Studies of proton-irradiated SO2 at low temperatures: Implications for lo

The infrared absorption spectrum from 3.3 to 27 μm (3030-370 cm^?) 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 Å. 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.     

Mass-injection rate from Io into the Io plasma torus

Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 10³ kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%.     

H-implantation in SO2 and CO2 ices

Ices in the solar system are observed on the surface of planets, satellites, comets and asteroids where they are continuously subordinate at particle fluxes (cosmic ions, solar wind and charged particles caught in the magnetosphere of the planets) that deeply modify their physical and structural properties. Each incoming ion destroys molecular bonds producing fragments that, by recombination, form new molecules also different from the original ones. Moreover, if the incoming ion is reactive (H⁺, Oⁿ⁺, Sⁿ⁺, etc.), it can concur to the formation of new molecules.Those effects can be studied by laboratory experiments where, with some limitation, it is possible to reproduce the astrophysical environments of planetary ices.In this work, we describe some experiments of 15-100 keV H⁺ and He⁺ implantation in pure sulfur dioxide (SO₂) at 16 and 80 K and carbon dioxide (CO₂) at 16 K ices aimed to search for the formation of new molecules. Among other results we confirm that carbonic acid (H₂CO₃) is formed after H-implantation in CO₂, vice versa H-implantation in SO₂ at both temperatures does not produce measurable quantity of sulfurous acid (H₂SO₃). The results are discussed in the light of their relevance to the chemistry of some solar system objects, particularly of Io, the innermost of Jupiter's Galilean satellites, that exhibits a surface very rich in frost SO₂ and it is continuously bombarded with H⁺ ions caught in Jupiter's magnetosphere.     

Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science288, 1208-1210) suggest that S₂ may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO₂ frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S₂, S₃, S₄, SO, and S₂O and a decreased abundance of O and O₂ in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO₂ ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO “sticking” probabilities at Io's surface; in that case, the SO/SO₂ ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S₂, S₃, S₄, and S₂O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S₄ radicals that are created and temporarily preserved when sulfur vapor (predominantly S₂) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity—during periods when volcanic outgassing of S₂ (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased.     

Evolution of Io's volatile inventory

Using Voyager results, we have made crude estimates of the rate at which Io loses volatiles by a variety of processes to the surrounding magnetosphere for both the current SO₂-dominated atmosphere as well as hypothetical paleoatmospheres in which other gases, such as N₂, may have been the dominant constituent. Loss rates are strongly influenced by the surface pressure on the night side, the relationship between the exobase and the Jovian magnetospheric boundary, the exospheric temperature, and the peak altitudes reached by volcanic plumes. Several mechanisms make significant contributions to the prodigious rate at which Io is currently losing volatiles. These include: interaction of the magnetospheric plasma with volcanic plume particles and the background atmosphere; sputtering of ices on the surface, if the nightside atmospheric pressure is low enough; and Jeans' escape of O, a dissociation product of SO₂ gas. For paleoatmospheres, only the first two of these mechanisms would have been effective. However, they are capable of eliminating large amounts of N₂ and other volatiles from Io over the satellite's lifetime. Io could have also lost large amounts of water over its lifetime due to the extensive recycling of water between its upper and lower crust, with the partial dissociation of water vapor in silicate magma chambers initiating this loss process. Significant amounts of water may also have been lost as a result of the interaction of the magnetospheric plasma with water ice particles in volcanic plumes. Once an SO₂-dominated atmosphere becomes established, much water may have also been lost through the sputtering of surface water ice.     

The SO2 atmosphere and ionosphere of Io: Ion chemistry,atmospheric escape,and models corresponding to the Pioneer 10 radio occultation measurements

Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO₂Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO₂ atmosphere with a surface density of 14 × 10¹⁰ cm^?3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO₂⁺, the major ion produced, is converted rapidly to SO⁺ and in turn to S⁺ by reactions with the dissociation products of SO₂. Ion chemistry leads to the formation of S⁺ as the dominant ion at and above the ionospheric peak. Na⁺ would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S⁺ ions would escape from Io in the wake region at a rate of up to 10²⁶ sec^?1.     

Observational tests for sulfur allotropes on Io

One of the intrinsic properties of particulate sulfur allotropes is a change in UV-visible reflectivity with temperature change of the material. The surface of Io experiences temperature changes during eclipse which are sufficient to cause a detectable change in the spectral reflectivity of sulfur; thus, if the surface of Io is composed primarily of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance. We observed four eclipse reappearances during July and August of 1983 and saw no posteclipse brightening effects in filter bands selected for sensitivity to color changes in sulfur. Our model of the brightness change for S₈ (“yellow” sulfur) implies that this material covers less than 50% of Io's surface. Negative posteclipse brightening observations were also obtained with a filter chosen for the high contrast between SO₂ frost and the average albedo of the surface of Io at that wavelength. We conclude that no significant condensation of optically thick SO₂ occurred on the surface of Io during these eclipses.     

Sodium in the Jovian magnetosphere

Observations of sodium D-line emission from Io and the magnetosphere of Jupiter are reported. A disk-shaped cloud of sodium is found to exist in the Jovian magnetosphere with an inner edge at about 4R and an outer edge at about 10R . The gravitational scale height above the equatorial plane is a few Jovian radii. The data are interpreted in terms of a sputtering model, in which the sodium required to maintain the cloud is sputtered off the surface of Io by trapped energetic radiation-belt protons. Conditions on the atmospheric density are obtained. The Keplerian orbits attainable by such escaping sputtered atoms can provide the observed spatial distribution. The required 500-keV proton flux required to provide the 1–10 keV protons which will sputter the sodium at the surface of Io is consistent with the limiting trapped flux determined by ion-cyclotron turbulence.Publication No. 1410, Institute of Geophysics and Planetary Physics, University of California, Los Angeles 90024, Cal., U.S.A.     

Magnetosphere,exosphere, and surface of mercury

The discovery of an atomic sodium exosphere at Mercury raises the question of whether Mercury, like Io at Jupiter, can maintain a heavy ion magnetosphere. We suggest that it does, and that heavy ions (mainly Na⁺) from the exosphere are typically accelerated to keV energies and make important or dominant contributions to the mass (∼300 g sec⁻¹) and energy (∼3 × 10⁹W) budgets of the magnetosphere. The sodium supply to the exosphere is largely from within Mercury itself, with external sources like meteroid infall and the solar wind being relatively unimportant. Therefore Mercury is in the process of losing its semivolatiles. Photosputtering dominates charged particle sputtering and can maintain an adequate rate of Na ejection from the surface.     

Alkali and Chlorine Photochemistry in a Volcanically Driven Atmosphere on Io

Observations of the Io plasma torus and neutral clouds indicate that the extended ionian atmosphere must contain sodium, potassium, and chlorine in atomic and/or molecular form. Models that consider sublimation of pure sulfur dioxide frost as the sole mechanism for generating an atmosphere on Io cannot explain the presence of alkali and halogen species in the atmosphere—active volcanoes or surface sputtering must also be considered, or the alkali and halide species must be discharged along with the SO₂ as the frost sublimates. To determine how volcanic outgassing can affect the chemistry of Io's atmosphere, we have developed a one-dimensional photochemical model in which active volcanoes release a rich suite of S-, O-, Na-, K-, and Cl-bearing vapor and in which photolysis, chemical reactions, condensation, and vertical eddy and molecular diffusion affect the subsequent evolution of the volcanic gases. Observations of Pele plume constituents, along with thermochemical equilibrium calculations of the composition of volcanic gases exsolved from high-temperature silicate magmas on Io, are used to constrain the composition of the volcanic vapor. We find that NaCl, Na, Cl, KCl, and K will be the dominant alkali and chlorine gases in atmospheres generated from Pele-like plume eruptions on Io. Although the relative abundances of these species will depend on uncertain model parameters and initial conditions, these five species remain dominant for a wide variety of realistic conditions. Other sodium and chlorine molecules such as NaS, NaO, Na₂, NaS₂, NaO₂, NaOS, NaSO₂, SCl, ClO, Cl₂, S₂Cl, and SO₂Cl₂ will be only minor constituents in the ionian atmosphere because of their low volcanic emission rates and their efficient photochemical destruction mechanisms. Our modeling has implications for the general appearance, properties, and variability of the neutral sodium clouds and jets observed near Io. The neutral NaCl molecules present at high altitudes in atmosph eres generated by active volcanoes might provide the NaX⁺ ion needed to help explain the morphology of the high-velocity sodium “stream” feature observed near Io.     

Oxidation state of the atmosphere and crust of Venus from pioneer Venus results

Recent spacecraft observations of Venus permit a detailed model of sulfur chemistry in the atmosphere-lithosphere system. Pioneer Venus experiments confirm that, as predicted, COS and H₂S are dominant over SO₂ in the lower atmosphere, and that the equilibrium concentrations of S₂ and S₃ are significant. Many criteria serve to bracket the oxidation state of the crust: it is nearly certain that the S₂^2?/SO₄^2? buffer regulates the oxygen fucagity, and that FeO is at least as abundant as Fe₂O₃ in crustal silicates. A highly oxidized crust (as, for example, would result from O₂ absorption complementary to escape of vast amounts of H₂) is incompatible with the gas-phase sulfur chemistry. If the Pioneer Venus mass spectrometer estimates of the abundance of sulfur gases are correct, Earth-like models for the bulk composition of Venus are seriously in error, and a far lower FeO content is required for Venus.     

S2O, polysulfuroxide and sulfur polymer on Io's surface?

To settle the question of disulfur monoxide and sulfur monoxide deposition and occurrence on Io's surface, we performed series of laboratory experiments reproducing the condensation of S₂O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO₂ and UV-visible radiation simulating that of Io's surface. Our experiments show that S₂O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S₂O outgassed by Io's volcanoes and condensing on Io's surface should lead to yellow polysulfuroxide deposits or to white deposits of S₂O diluted in sulfur dioxide frost (i.e., S₂O/SO₂ < 0.1%). Thus S₂O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S₂O and polysulfuroxide with NIMS/Galileo infrared spectra of Io's surface leads us to discuss the possible identification of polysulfuroxide. We also recorded the visible transmission spectra of sulfur samples resulting from polysulfuroxide decomposition. These samples consist in a mixture of sulfur polymer and orthorhombic sulfur. Using the optical constants extracted from these measurements, we show that a linear combination of the reflectance spectra of our samples, the reflectance spectrum of orthorhombic S₈ sulfur and SO₂ reflectance spectrum, leads to a very good matching of Io's visible spectrum between 330 and 520 nm. We conclude then that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S₈ and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas.     

Sulfuric Acid Production on Europa: The Radiolysis of Sulfur in Water Ice

Europa's surface is chemically altered by radiolysis from energetic charged particle bombardment. It has been suggested that hydrated sulfuric acid (H₂SO₄·nH₂O) is a major surface species and is part of a radiolytic sulfur cycle, where a dynamic equilibrium exists between continuous production and destruction of sulfur polymers S_x, sulfur dioxide SO₂, hydrogen sulfide H₂S, and H₂SO₄·nH₂O. We measured the rate of sulfate anion production for cyclo-octal sulfur grains in frozen water at temperatures, energies, and dose rates appropriate for Europa using energetic electrons. The measured rate is G_Mixture(SO₄²⁻)=f_Sulfur (r₀/r)^βG₁ molecules (100 eV)⁻¹, where f_Sulfur is the sulfur weight fraction, r is the grain radius, r₀=50 μm, β≈1.9, and G₁=0.4±0.1. Equilibrium column densities N are derived for Europa's surface and follow the ordering N(H₂SO₄) » N(S)>N(SO₂)>N(H₂S). The lifetime of a sulfur atom on Europa's surface for radiolysis to H₂SO₄ is τ(−S)=120(r/r₀)^β years. Rapid radiolytic processing hides the identity of the original source of the sulfurous material, but Iogenic plasma ion implantation and an acidic or salty ocean are candidate sources. Sulfate salts, if present, would be decomposed in <3800 years and be rapidly assimilated into the sulfur cycle.     

The Dual Sources of Io's Sodium Clouds

Io's sodium clouds result mostly from a combination of two atmospheric escape processes at Io. Neutralization of Na⁺ and/or NaX⁺ pickup ions produces the “stream” and the “jet” and results in a rectangular-shaped sodium nebula around Jupiter. Atmospheric sputtering of Na by plasma torus ions produces the “banana cloud” near Io and a diamond-shaped sodium nebula. Charge exchange of thermal Na⁺ with Na in Io's atmosphere does not appear to be a major atmospheric ejection process. The total ejection rate of sodium from Io varied from 3×10²⁶ to 25×10²⁶ atoms/s over seven years of observations. Our results provide further evidence that Io's atmospheric escape is driven from collisionally thick regions of the atmosphere rather than from the exosphere.     

Boundary layer ion composition at Jupiter during the inbound pass of the Ulysses flyby

During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. (Science257, 1539–1542, 1992) using plasma electron data. In the present study, ion composition and spectral measurements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the energy-per-charge range of 0.6–60 keV/e and involve mass as well as mass-per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O⁺, O²⁺, S²⁺, S³⁺) and sheath origin (including He²⁺ and high charge state CNO) have been directly identified for the first time in the Jovian magnetospheric boundary layer. Protons of probably mixed origin and He⁺ of possibly sheath (ultimately interstellar pickup) origin were also observed in the boundary layer. Sheath-like ions are observed throughout the boundary layer; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed within the outer magnetosphere. and ions of magnetospheric origin are found within the sheath, indicating that transport across the magnetopause boundary can work both ways, at least under some conditions. Although their source cannot be uniquely identified, the proton energy spectrum in the boundary layer suggests a sheath origin for the lower energy protons.     

On the structure of the Io torus

It is now recognized that a number of neutral-plasma interaction processes are of great importance in the formation of the Io torus. One effect not yet considered in detail is the charge exchange between fast torus ions and the atmospheric neutrals producing fast neutrals energetic enough to escape from Io. Since near Io the plasma flow is reduced, the neutrals of charge exchange origin are not energetic enough to leave the Jovian system; these neutrals are therefore distributed over an extensive region as indicated by the sodium cloud. It is estimated here that the total neutral injection rate can reach 10²⁷ s^?1 if not more. New ions subsequently created in the distributed neutral atomic cloud as a result of charge exchange or electron impact ionization are picked up by the corotating magnetic field. The pick-up ions are hot with initial gyration speed near the corotation speed. The radial current driven by the pickup process cannot close in the torus but must be connected to the planetary ionosphere by field-aligned currents. These field-aligned currents will flow away from the equator at the outer edge of the neutral cloud and towards it at the inner edge. We find that the Jovian ionospheric photoelectrons alone cannot supply the current flowing away from the equator, and torus ions accelerated by a parallel electric field could be involved. The parallel potential drop is estimated to be several kV which is large enough to push the torus ions into the Jovian atmosphere. This loss could explain the sharp discontinuous change of flux tube content and ion temperature at L = 5.6 as well as the generation of auroral type hiss there. Finally we show that the inner torus should be denser at system III longitudes near 240° as a result of the enhanced secondary electron flux in this region. This effect may be related to the longitudinal brightness variation observed in the SII optical emissions.     

The atmospheric signature of Io's Prometheus plume and anti-jovian hemisphere: evidence for a sublimation atmosphere

Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Å spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60° N-60° S, using a 0.1″ slit centered on Prometheus and tilted roughly 45° to the spin axis. The SO₂ column density peaked at 1.25×10¹⁷ cm⁻² near the equator, with an additional 5×10¹⁶ cm⁻² enhancement over Prometheus corresponding to a model volcanic SO₂ output of 10⁵ kg s⁻¹. Apart from the Prometheus peak, the SO₂ column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than ±30°, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3±0.6 K, though SO₂ abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO₂ frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45° tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO₂ gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO₂ ratio in the range 1-10%, roughly consistent with previous observations. S₂ gas was not seen anywhere, with an upper limit of 7.5×10¹⁴ cm⁻² for the Prometheus plume, confirming that this plume is significantly poorer in S₂ than the Pele plume (S₂ /SO₂<0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Å) for the first time. The brightness of the observed emission was directly correlated with the SO₂ abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator.     

Sulfur in vacuum: Sublimation effects on frozen melts,and applications to Io's surface and torus

A form of sulfur that is white at room temperature, shows almost no color change on cooling, and is fluffy in texture has been found in experiments on the effects of vacuum sublimation on solid sulfur. The white sulfur is a residual skin that forms on frozen sulfur in vacuum by differential evaporation of molecular species in the solid. S₈ ring sulfur is the dominant sublimation phase lost to the vacuum sink, and polymeric sulfur is the dominant residual phase. The microtexture of the fluffy sulfur layer is skeletal with an organized structure of filamentary components constructed of chains and clumps of submicron polyhedra. The layer is very porous (∼98%) and attains a thickness of ∼0.5 mm after 800 hr at 10⁻⁷ Torr (∼10⁻¹⁰ atm), and does not thicken much thereafter. Its color changes from that of the original melt freeze—yellow, tan, or brown depending on the prefreeze melt temperature—to white at room temperature. The UV/VIS reflectance spectrum (0.35 to 0.70 μm) of the original sulfur is greatly modified by formation of the vacuum surface layer: the blue absorption band edge moves toward the UV resulting in an increase in reflectivity in the range 0.42−0.46 μm as much as 400% and the UV reflectivity below 0.40 μm is reduced to one-third its original level to as low as 2%. Initially the changing band-edge position remains temperature sensitive, as in unmodified sulfur, shifting to shorter wavelengths with decreasing temperature, and returning to its precooled wavelength with temperature recovery; but once vacuum “maturity” is reached the temperature-induced excursion range of the absorption edge is reduced by an order of magnitude and is mostly in the UV whereas for ordinary sulfur (S₈) it is mostly in the blue. The sublimation rate from fresh frozen sulfur at initial exposure to high vacuum (∼10⁻⁷ Torr) is ∼3 × 10¹⁵ S cm⁻² sec⁻¹ at 300°K, increases steeply with temperature, decreases with higher vacuum pressure, and decreases with vacuum exposure time reaching an equilibrium flux of ∼3 × 10¹⁴ S cm⁻² sec⁻¹ after ∼1200 hr. For fresh frozen sulfur evaporating at ∼300°K and ∼10⁻⁷ Torr there occur significant spectral, color, and albedo effects in as little as 10 hr; samples become uniformly whitened within ∼100 hr, and progressive whitening and change in surface spectral properties continue for at least 1200 hr.This vacuum sulfur should exist in large quantity on Jupiter's satellite Io if there is solid free sulfur there that has solidified from a melt. A sulfur volcanism model for Io based on these findings is outlined. Color and spectra of different sulfur areas of Io may indicate relative crystallization age and cooling history. Concepts to be developed from this work on vacuum sulfur may help in understanding properties of Io's surface such as composition, texture, adsorbtivity, thermal inertia, photometry, and posteclipse brightening. The inferred flux of subliming sulfur from hotspots on Io is consistent with estimated turnover rates of the surface and is sufficient to supply the requisite sulfur to the Io plasma torus.     

Sulfur dioxide on Io: Spatial distribution and physical state

Observations of the 4-μm SO₂ band on Jupiter's satellite Io and laboratory measurements of SO₂ frost are presented. The observations confirm the existence of a large longitudinal variation in band strength but show no evidence of temporal changes. Comparison of the band position and shape in Io's spectrum with those in the laboratory frost's suggests that the bulk of the absorption on Io is due to frost, not adsorbed gas. The derived SO₂ coverage is large enough to require that SO₂ be present in most terrain types on Io and not just in the white plains unit. To reconcile the infrared observations that indicate large amounts of SO₂ with the ultraviolet observations of Voyager and IUE that show little, the SO₂ must be mixed intimately with the sulfur (or other material) so that at each wavelength the darker component dominates the spectrum.     

The radiolysis of SO2 and H2S in water ice: Implications for the icy jovian satellites

Spectra of Europa, Ganymede, and Callisto reveal surfaces dominated by frozen water, hydrated materials, and minor amounts of SO₂, CO₂, and H₂O₂. These icy moons undergo significant bombardment by jovian magnetospheric radiation (protons, electrons, and sulfur and oxygen ions) which alters their surface compositions. In order to understand radiation-induced changes on icy moons, we have measured the mid-infrared spectra of 0.8 MeV proton-irradiated SO₂, H₂S, and H₂O-ice mixtures containing either SO₂ or H₂S. Samples with H₂O/SO₂ or H₂O/H₂S ratios in the 3-30 range have been irradiated at 86, 110, and 132 K, and the radiation half-lives of SO₂ and H₂S have been determined. New radiation products include the H₂S₂ molecule and HSO⁻₃, HSO⁻₄, and SO²⁻₄ ions, all with spectral features that make them candidates for future laboratory work and, perhaps, astronomical observations. Spectra of both unirradiated and irradiated ices have been recorded as a function of temperature, to examine thermal stability and phase changes. The formation of hydrated sulfuric acid in irradiated ice mixtures has been observed, along with the thermal evolution of hydrates to form pure sulfuric acid. These laboratory studies provide fundamental information on likely processes affecting the outer icy shells of Europa, Ganymede, and Callisto.