Spectrum Of Io Research Articles

Spectroscopic Mapping of Io’s Surface with HST/STIS: SO2 Frost, Sulfur Allotropes, and Large-scale Compositional Patterns

Io’s intense volcanic activity results in one of the most colorful surfaces in the solar system. Ultraviolet and visible-wavelength observations of Io are critical to uncovering the chemistry behind its volcanic hues. Here, we present global, spatially resolved ultraviolet-visible spectra of Io from the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, which bridge the gap between previous highly resolved imagery and disk-integrated spectroscopy, to provide an unprecedented combination of spatial and spectral detail. We use this comprehensive data set to investigate spectral endmembers, map observed spectral features associated with SO2 frost and other sulfur species, and explore possible compositions in the context of Io surface processes. In agreement with past observations, our results are consistent with extensive equatorial SO2 frost deposits that are stable over multidecade timescales, widespread sulfur-rich plains surrounding the SO2 deposits, and the enrichment of Pele’s pyroclastic ring and the high-latitude regions in metastable short-chain sulfur allotropes.

Infrared observations of Io from Juno

The Jovian InfraRed Auroral Mapper (JIRAM) on board the NASA Juno spacecraft is a dual-band imager and spectrometer, primarily designed to study the Jovian atmosphere and aurorae. In addition to its primary goal, JIRAM has been used to obtain images and spectra of the Galilean satellites, Jupiter's largest moons, when the spacecraft attitude was favourable to achieve this goal. Here we present JIRAM's first images and spectra of Io. These observations are used to characterize the location and possible morphology, and some temperatures, of Io's volcanic thermal sources; the identification of SO2 and the possible identification of other materials. A new hot spot/volcano is identified close to the South Pole of Io, and others are seen in lower latitude regions, which were previously unmapped. Images of the same region taken 2 months apart also show variations of hot spot intensity, possibly due to new lava flows or to lava flow breakouts.

A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 μm

Abstract We present hemispherically resolved spectra of the surface of Europa from ∼3.1–4.13 , which we obtained using the near-infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface to extend beyond 4 . In our data, we identify a previously unseen spectral feature at 3.78 on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 feature of SO2 frost is absent from our data. This result is contrary to the suggested detection of SO2 at 4.05 in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 feature is not easily explained by the presence of SO2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.

Detection and characterization of Io’s atmosphere from high-resolution 4-μm spectroscopy

We report on high-resolution and spatially-resolved spectra of Io in the 4.0μm region, recorded with the VLT/CRIRES instrument in 2008 and 2010, which provide the first detection of the ν1+ν3 band of SO2 in Io’s atmosphere. Data are analyzed to constrain the latitudinal, longitudinal, and diurnal distribution of Io’s SO2 atmosphere as well as its characteristic temperature. Equatorial SO2 column densities clearly show longitudinal asymmetry, but with a maximum of ∼1.5×1017cm−2 at central meridian longitude L=200–220 and a minimum of ∼3×1016cm−2 at L=285–300, the longitudinal pattern somewhat differs from earlier inferences from Ly α and thermal IR measurements. Within the accuracy of the measurements, no evolution of the atmospheric density from mid-2008 to mid-2010 can be distinguished. The decrease of the SO2 column density towards high latitudes is apparent, and the typical latitudinal extent of the atmosphere found to be ±40° at half-maximum. The data show moderate diurnal variations of the equatorial atmosphere, which is evidence for a partially sublimation-supported atmospheric component. Compared to local noon, factor of 2 lower densities are observed ∼40° before and ∼80° after noon. Best-fit gas temperatures range from 150 to 220K, with a weighted mean value of 170±20K, which should represent the column-weighted mean kinetic temperature of Io’s atmosphere. Finally, although the data include clear thermal emission due to Pillan (in outburst in July 2008) and Loki, no detectable enhancements in the SO2 atmosphere above these volcanic regions are found, with an upper limit of 4×1016cm−2 at Pillan and 1×1017cm−2 at Loki.

Io’s atmosphere: Constraints on sublimation support from density variations on seasonal timescales using NASA IRTF/TEXES observations from 2001 to 2010

We present an analysis of 19 μm spectra of Io’s SO 2 atmosphere from the TEXES mid-infrared high spectral resolution spectrograph on NASA’s Infrared Telescope Facility, incorporating new data taken between January 2005 and June 2010 and a re-analysis of earlier data taken from November 2001 to January 2004. This is the longest set of contiguous observations of Io’s atmosphere using the same instrument and technique thus far. We have fitted all 16 detected blended absorption lines of the ν 2 SO 2 vibrational band to retrieve the subsolar values of SO 2 column abundance and the gas kinetic temperature. By incorporating an existing model of Io’s surface temperatures and atmosphere, we retrieve sub-solar column densities from the disk-integrated data. Spectra from all years are best fit by atmospheric temperatures <150 K. Best-fit gas kinetic temperatures on the anti-Jupiter hemisphere, where SO 2 gas abundance is highest, are low and stable, with a mean of 108 (±18) K. The sub-solar SO 2 column density between longitudes of 90–220° varies from a low of 0.61 (±0.145) × 10 −17 cm −2, near aphelion in 2004, to a high of 1.51 (±0.215) × 10 17 cm −2 in 2010 when Jupiter was approaching its early 2011 perihelion. No correlation in the gas temperature was seen with the increasing SO 2 column densities outside the errors. Assuming that any volcanic component of the atmosphere is constant with time, the correlation of increasing SO 2 abundance with decreasing heliocentric distance provides good evidence that the atmosphere is at least partially supported by frost sublimation. The SO 2 frost thermal inertias and albedos that fit the variation in atmospheric density best are between 150–1250 W m −2 s −1/2 K −1 and 0.613–0.425 respectively. Photometric evidence favors albedos near the upper end of this range, corresponding to thermal inertias near the lower end. This relatively low frost thermal inertia produces larger amplitude seasonal variations than are observed, which in turn implies a substantial additional volcanic atmospheric component to moderate the amplitude of the seasonal variations of the total atmosphere on the anti-Jupiter hemisphere. The seasonal thermal inertia we measure is unique both because it refers exclusively to the SO 2 frost surface component, and also because it refers to relatively deep subsurface layers (few meters) due to the timescales of many years, while previous studies have determined thermal inertias at shallower levels (few centimeters), relevant for timescales of ∼2 h (eclipse) or ∼2 days (diurnal curves).

Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model

Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io’s unique atmospheric structure and origin. Improving upon previous models, Walker et al. (Walker, A.C., Gratiy, S.L., Levin, D.A., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Moore, C.H., Stewart, B. [2009]. Icarus) developed a fully 3-D global rarefied gas dynamics model of Io’s atmosphere including both sublimation and volcanic sources of SO 2 gas. The fidelity of the model is tested by simulating remote observations at selected wavelength bands and comparing them to the corresponding astronomical observations of Io’s atmosphere. The simulations are performed with a new 3-D spherical-shell radiative transfer code utilizing a backward Monte Carlo method. We present: (1) simulations of the mid-infrared disk-integrated spectra of Io’s sunlit hemisphere at 19 μm, obtained with TEXES during 2001–2004; (2) simulations of disk-resolved images at Lyman- α obtained with the Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) during 1997–2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range obtained with the IRAM-30 m telescope in October–November 1999. We found that the atmospheric model generally reproduces the longitudinal variation in band depth from the mid-infrared data; however, the best match is obtained when our simulation results are shifted ∼30° toward lower orbital longitudes. The simulations of Lyman- α images do not reproduce the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere sustains columns that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explains the observed line widths, which are too wide to be formed by thermal Doppler broadening alone.

Eclipse reappearances of Io: Time-resolved spectroscopy (1.9–4.2 μm)

We obtained time-resolved, near-infrared spectra of Io during the 60–90 min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO 2 frost absorption bands, that would indicate surface–atmosphere exchange of gaseous SO 2 induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141–148] reported significant changes in the strengths of two strong SO 2 bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07 μm [ ν 1 + ν 3]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io’s spectrum at any of the five observed events during the approximately 60–90 min during which spectra were obtained following Io’s emergence from Jupiter’s shadow. The areas of the three strongest SO 2 bands in the region 3.5–4.15 μm were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io’s few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198 μm (4549.6 cm −1) 4 ν 1 band of SO 2 frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra ∼6 min before Io reappeared from Jupiter’s shadow, during which time it was detected through its own thermal emission. No SO 2 bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO 2 in the vertical column above Io’s surface was ∼4 × 10 −5 g cm −2.

Development of optical methods for simultaneous detection of I2, I−, IO 3 − , and I 3 − in liquid media in real Time

The absorption spectra of IO 3 − in liquid media are experimentally studied in the UV spectral range. The absorption cross sections are determined in the range 200–265 nm. The results are used to propose an optical method for the real-time simultaneous detection of molecular iodine and the I−, IO 3 − and I 3 −1 anions in liquid media. The measurement sensitivity for the above substances is sufficient for monitoring of the kinetics of physicochemical reactions that take place at various stages of processing of the spent nuclear fuel. This circumstance makes it possible to increase the efficiency and improve the ecological safety of the process.

S 2O, 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 S 2O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO 2 and UV–visible radiation simulating that of Io's surface. Our experiments show that S 2O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S 2O outgassed by Io's volcanoes and condensing on Io's surface should lead to yellow polysulfuroxide deposits or to white deposits of S 2O diluted in sulfur dioxide frost (i.e., S 2O/SO 2 < 0.1%). Thus S 2O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S 2O 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 S 8 sulfur and SO 2 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 S 8 and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas.

Spectroscopic studies of the I 2/O 3 photochemistry : Part 2. Improved spectra of iodine oxides and analysis of the IO absorption spectrum

Based on multivariate statistical separation techniques spectra of iodine oxides formed in the photolysis of I 2 + O 3 have been obtained. Data was recorded by time resolved UV–vis absorption spectroscopy. Overlapped spectra of ground state IO( ν′ ← 0), vibrationally excited IO( ν′ ← ν″) with ν″ > 0, and OIO have been separated from each other and from underlying absorptions of further iodine oxides. The uncertainty due to uncorrected foreign absorptions is of no more than ±3%. Relative errors of spectra are of the order of a few percent for the main parts of the spectra. The up to now unknown continuous parts of spectra are thereby determined. Previous uncertainties in differential absolute absorption cross-section are removed. By the separation of the ground state IO spectrum from that of vibrationally excited IO spectroscopic measurements under non-equilibrium conditions are enabled. Three further absorber spectra have been extracted in the 200–600 nm window. Two of them are most likely caused by I 2O 2 and possibly I 2O 3 thereby providing part of the missing link between IO and OIO consumption and the formation of higher oxides and possibly aerosol. All spectra are available as supplementary data. Given the unprecedented quality of extracted spectra for the first time an analysis of band strength of the IO(A 2Π 3/2 ← X 2Π 3/2) transition could be made. The continuum absorption of the ground state IO spectrum was resolved into overlapped bands of bound–bound transitions and two bound-free transitions. From the bound-free transitions found the existence of two optically active repulsive states intersecting with the IO(A 2Π 3/2) potential has been inferred and tangents to the two repulsive potentials have been determined. Correlating the tangents to probable dissociation products gives a plausible rough picture of the shape of the repulsive states which is in good agreement with previous observations on predissociation of states. There is evidence that one of the continuous absorptions observed could have its origin in the IO( 2Π 1/2) sub-system. Relative band strength was determined for absorption bands of IO(A 2Π 3/2 ← X 2Π 3/2). They are in good agreement with literature and with simple calculations based on a Morse approximation. An anomalous behaviour of IO(2 ← 0) in time resolved measurements was observed and studied. It could be explained by a hypothetical partial population inversion of the IO(A 2Π 3/2) state, but the source of it remains unclear.

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 SO 2 column density peaked at 1.25×10 17 cm −2 near the equator, with an additional 5×10 16 cm −2 enhancement over Prometheus corresponding to a model volcanic SO 2 output of 10 5 kg s −1. Apart from the Prometheus peak, the SO 2 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 SO 2 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 SO 2 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. SO 2 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/SO 2 ratio in the range 1–10%, roughly consistent with previous observations. S 2 gas was not seen anywhere, with an upper limit of 7.5×10 14 cm −2 for the Prometheus plume, confirming that this plume is significantly poorer in S 2 than the Pele plume (S 2 /SO 2<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 SO 2 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.

Sulfurous acid (H 2SO 3) on Io?

Sulfurous acid (H 2SO 3) has never been characterized or isolated on Earth. This is caused by the unfavorable conditions for H 2SO 3 within Earth's atmosphere due to the high temperatures, the high water content and the oxidizing environment. Kinetic investigations by means of transition state theory showed that the half-life of H 2SO 3 at 300 K is 1 day but at 100 K it is increased to 2.7 billion years. Natural conditions to form H 2SO 3 presumably require cryogenic SO 2 or SO 2/H 2O mixtures and high energy proton irradiation at temperatures around 100 K. Such conditions can be found on the Jupiter moons Io and Europa. Therefore, we calculated IR-spectra of H 2SO 3 which we compared with Galileo's spectra of Io and Europa. From the available data we surmise that H 2SO 3 is present on Io and probably but to a smaller extent on Europa.

Physical properties and spectra of IO, IO − and HOI studied by ab initio methods

Structure and properties of the IO, IO − and HOI species, which are of potential importance for the ozone destruction catalytic cycle in the troposphere, have been calculated together with the EPR, NMR and UV–visible spectra by ab initio methodology with account of spin–orbit coupling (SOC) effects. Multi-configuration self-consistent field calculations with linear and quadratic response techniques and the multi-reference configuration interaction method have been employed. Photodissociation of these species, crucial for the catalytic ozone-destruction cycle, is critically reviewed and analyzed. Calculations predict that the singlet–triplet ( S– T) transition to the lowest triplet state ( X 1 A′ → 3 A″) should be responsible for the weak long-wavelength tail absorption (∼450–560 nm) and photodissociation of the HOI molecule. The second, more intense, band around 400 nm is produced by two overlapping S– S and S– T transitions. In order to check this assignement of the HOI photodissociation the isoelectronic IO − anion and IO radical have been studied by the same methods. Comparison with the EPR spectrum of the IO radical indicates that the methods are reliable which gives credit to the accuracy of the HOI spectral interpretation. NMR spectra of HOI and IO − molecules and some other properties are calculated for the first time.

Eclipse Spectroscopy of Io's Atmosphere

High resolution optical (4193–6619 Å) spectra of Io in eclipse show auroral emission in five lines: [O I ] 6300, 6363, 5577 Å and Na 5889, 5896 Å. We conclude that Io's diffuse red emissions imaged by the Galileo Solid State Imager (P. E. Geissler et al. 1999, Science 285, 870–874) are due to impact or dissociative excitation of oxygen, while diffuse green emissions are caused by the impact or dissociative excitation of sodium. No emission lines were detected in the blue region of the spectra, suggesting that the intense emission observed above Io's equatorial plumes is due to a molecular species such as SO 2 or SO.

Search for Proton Aurora and Ambient Hydrogen on Io

A search was conducted using the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) in 1994 August for Iogenic H Lyα emission predicted from electron capture by precipitating trapped magnetospheric protons. The Doppler-shifted diffusely reflected solar Lyα component was detected in the combined high-resolution spectra, partly resolved from the geocoronal emission. This component had a disk-averaged intensity of 2.5 ± 0.7 kR. Io's corresponding geometric albedo in this line, which must include the effects of atmospheric SO2 absorption, was 0.055 ± 0.015. Iogenic emission from ambient H atoms was probably also detected, with a disk-averaged intensity greater than 280 R, most likely ≈485 R, which suggests a significant column of atmospheric or coronal H. The dominant brightness of the reflected solar line in our GHRS data supports the proposed hypothesis that the primary source of Io's H Lyα emission observed more recently (1997–1998) by the HST Space Telescope Imaging Spectrograph at low spectral resolution is diffusely reflected sunlight, rather than Iogenic H. The consequence is a significant increase in the absorbing SO2 column of Io's trailing atmosphere as Jupiter moved 0.37–0.44 AU closer to the Sun; this could result from the higher SO2 equilibrium vapor pressure over the 4–5 K warmer ice. Averaging low-resolution HST GHRS archival far-ultraviolet spectra of Io obtained during 1994–1996 provided a 2 σ upper limit of 5.2 R A-1 for the disk-averaged intensity of Lyα emission from energetic (27–223 keV) precipitating protons. This corresponds to an estimated upper limit of 47 R, integrated over the red Doppler wing of the thermal distribution for protons of mean energy 30–60 keV. This is close to estimates for a proton aurora generated in the SO2 column of Io's trailing atmosphere, indicating that SO2 may be Io's dominant atmospheric gas. A 2 σ upper limit of 123 R per line is derived for nearby fluorescence of extreme-UV radiation by ambient atmospheric sulfur.

Spin−Orbit Configuration Interaction Calculation of the Potential Energy Curves of Iodine Oxide

An ab initio configuration interaction (CI) study including spin−orbit coupling is carried out for the ground and excited states of the IO radical by employing relativistic effective core potentials. The computed spectroscopic constants are in good agreement with available experimental data, with some tendency to underestimate the strength of bonding. The first excited state, a4Σ-, which has not yet been observed experimentally, is predicted to be bound by 30.1 kJ/mol and to have a significantly larger equilibrium distance than the ground state. It is split by spin−orbit interaction into 1/2 and 3/2 components, with the 1/2 component being the lower one with a calculated spin−orbit splitting of 210 cm-1. The most interesting state in the low-energy IO spectrum, A12Π3/2, is shown to be predissociated due to interaction with a number of repulsive electronic states. Predissociation of the A1, v‘ = 0, 1 vibrational levels is attributed to a fairly weak spin−orbit coupling with the 2Δ3/2 state, while rotationally dependent predissociation of the v‘ = 2 level is explained by the coupling with the 1/2(III) state having mainly 2Σ- character. Strong predissociation of the v‘ ≥ 4 levels is attributed to interaction with the higher-lying Ω = 3/2 states, with predominantly 4Σ+ and 4Δ origin.

Volcanogenic Sulfur on Earth and Io: Composition and Spectroscopy

The causes of Io's variegated surface, especially the roles of sulfur, and the geochemical history of sulfur compounds on Io are not well understood. Suspecting that minor impurities in sulfur might be important, we have investigated the major and trace element chemistry and spectroscopic reflectance of natural sulfur from a variety of terrestrial volcanic–hydrothermal environments. Evidence suggests that Io may be substantially coated with impure sulfur. On Earth, a few tenths of a percent to a few percent of chalcophile trace elements (e.g., As and Se) comonly occur in sulfur and appear to stabilize material of yellow, brown, orange, and red hues, which may persist even at low temperatures. Percentage levels of chalcophile impurities are reasonably expected to occur on Io in vapor sublimate deposits and flows derived from such deposits. Such impurities join a host of other mechanisms that might explain Io's reds and yellows. Two-tenths to two percent opaque crystalline impurities, particularly pyrite (FeS 2), commonly produces green, gray, and black volcanic sulfur on Earth and might explain areas of Io having deposits of these colors. Pyrite produces a broad absorption near 1 μm that gradually diminishes out to 1.6 μm—similar but not identical to the spectrum of Io seen in Galileo NIMS data. Percentage amounts of carbonaceous impurities and tens of percent SiO 2 (as silicates) also strongly affect the spectral properties of Earth's sulfur. Io's broad absorption between 0.52 and 0.64 μm remains unexplained by these data but could be due to sodium sulfides, as suggested previously by others, or to As, Se, or other impurities. These impurities and others, such as P and Cl (which could exist on Io's surface in amounts over 1% that of sulfur), greatly alter the molecular structure of molten and solid sulfur. Minor impurities could impact Io's geology, such as the morphology of sulfur lava flows and the ability of sulfur to sustain high relief. We have not found any natural sulfur containing significant Na beyond that attributable to silicate inclusions. In sum, the unique physical–chemical properties of S-rich systems and the strong affinity of certain elements for S may have broad implications for the appearance, spectroscopic interpretation, and geologic processes of Io. Identification of impurities in sulfur may be helpful in tracing the geochemical evolution of surface deposits on Io. Perhaps foretelling of new areas of investigation, Cl has recently been reported in the Io torus (M. Kueppers and N. M. Schneider 1999, Eos Trans. 80, 5207), suggesting the presence on Io of either salts, such as halite, or sulfur chlorides. Further evidence of minor iogenic impurities should be sought in Io's neutral cloud and plasma torus as well as in further scrutiny of Io's reflectance spectra.

Measurement of tropospheric trace gases by long‐path differential absorption spectroscopy during the 1993 OH Photochemistry Experiment

In August and September 1993 a comparative study of tropospheric long‐path absorption techniques with in situ methods was performed for both the hydroxyl radical and the other important trace species. Long‐path measurements were made over the 10.3 km path between Fritz Peak Observatory and Caribou Mine in the mountains 17 km west of Boulder, Colorado. At Caribou Mine, a 121 element, 1 m2 retroreflector array folds the optical path to give a total path of 20.6 km. The in situ instruments were located at Idaho Hill 0.5 km northwest of Caribou Mine. The optical design and analysis techniques used to obtain the path‐integrated concentrations of O3, CH2O, SO2, and NO2 will be presented. The spectrograph used in this study is a 1/4 m double, crossed Czerny‐Turner that employs a diode array detector allowing the acquisition of 40 nm spectral bands in the near UV and visible spectral regions. This system also utilizes automatic alignment and self‐adjusting time integration so the system will acquire data in an unattended mode. The spectral bands selected for this study permit the simultaneous measurement of O3, NO2, CH2O, and SO2; NO2 and H2O; and NO3 and H2O. The data analysis uses a nonlinear least squares regression procedure to deduce the concentration of each of the species present in the atmosphere and also provides an effective method for removing the influence of scattered solar light for daytime measurements. An estimate of the measurement precision can be found by comparing atmospheric spectra analyzed with two different IO spectra; one measured through the atmosphere and the other a direct arc lamp spectrum.

Experimental Determination of the Ionization Energy of IO(X2Π3/2) and Estimations of ΔfH°0(IO+) and PA(IO)

Photoionization efficiency (PIE) spectra of IO(X2Πi) were measured over the wavelength range λ = 115.0−130.0 nm and in the ionization threshold region λ = 126.0−130.0 nm, using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. Iodine oxide was generated by the reactions of O(3P) atoms with I2 and CF3I. The PIE spectra displayed step-function behavior. From the half-rise point of the initial step, a value of 9.735 ± 0.017 eV was obtained for the adiabatic ionization energy (IE) of IO, corresponding to the IO+(X3Σ-) ← IO(X2Π3/2) transition. As this appears to be the first experimental determination of IE(IO), a trend analysis has been employed to provide further insight into the experimentally derived value. The separation between the first two steps in the PIE spectrum (which might be perturbed slightly due to autoionization) was used to derive the vibrational spacing of the cation, 1060 ± 160 cm-1. A Rydberg progression, presumably converging to the ex...

Laboratory Infrared Spectra (2.3-23 μm) of SO 2 Phases: Applications to Io Surface Analysis

Mid-infrared spectra (∼2-25 μm) of various SO 2 phases (some known to be present on Jupiter's satellite Io) contain diagnostic band features that can be used with observational data of Io to learn about its surface and atmospheric composition. A key portion of this spectral range (2-5 μm) will be accessible to the Near Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft now headed for Jupiter. Our lab work provides reflectance spectra of SO 2 phases for: (1) comparison with IR spectra of Io, (2) identification of compositional species on Io, and (3) mapping the spatial distribution of SO 2 phases on Io. Spectra at 4 cm -1 resolution were measured for a variety of phase states of SO 2 including SO 2 gas, SO 2 frost, SO 2 slab ice, SO 2 liquid, and adsorbed SO 2. Detailed lab spectra and band identifications including some new identifications are given for several varieties of each of these phases with particular emphasis on the 2- to 6-μm regions. We also produced a variety of surface textures for condensed SO 2, ranging from frosts with many peculiar (reticulated) textures to solid slab or glaze ice and liquid. The different frost textures result from different growth conditions (such as temperature, pressure, gas-supply rate, and substrate composition). All the optically thick frosts and ices that we produced have reflectance spectra that are essentially identical, regardless of texture. Previously reported IR spectra of solid SO 2 in other investigations were usually produced as transmission spectra of films of ice, not reflectance spectra of frost or ice surfaces. Our work shows that transmission spectra of thin films are not adequate to represent all features that may be present in reflectance spectra of macroscopic frost or ice on planetary bodies like Io; this is because thin-film samples provide optical pathlength insufficient to produce detectable absorption bands at wavelengths where bands are strong in reflectance spectra of optically thick deposits (or in transmission spectra of thick (few millimeters) monocrystals). Reflection spectra, as opposed to transmission spectra, also allow easier simulation of frost thickness effects and better reproduction of hand shapes that will actually be observed in planetary spectral observations. Our results indicate that one can estimate phase state and concentration level for condensed SO 2 on Io using the principal band ( v1 + v3) in the 4-μm region; for example, we found that the v1 + v3 band-minimum position is dependent on: (a) phase state of the SO 2, (b) thickness of frost or ice, and (c) instrument resolution. SO 2 ice or frost thickness (on a millimeter scale or less) can also be assessed by the 4-μm band shape characteristics and by the presence or absence of several other weaker bands; we have done this for In spectra and find that significant fractions of In must be covered by thick (>2 mm) frost. We have successfully used our lab spectra to identify and assign new hands in Io observational data reported previously by other researchers. We developed NIMS-equivalent spectra of various SO 2 phases showing what NIMS should "see" on Io at various spectral sampling strategies (resolutions): our lab data will provide a basis for NIMS to distinguish and map the distribution of various SO 2 phases on Io's surface and also identify such phases on other Galilean satellites if detectable quantities exist there.