F.↗

NASA NTRS1988

Background heatflow on hotspot planets - Io and Venus

It is suggested that there is no simple relationship between lithospheric thickness and heatflow on planets where volcanism dominates the heatflow. This applies locally and globally, even away from regions of volcanic activity. This indicates that there is no basis for the assumption that the Io heatflow is as low as (or lower than) the hotspot component alone would suggest. A model is presented to describe the heatflow on hotspot planets. The model is applied to Io and Venus.

Stevenson, David J.↗

NASA NTRS1989

High-resolution infrared spectroscopy of Io and possible surface materials

A comparison of new spectra of Io with laboratory-simulated frosts confirms that the dominant materials on Io are SO2 frost in conjunction with a spectrally neutral material presumed to be sulfur. While the 4-micron region spectra are largely explainable in these terms, attention is drawn to a shoulder in the spectrum at 4.04 microns that is suggestive of adsorbed SO2 gas; two shallow, unidentified bands are also noted at 3.85 and 3.91 microns. The isotopic ratios of oxygen and sulfur appear to be normal. The absence of distinct bands in the new spectra in the 5-micron region limits the abundance of sulfate and sulfite compounds.

Howell, Robert R.↗

NASA NTRS1989

Charge exchange cross sections for the Io plasma torus

An impact parameter method for calculating cross sections as a function of incident ion energy is used in conjunction with an improved exchange energy formulation to update several of the charge exchange cross sections currently used in Io plasma torus modeling. New cross sections for S(+) + S(2+) yielding S(2+) + S(+) and Na(+) on neutral targets, useful in analyzing the fast Na jets observed at Io, are also calculated.

Mcgrath, M. A.↗

NASA NTRS1989

The surface of Io - A new model

The role of elemental sulfur on Io is evaluated in light of duplications of its spectral properties by combinations of basalt and condensates of SO2 and its S2O and polysulfur oxide (PSO) dissociation products. Elemental sulfur is not seen to be present in spectrally significant amounts. It is instead suggested that Io's exposed surface consists of mafic silicates which have been partially covered by thin deposits of SO2, PSO, and S2O. A model in which most of the spectrally active frost occurs in the form of thin and ephemeral partial coatings on the topmost regolith particles is seen as most consistent with observations. Attention is drawn to similarities with the surface of Europa.

Hapke, Bruce↗

NASA NTRS1990

The 2.5-5.0 micron spectra of Io - Evidence for H2S and H2O frozen in SO2

The present analysis of IR spectra of Io in the 2.5-5.0 micron region with laboratory data on plausible surface ices gives attention to four unidentified features exhibiting spatial and temporal band strength variations; these fall close to the fundamental stretching modes in H2S and H2O, respectively. It is suggested that the 3.85- and 3.91-micron bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration in H2S clusters and isolated molecules in an SO2-dominated ice. The observations resemble the transmission spectra of SO2 ices that contain about 3 percent H2S and 0.1 percent H2O.

Salama, F.↗

NASA NTRS1990

Observations of industrial sulfur flows - Implications for Io

A study of industrial liquid sulfur flows at about 130 C yields information on the thermal and rheological characteristics of the sulfur encountered in Io's volcanic activity. As the local sulfur flow cools and/or the local flow rate decreases, the formation of a crust over much of the surface does not prevent a continuation of flow underneath. Subsequent rapid development of a durable crust implies the formation of extensive crusts on Ionian sulfur flows, significantly reducing heat loss and extending flow length. It is suggested on the basis of the present laboratory results that the colors of proposed sulfur flows on Io may correlate with neither the temperature nor the inferred rheological properties attributed to the colors.

Greeley, Ronald↗

NASA NTRS1990

Internal structure of Io and the global distribution of its topography

A global topography is presently calculated for two multilayer Io models in which dissipation occurs in a viscous asthenosphere and a solid mantle: (1) a 'thermal swell' model, in which topography and heat flow are positively correlated, and (2) a 'differentiated lithosphere' model, in which topography and heat flow are negatively correlated. Both the polar topography and the hypsometric distribution of elevations in the differentiated lithosphere model are better matched with observations than the thermal swell model. The shift of the equatorial basin-swell pattern indicates a recent zonal rotation of about 25 deg for Io's lithosphere.

Ross, M. N.↗

NASA NTRS1990

The atmospheric abundance of SO2 on Io

The IUE satellite has obtained near-UV spectra of Io with sufficient resolution to ascertain the east, or leading and west, or trailing hemispheres' dayside atmosphere SO2 abundance. The derived geometric albedos are compared with various model albedos that might result from proposed SO2 atmospheres, as well as from localized, sublimation- or volcanism-generated atmospheres. A homogeneous-layer alternative atmosphere is introduced whose upper limit on the average SO2 column density for both hemispheres implies that a collisionally thick SO2 atmosphere of intermediate density may have been present on Io's dayside during the present observations.

Ballester, Gilda E.↗