Spectrum Of Io Research Articles

Charge‐coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede

We have obtained 3200–7800 Å CCD spectra of the Galilean satellites at a variety of orbital longitudes, with spectral resolution between 3 and 18 Å and signal‐to‐noise ratios of up to 2000 at 6000 Å. Despite the higher resolution and signal‐to‐noise ratio than previous published spectra, no new features are seen on Io, Europa, and Callisto. However, Ganymede shows an unusual and previously unreported shallow absorption feature at 5773 Å, which is much stronger on the trailing side, and a weaker band at 6275 Å. The features are apparently due to diatomic oxygen, and require simultaneous electronic transitions in two adjacent molecules: they are the strongest visible‐wavelengt h absorption bands in solid or liquid oxygen. Because condensed pure oxygen is not stable at Ganymede surface temperatures and pressures, the oxygen must be trapped in other surface materials, perhaps in water ice, with the constraint that O2 molecules must be close enough together for simultaneous electronic transitions. Magnetospheric bombardment of water ice is a plausible production mechanism for the O2. Continuum shapes on all four satellites are more clearly seen than in previous spectra. The shape of the continuum is very similar on Ganymede and Callisto but is strikingly different on Europa, indicating a different origin for the nonice component on Europa. The shape of the Europa continuum suggests that allotropes or compounds of sulfur may be the dominant spectrally active materials in the visible spectrum. No convincing changes since 1978 are visible in Io's spectrum, despite the high resurfacing rates, suggesting that volcanic resurfacing tends to overpaint areas with more material of the same composition.

Infrared Spectra and Structure of Solid Phases of Sulfur Trioxide: Possible Identification of Solid SO 3 on Io's Surface

The IR spectra of solid sulfur trioxide (SO 3) have been examined between 20 and 290 K. The spectral features of an amorphous low-temperature deposit and four additional phases (monomeric, γ, α, and β) are presented. Two of the emission peaks in the thermal spectrum of Io recorded by the Voyager IRIS instrument are suggested to be possibly due to solid SO 3 mixed with crystalline SO 2.

Identification of Three Absorption Bands in the 2-μm Spectrum of Io

Spectroscopic observations of the trailing face of Io at a resolution of 1.02 cm-1 performed with the FTS on the 3.6-m Canada-France-Hawaii telescope on Mauna Kea confirm the presence of two weak absorption features in the 2-μm region. The first feature occurs at 4704.9 ± 0.2 cm-1 (2.12545 ± 0.00010 μm); it is about 6.8% deep and 3 cm -twide (FWHM). The second feature is at 5047.1 ±1 cm-1 (1.98135 ± 0.0004 μm) with a broad central core; its depth is about 8% and its width approximately 7 cm-1. A laboratory investigation of spectra of solid SO2 with relatively thick samples, as well as of mixtures of SO2 with CO2 and H2S, indicates that the two features at 4705 cm-1 and 5047 cm-1 are best explained by the 3v1 + v3 and v1 + 3v3 modes of solid SO2 around 130 K, respectively. Previous work tentatively proposing CO2 clusters as an explanation for the first feature and condensed H2S for the second can no longer be supported. In addition, a careful a posteriori look at our Io spectrum shows the presence of a band at 3933 ± 1 cm-1 (2.5426 ± 0.0007 μm) (depth ≈ 30%, FWHM ≈ 8 cm-1) due to the 3 v3 band of solid SO2, as predicted by our laboratory experiments. The positions and widths of these bands indicate that a temperature gradient may exist between the surface of the frost and several centimeters below and suggest that some SO2 may be mixed at the molecular level with some neutral component. Three different models are proposed to explain the apparent discrepancy between the large variability with 1ongitude of the strong 2457 cm-1 (4.07 μm) band and the nearly constant depth of the weak 4705 cm-1 (2.1254 μm) band. All three models lead to mean grain sizes from a few hundreds of micrometers to about 1 mm. The first model invokes a large variation in grain sizes (factor 2-3) over lo's hemispheres. This model correctly fits the observed depths of the 2. and 4.07-μm bands but not that of the 3.78-μm (2645 cm-1) band. The second model assumes a considerable variation in thermal flux with Io's longitude (at 2457 cm-1, up to 20% of the reflected continuum flux). This model can reconcile the band strengths in the whole 2- to 4-μm range with about the same grain size at all longitudes and with moderate variations in frost coverage, but does not explain the albedo variability in the UV range. The third model assumes an uniform layer of coarse grained SO2 frost (a few hundred micrometers in size) with a variable longitudinal coverage of a thin layer of very fine frost grains (less than a few millimeters of micrometer-sized grains). This model qualitatively explains the observed IR and UV variations. Using the optical constants of solid SO2 measured in the laboratory, we predict the position and depth (≥-1%) of about 25 additional absorption features of SO2 frost, in the 2800-5000 cm-1 (2.0-3.6 μm) range, that should be observed in future high quality spectra of Io.

New Narrow Infrared Absorption Features in the Spectrum of Io between 3600 and 3100 cm -1 (2.8-3.2 μm)

We report the discovery of a series of infrared absorption bands between 3600 and 3100 cm -1 (2.8-3.2 μm) in the spectrum of Io. Individual narrow bands are detected at 3553, 3514.5, 3438, 3423, 3411.5, and 3401 cm -1 (2.815, 2.845, 2.909, 2.921, 2.931, and 2.940 μm, respectively). The positions and relative strengths of these bands, and the difference of their absolute strengths between the leading and trailing faces of Io, indicate that they are due to SO 2. The band at 3438 cm -1 (2.909 μm) could potentially have a contribution from an additional molecular species. The existence of these bands in the spectrum of Io indicates that a substantial fraction of the SO 2 on Io must reside in transparent ices having relatively large crystal sizes. The decrease in the continuum observed at the high frequency ends of the spectra is probably due to the low frequency side of the recently detected, strong 3590 cm -1 (2.79 μm) feature. This band is likely due to the combination of a moderately strong SO 2 band and an additional absorption from another molecular species, perhaps H 2O isolated in SO 2 at low concentrations. A broad (FWHM ≈ 40-60 cm -1), weak band is seen near 3160 cm -1 (3.16 μm) and is consistent with the presence of small quantities of H 2O isolated in SO 2-rich ices. There is no evidence in the spectra for the presence of H 2O vapor on Io. Thus, the spectra presented here neither provide unequivocal evidence for the presence of H 2O on Io nor preclude it at the low concentrations suggested by past studies.

Hubble Space Telescope UV spectral observations of Io passing into eclipse

Time‐resolved spectra of Io have been obtained with the Faint Object Spectrograph on the Hubble Space Telescope in January 1992 at times centered on the passage of Io into Jupiter's shadow. Two different eclipse observations covered 1100–1600 Å and 2250–3300 Å. In the far‐UV range, emission lines of atomic sulfur and oxygen from Io's atmosphere (similar to those previously detected with IUE) have been observed from Io in sunlight, and the spatial extent of the emitting region has been resolved for the first time: this is 0.5–1 Io radii (RIo) above the surface. The emission lines are typically 1 kR in brightness while Io is in sunlight, and decrease to a few hundred Rayleighs within 20 min or less of Io's passing into shadow. If the emissions are produced in Io's ionosphere, the decrease in shadow appears consistent with the collisional slowing and recombination of photoelectrons in 100–1000 s, with recombination an important quenching process if the dominant ion is molecular (i.e., SO2+). By contrast, the impact of corotating torus electrons is expected to continue when Io is in shadow. If impact by torus plasma dominates the emission, the decrease in shadow may be due to surface SO2 condensation, with the residual emission in shadow due either to plasma impact of gas above the hot volcanic calderas or electron impact on S and O. In the near‐UV range, we have not detected any airglow emissions from Io's atmosphere in shadow, with the main limitation being a high level of scattered light from Jupiter. We derive a 3σ upper limit to the 2560 Å SO emission feature of 1 kR, which is close to what is expected from electron impact on SO2 based on the observed brightness of the FUV S and O lines in shadow. A high signal‐to‐noise spectrum of Io's albedo in sunlight reveals a spectral shape similar to laboratory spectra of SO2 frost reflectivity, and the relative albedo spectrum changed as Io passed into eclipse and part of the disk was in shadow. No specific SO2 gas absorption features appear in the albedo spectrum, although there could be substantial gas absorption near 2800 Å if the individual lines are narrow and saturated. Finally, we present preliminary models for the near‐UV spectrum of Io as functions of SO2 frost areal coverage and SO2 gas density.

Is H 2O Present on Io? The Detection of a New Strong Band Near 3590 cm -1 (2.79 μm)

A strong absorption band at 3590 ± 20 cm -1 (2.790 ± 0.015 μm) has been discovered in the spectrum of Io using the Kuiper Airborne Observatory (KAO). The 2 v 1 + v 3 combination mode of solid SO 2 falls at this position. Since SO 2 is abundant on Io it must contribute to the new band. However, a band due to H 2O was predicted near this frequency in Io's spectrum based on laboratory experiments of H 2O: SO 2 mixed Io ice analogs which were used to assign the two weak, variable features at 3370 and 3170 cm -1 (2.97 and 3.15 μm) to trace amounts of H 2O frozen in solid SO 2 on Io. The new band probably originates from both SO 2 and H 2O. Unfortunately, the spectral resolution of the data is insufficient to settle the issue of whether there are two resolvable components.

3- to 13-μm Spectra of Io

We obtained 3- to 13-μm spectra of Io on three nights in June 1991, observing both leading and trailing hemispheres. The spectrum from the hemisphere centered at 287° containing the volcanoes Loki and Pele had higher flux than the opposite hemisphere by nearly a factor of 3 from 9 to 11 μm. We performed a multivariable least squares fit to the data for all three nights with a cool component ( T = 110-125 K) and a warm component ( T = 189-369 K). We found that the warm component is more extensive on the Loki hemisphere, covering as much as 5.5 × 10 5 km 2. Fits using more than two temperature components can be constructed, but are insufficiently constrained by the data. The 4.1-μm SO 2 absorption feature was detected, but no additional spectral features were observed.

Discovery of a second narrow absorption feature in the near-infrared spectrum of Io

Extension of our high resolution survey of the near-infrared reflectance spectrum of Io to shorter wavelengths has revealed a second relatively sharp absorption feature that, in strength, width, and constancy, appears similar to the one discovered in 1990. The new feature is centered at vacuum wavelength 1.9820 ± 0.0005 μm (5045 ± 1 cm −1) and has an equivalent width of about 0.5 cm −1 (5% deep in our spectra). This new feature is not present in the laboratory spectra of dilute CO 2 in a matrix, and therefore CO clusters, which have been proposed as an identification for the carrier of the original narrow absorption feature at 2.125 μm, do not appear to be responsible for the second feature. The spectrum of cold H 2S ice crystals shows considerable structure near this wavelength. The difference in width between this feature and that of the much broader Ionian H 2S ice features at longer wavelengths may be the result of different phases of ice, at different temperatures, that are emphasized in the different spectral regions. Alternatively, the trapping of H 2S in an SO 2 matrix may account for the small discrepancy in observed wavelength. Possible tests for the H 2S identification of the carrier are discussed.

The complete UV spectrum of SO2 by electron impact, 1, The vacuum ultraviolet Spectrum

We have measured the vacuum ultraviolet (VUV) emission cross sections of SO2 from 40 to 200 nm in the laboratory in a crossed beam experiment. The wavelength range comprised the extreme ultraviolet (EUV) spectrum, from 40 to 120 nm, and the far ultraviolet (FUV) spectrum, from 120 to 200 nm. The EUV and FUV emission spectra induced by electron impact consist of atomic multiplets of S I, II, and III and O I, II, and III. The studies included the measurement of excitation functions of the strongest multiplets from 0 to 1 keV impact energy and the identification of dissociation processes from the analysis of the threshold excitation function. The O I (98.9 nm) resonance multiplet (g3 P‐3D) is the strongest EUV feature from electron impact induced fluorescence of SO2 with a peak cross section of 7.82 ± 1.59 × 10‐19 cm2 at 125 eV electron impact energy. The O I (130.4 nm) multiplet (g3P‐3S) resonance transition is the strongest feature in the FUV, with a peak cross section of 1.98 ± 0.44 × 10‐18 cm2 at 90 eV. The strongest feature of S I in the VUV is the S I (147.9 nm) multiplet (g3 P‐3D) resonance transition, with a peak cross section of 1.52 ± 0.33 × 10‐18 cm2 at 90 eV. Dissociation‐excitation cross sections of SO2 are important contributors to the total inelastic cross section of SO2 gas interacting with electrons. These cross sections are used in neutral cloud theory models of the Io atmosphere. The laboratory spectrum at low energy (~22 eV) matches a recently obtained IUE FUV spectrum of Io. The comparison represents evidence of a thin or intermediate atmosphere of SO2 that is directly impacted by the plasma torus. The existence of an atmosphere on Io embedded in the corotating torus plasma is an important remote sensing target for the Hubble Space Telescope, Goddard High Resolution Spectrograph and Faint Object Spectrograph, and Galileo EUV and FUV spectrometers, among other present and planned VUV studies of the Jovian system.

High resolution observations of the 2.125-μm feature in Io's spectrum during 1975 and 1976

We confirm the presence of a weak absorption feature in Io's spectrum at 2.125 μm using high resolution observations acquired in 1975 and 1976. If this feature is due to a new class of material on Io, it appears to be the only diagnostic spectral feature of it anywhere in those regions of Io's near-IR spectrum that are accessible at groundbased telescopes. We cannot definitively assign the feature to any atom or molecule.

Laboratory studies of the newly discovered infrared band at 4705.2 cm −1 (2.1253 μm) in the spectrum of Io: The tentative identification of CO 2

We discuss over 120 laboratory experiments pertaining to the identification of the new absorption band discovered by Trafton et al. (1991) at 4705.2 cm −1 (2.1253 μm) in the spectrum of Io. It is shown that this band is not due to overtones or combinations of the fundamental bands associated with the molecules (or their chemical complexes) already identified on Io, namely, SO 2, H 2S, and H 2O. Thus, this band is due to a new, previously unidentified, component of Io. Experiments also demonstrate that the band is not due to molecular H 2 frozen in SO 2 frosts. Since the frequency of this band is very close to the first overtone of the ν 3 asymmetric stretching mode of CO 2, we have investigated the spectral behavior of CO 2 under a variety of conditions appropriate for Io. The profile of the Io band is not consistent with the rotational envelope expected for single, freely rotating, gaseous CO 2 under Io-like conditions. It was found that pure, solid CO 2 and CO 2 intimately mixed in a matrix of solid SO 2 and H 2S produce bands with similar widths (5–10 cm −1), but that these bands consistently fall at frequencies about 10−20 cm −1 (∼0.007 μm) lower than the Io band. CO 2 in SO 2: H 2S ices also produces several additional bands that are not in the Io spectra. The spectral fit improves, however, as the CO 2 concentration in SO 2 increases, suggesting that CO 2CO 2 interactions might be involved. A series of Ar: CO 2 and Kr: CO 2 matrix isolation experiments, as well as laboratory work done elsewhere, show that CO 2 clustering shifts the ands position to higher frequencies and provides a better fit to the Io band. Various laboratory experiments have shown that gaseous CO 2 molecules have a propensity to cluster between 80 and 100 K, temperatures similar to those found on the colder regions of Io. We thus tentatively identify the newly discovered Io band at 4705.2 cm −1 (2.1253 μm) with CO 2 multimers or “clusters” on Io. Whether these clusters are buried within an SO 2 frost, reside on the surface, or are in a residual, steady-state “atmospheric aerosol” population over local coldtraps is not entirely clear, although we presently favor the latter possibility. The size of these clusters is not well defined, but evidence suggests groups of more than four molecules are required. The absorption strength of the 2ν 3CO 2 cluster overtone determined in the laboratory, in conjunction with the observed strength of the Io band, suggests that the disk-integrated abundance of CO 2 is less than 1% that of the SO 2. Studies of the sublimation behavior of CO 2 indicate that it probably resides predominantly in the cooler areas (< 100 K) of Io. The relative constancy of the Io feature over a variety of orbital phases suggests that the polar regions may contain much of the material. Some consequences of the physical properties of CO 2 under conditions pertinent to Io are discussed. The presence of CO 2 clusters on Io could be verified by the detection of any one of several other infrared bands associated with the CO 2 molecule, of which the strongest are the ν 3 12CO 2 asymmetric stretch fundamental near 2350 cm −1 (4.25 μm) and the ν 2 bending mode fundamental near 660 cm −1 (15.1 μm). Weaker bands that may also be detactable include the ν 3 13CO 2 asymmetric stretch fundamental near 2280 cm −1 (4.39 μm), the 2ν 2 + ν 3 combination/overtone band near 3600 cm −1 (2.78 μm), and the ν 1 + ν 3 combination band near 3705 cm −1 (2.70 μm).

A new class of absorption feature in lo's near-infrared spectrum

We report our discovery of an absorption feature in the infrared spectrum of Io centered at 2.1253 μm (4705.2 cm −2). This contrasts with the 30- to 50-cm −1 widths of the board absorption features previously detected at longer wavelengths which arise from mixtures of SO 2 with H 2S and H 2O. This newly discovered feature is relatively weak, having a core only 5% below the continuum at this resolving power. Our survey from 1.98 to 2.46 μm (5050-4065 cm −1) at this same resolving power revealed no other feature greater than 1% of the continuum level shortward of 2.35 μm, and 3% elsewhere. The feature does not correspond to any gas-or solid-phase absorption that might be expected from previously identified constituents of Io's surface. No temporal of longitudinal variational has been detected in the course of 18 nights of observation over the past year and no significant variation in the strength of the feature was seen during an emergence from eclipse. These observations indicate that the source material of the feature is reasonably stable, and is more uniformly distibuted in longitude than Io's hot spots. These characteristics all indicate that the feature belongs to a class different from those characterizing other known absorption feature in Io's spectrum. Consequently, it should reveal important new information about Io's atmosphere-surface composition and interation. A series of laboratory experiments of plausible surface ices indicates that (i) the band does not arise from overtones or combinations of any of the molecular variations associated with species already identified on Io (SO 2, H 2S, H 2O) or from chemical complexes of these molecules, (ii) the band does not arise from H 2 trapped in SO 2, and (iii) the band may arise from the 2 v 3 mode of CO 2. If the band arises from CO 2, it is clear from its detailed shape and position that the molecdules are not embedded in an SO 2 matrix, as are H 2S and H 2O, but may be present as multimers or “clusters.”

The atmospheric abundance of SO 2 on Io

Near-ultraviolet spectra of Io have been obtained for the first time at high-resolution with the International Ultraviolet Explorer (IUE) satellite in order to determine the SO 2 abundance in the dayside atmosphere for both the leading (east) and trailing (west_ hemispheres. The improved resolution ( ∼0.2 A ̊ ), compared with previous low-resolution observations ( ∼8 A ̊ ), is adequate for determination of the spectral contrast of the SO 2 gas absorption. We have compared the derived geometric albedos with various model albedos that would result from proposed SO 2 atmospheres, as well as from localized atmospheres produced by sublimation (on the basis of recent estimates of the distribution of optically thick SO 2 frost and temperature) and by direct volcanic output. Alternatively with a model atmosphere composed of a homogeneous layer, we have placed an upper limit of 2 × 10 17 cm −2 (or 0.0074 cm-atm, corresponding to 4 × 10 −9 bar surface pressure and 2 × 10 11 cm −3 surface density for a 10-km scale height) on the average SO 2 column density for both hemispheres. This upper limit implies that a collisionally thick SO 2 atmosphere of intermediate density may have been present on Io's dayside at the time of our observations. In addition, small atmospheric regions of relatively large density (about an order of magnitude higher than the average upper limit) associated with volcanic plumes are also compatible with the observations. Some intermediate atmospheres may contain sufficiently large SO 2 densities near the evening terminator to reproduce the evening ionospheric profile measured by Pioneer 10 on the leading hemisphere (away from any known volcanic plumes).

The surface of Io: A new model

The spectral properties of Io can be duplicated by combinations of basalt and condensates of SO 2 and its dissociation products, polysulfur oxide and S 2O, without elemental sulfur. The arguments concerning elemental sulfur on Io are reviewed and it is concluded that this material probably is not present in spectrally significant amounts. These results imply that the hot spots and plumes probably are silicate volcanoes and SO 2 fumaroles and the dark caldera are fresh mafic silicate extrusives. It is suggested that the exposed surface consists of mafic silicates partially covered with thin deposits of SO 2, polysulfur oxide, and S 2O. A model for the distribution and state of SO 2 that is consistent with observations is one in which most of the spectrally active frost occurs as thin, ephemeral, partial coatings on the topmost particles of the regolith. The composition of the torus can be explained without requiring unusual Na compounds on the surface. Several features of Io's spectrum are also displayed by Europa, although weakly. It is suggested that they have the same source as on Io: endogenic SO 2, polysulfur oxide, and S 2O, rather than magnetospherically imbedded sulfur from Io. The leading-trailing side asymmetries on both bodies can be explained by preferential magnetospheric sputter erosion of the condensates, rather than ion imbedding.

Hydrogen Sulfide on IO: Evidence from Telescopic and Laboratory Infrared Spectra

Evidence is reported for hydrogen sulfide (H(2)S) on Io's surface. An infrared band at 3.915 (+/- 0.015) micrometers in several ground-based spectra of Io can be accounted for by reflectance from H(2)S frost deposited on or cocondensed with sulfur dioxide (SO(2)) frost. Temporal variation in the occurrence and intensity of the band suggests that condensed H(2)S on Io's surface is transient, implying a similar variation of H(2)S abundance in Io's atmosphere. The band was observed in full-disk measurements of Io at several orbital longitudes, including once at 24 degrees ( approximately 0.5 hour after Io's reappearance after an eclipse)-but not after another reappearance at 22 degrees -and once at 95 degrees (on Io's leading hemisphere). These results suggest that condensed H(2)S is sparse and variable but can be widespread on Io's surface. When present, it would not only produce the infrared band but would brighten Io's typical surface at ultraviolet and visible wavelengths.

High-resolution infrared spectroscopy of Io and possible surface materials

We have obtained new spectra of Io in the 3.5- to 4.2- and 4.5- to 5.4-μm regions with a resolution (λ/Δλ) of roughly 500. The Io spectra cover a range of longitudes, and times from 1983 through 1985. Laboratory spectra of various materials have also been obtained. In this wavelength region are located several strong bands of SO 2, known to be a major component of Io's surface, as well as the bands of several potential surface materials. In the Io spectra we identify several new features attributable to SO 2, and have obtained strengths of the 33S, 34S, and 18O isotopic bands, which appear normal. We place limits on the amounts of Na 2SO 3, and Na 2SO 4 present. Finally, we use the data to place limits on the SO 2 gas abundance in Io's atmosphere.

Vacuum weathering of sulfur: Temperature effects and applications to Io

We have conducted laboratory experiments on frozen sulfur to further study the vacuum‐weathering phenomenon discovered by Nash [1987]. In this report we discuss the effect of surface temperature on the initial transient and subsequent steady‐state behavior of the vacuum‐sublimation rate and the UV/VIS spectral reflectance of frozen sulfur. We show that surface temperature is a dominant parameter controlling the rate of vacuum weathering of sulfur, a finding consistent with the steep vapor pressure curve of sulfur. We set limits on the prodigious sublimation rates expected for sulfur from hot spots on Io's surface and further define the range of changes that occur in the spectral reflectance of freshly frozen sulfur. These results imply that any freshly‐frozen sulfur deposits on Io at typical Io hotspot temperatures should display major transient changes in color (and microtexture) in a matter of hours to days. These rapid changes in spectral reflectivity should be readily detectable by high time‐resolution synoptic observations of Io's spectrum in the UV/VIS by spacecraft optical instruments such as on Galileo and, in certain instances (e.g., during or immediately after large‐scale Io volcanic activity), by Earthbased telescopes. Such observations should be useful in determining the frequency of occurrence, surface location, and areal scale of any sulfur volcanic activity on Io.

Disk-resolved photometry of Io II. Opposition Surges and Normal Reflectances

Near-opposition ( α = 2–8°) phase curves have been derived for individual regions on Io in three color classes using the limb-darkening results from Paper I. Analysis of the phase curves is complicated by a slight inconsistency of uncertain origin between data from Voyager 1 ( α = 2–5°) and Voyager 2 ( α = 7–8°). However, disk-integrated data from the same imaging sequences can be used to approximately “correct” the disk-resolved phase curves. Phase coefficients derived from the Voyager 1 data alone ( α = 2−5°) are typically 0.01−0.02 mag/deg for Bright regions and ≈0.03−0.04 mag/deg for Average and Polar materials, a difference suggestive of a contrast in surface texture. Accurate normal reflectances derived for the three color classes using the information on near-opposition limb darkening and phase behavior are generally consistent with earlier preliminary determinations. Careful comparison with laboratory spectra supports suggestions that Voyager disk-resolved spectra of Io can be explained by mixtures of sulfurs and SO 2 frost. However, given the coarse spectral resolution and limited wavelength coverage of the Voyager cameras, this consistency should not be misinterpreted as a unique diagnostic identification, nor should one assume that sulfurous materials must make up the bulk of Io's outer crust.

Spectral reflectance of solid sulfur trioxide (0.25–5.2 μm): Implications for Jupiter's satellite Io

Laboratory data on the UV and near-IR spectra of SO3 are compared with spectra obtained for the Jovian moon Io by the Voyager and IUE satellites. The experimental data were taken at wavelengths from 0.25 micron to nearly 6 microns, although only order-of-magnitude estimates could be made of the associated particle sizes. Tests were also performed to identify the spectral characteristics with succeedingly lower concentrations of SO3 relative to SO2, which is apparently abundant on the Io surface. No evidence was found for the presence of solid SO3 in the spectra of Io down to projected concentrations lower than 1 percent. The findings are considered surprising since it is known that particle bombardment will cause SO3 to form in SO2 concentrations. It is expected that SO3 will be detected by the IR mapping instrument to be carried by the Galileo spacecraft.

Sulfur dioxide on Io: Spatial distribution and physical state

Observations of the 4-μm SO 2 band on Jupiter's satellite Io and laboratory measurements of SO 2 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 SO 2 coverage is large enough to require that SO 2 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 SO 2 with the ultraviolet observations of Voyager and IUE that show little, the SO 2 must be mixed intimately with the sulfur (or other material) so that at each wavelength the darker component dominates the spectrum.