Near-Infrared Mapping Spectrometer Research Articles

Near Infrared Spectral Radiance at Multiple Wavelengths From Io's Volcanoes 1: The Low Spatial Resolution Night‐Time Galileo NIMS Data Set

AbstractWe have calculated surface‐leaving spectral radiances for 285 detections of volcanic thermal emission from Io observations made by the Galileo Near Infrared Mapping Spectrometer (NIMS) in the 0.7‐ – 5.2‐μm wavelength range, the first such analysis of the entire low‐spatial‐resolution night‐time data set. Most of these detections are of 53 individual hot‐spots. We establish that 4.8‐μm spectral radiance is a good proxy for total thermal emission from a hot spot. Many detections reported here have not been previously modeled. This set of hot‐spot detections is derived from NIMS night‐time observations obtained between June 1996 (orbit G1) and October 2001 (Orbit I32), where the hot‐spot is sub‐pixel. Modeling of the spectra yields total thermal emission from each volcano in each observation, as well as the 2:5‐μm spectral radiance ratio and radiant flux density. Surface‐leaving spectral radiance is derived at 2.0, 3.5, 3.8, 4.8, and 5.0 μm allowing direct comparison with data obtained by other spacecraft and ground‐based telescope data. The most energetic volcanic eruptions are identified. We find an unusual number of sources in a C30 observation suggesting that a vigorous volcanic activity was taking place simultaneously in multiple locations, possibly tidally controlled.

Dark ray craters on Ganymede: Impactor or endogenous origin

Through the spectral analysis of Galileo Near Infrared Mapping Spectrometer (NIMS) observations we now have a better understanding of the origin of dark ray craters (DRCs) on Ganymede. The analysis of NIMS observations show that DRCs are not a single compositional group, and at least some DRCs are the result of the excavation of endogenous material. DRCs overlying bright terrain may be entirely due to the excavation of the darker Ganymede hydrate material; the dark ejecta associated with the Tammuz impact is an example with its rays composed entirely of Ganymede hydrate. In contrast, three DRCs on the trailing hemisphere imaged by NIMS (Kittu, Amon, and Mir craters) show that a third component is present, with the ejecta being a mixture of water-ice, the hydrated non-ice material endogenous to Ganymede, and a less-hydrated, red, non-ice component. The spectrum of this second non-ice component is consistent with hydrated carbonaceous material, implying either a primitive impactor origin or excavation of primordial Ganymede non-ice material. This carbonaceous component is required at an abundance of about 40 to 50% in the ejecta of the three DRCs, assuming discrete mixing. It is absent in icy terrain but often present in regiones unassociated with DRCs, ranging in abundance in that terrain from 0 to again, about 50%. Its abundance in older, more primordial terrain suggests the carbonaceous material is endogenous to Ganymede. Thus, excavation of subsurface material appears capable of accounting for the diversity of DRCs. Additional spectral imaging observations of these and other DRCs are needed to verify this conclusion.

Composition and possible origins of dark crater ejecta on Europa

The origins of low-albedo material on Europa’s surface have been of interest since Voyager first returned close-up images of the icy moon. Material ejected from Io is known to contribute an exogenic flux of dark material to Europa’s trailing hemisphere, and hydrated salt compounds concentrated within chaos terrain, ridges, and pits may be endogenous to the subsurface ocean. Many of Europa’s impact craters also exhibit dark ejecta, the origins of which are unknown. Our study examines the ejecta of several large impact craters to determine possible origins for this dark material. We compared the dark material found in crater ejecta to other surface materials using Galileo Near-Infrared Mapping Spectrometer data to assess similarities in composition between ejecta material and other dark materials on Europa’s surface. Our analysis shows that dark material found in crater ejecta has similar composition to other dark features on Europa and may be the result of comparable sources or alteration processes. We also considered dark impactors as sources for the dark ejecta material. Using crater scaling laws, we estimated the impactor size for each crater and determined the impactor’s potential contribution of dark material to the ejecta. We then compared these quantities to those derived using a radiative transfer model and the measured reflectance values of each dark-ejecta crater. Our model results show that the lower albedo of the ejecta of these craters cannot be solely attributed to an intimate mixture of the impactor material with the target material. In contrast, modeled impact heating and vaporization suggest sufficient amounts of ice could be removed in order to explain the observed low-albedo patterns, if preexisting or impactor-derived dark material is just 0.1% more resistant to vaporization than the ice. Given the lack of spatial correlation, and the presence of similar-sized craters without dark ejecta, these results point to either localized differences in the concentration of dark non-ice materials in Europa’s shell, or variations in impact velocities and geometries leading to differences in the amount of vaporized ejecta.

Europa’s Surface Water-ice Crystallinity and Correlations between Lineae and Hydrate Composition

Abstract Europa’s surface composition and evidence for cryovolcanic activity can provide insight into the properties and composition of the subsurface ocean, allowing the evaluation of its potential habitability. One promising avenue for revealing the surface processing and subsurface activity are the relative fractions of crystalline and amorphous water-ice observed on the surface, which are influenced by temperature, charged particle bombardment, vapor deposition, and cryovolcanic activity. The crystallinity observed on Europa’s leading hemisphere cannot be reproduced by thermophysical and particle flux modeling alone, indicating that there may be additional processes influencing the surface. We performed a spectral mixture analysis on hyperspectral image cubes from the Galileo Near-Infrared Mapping Spectrometer (NIMS) to identify how surface crystallinity is influenced by physical processing at a high spatial resolution scale. We focus specifically on two image cubes, 15e015 closer to the equator and 17e009 closer to the south pole, both on the leading hemisphere. We performed a nonnegative least-squares spectral mixture analysis to reveal both the non-ice composition and the water-ice crystallinity of the surface. We found that amorphous water-ice dominates the spectrum at the equator and the south pole. We estimated a mean crystallinity of ∼35% within the 15e015 NIMS cube and a mean crystallinity of ∼15% within the 17e009 NIMS cube, which is consistent with ground-based spectroscopically derived crystallinities. We also identified a correlation of magnesium sulfate, magnesium chloride, and hydrated sulfuric acid with lineae and ridges, which may provide evidence for surface processing by upwelling subsurface material.

Cautionary Analysis of Spectral Radiance from Io’s Active Volcanoes Derived from Galileo Near-Infrared Mapping Spectrometer Data

Abstract Between 1996 and 2001, the Galileo Near-Infrared Mapping Spectrometer (NIMS) obtained 190 observations of the volcanic Jovian moon Io. Rathbun et al. (2018) [Astron. J., 156, 207] published a list of 287 measurements of 3.5 μm spectral radiance from some of Io’s active volcanoes, derived from a subset of the NIMS data. However, the spectral radiances reported by Rathbun et al. are lower, in some cases by multiple orders of magnitude, than other analyses of the same observations and spectral radiances derived from contemporaneous ground-based data. In many cases, the Rathbun et al. hot-spot radiances are underreported by a factor of π, likely due to a mistake in unit conversion. For a small number of powerful hot spots, additional discrepancies appear to be the result of poor fits to data limited in wavelength range by NIMS detector saturation and a methodology that discards short-wavelength NIMS data that otherwise would have provided more robust temperature model fits.

A Comprehensive Revisit of Select Galileo/NIMS Observations of Europa

Abstract The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of Europa in the 0.7–5.2 μm wavelength region, which have been critical to improving our understanding of the surface composition of this moon. However, most of the work done to get constraints on abundances of species like water ice, hydrated sulfuric acid, hydrated salts, and oxides has used proxy methods, such as absorption strength of spectral features or fitting a linear mixture of laboratory-generated spectra. Such techniques neglect the effect of parameters degenerate with the abundances, such as the average grain size of particles, or the porosity of the regolith. In this work we revisit three Galileo NIMS spectra, collected from observations of the trailing hemisphere of Europa, and use a Bayesian inference framework, with the Hapke reflectance model, to reassess Europa’s surface composition. Our framework has several quantitative improvements relative to prior analyses: (1) simultaneous inclusion of amorphous and crystalline water ice, sulfuric-acid-octahydrate (SAO), CO2, and SO2; (2) physical parameters like regolith porosity and radiation-induced band-center shift; and (3) tools to quantify confidence in the presence of each species included in the model, constrain their parameters, and explore solution degeneracies. We find that SAO strongly dominates the composition in the spectra considered in this study, while both forms of water ice are detected at varying confidence levels. We find no evidence of either CO2 or SO2 in any of the spectra; we further show through a theoretical analysis that it is highly unlikely that these species are detectable in any 1–2.5 μm Galileo NIMS data.

The Science Case for Spacecraft Exploration of the Uranian Satellites: Candidate Ocean Worlds in an Ice Giant System

Abstract The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus’s five largest “classical” moons—Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon, Puck—but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved data sets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons’ surface compositions, and the processes that modify them, is limited to disk-integrated data sets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and to investigate Uranus’s ring moons and irregular satellites, close-up observations and measurements made by instruments on board a Uranus orbiter are needed.

H2O-ice particle size variations across Ganymede's and Callisto's surface

The detailed investigation of the variations in the H2O-ice spectral signatures across Ganymede's and Callisto's surfaces derived from the NIMS (Near Infrared Mapping Spectrometer) data of the Galileo mission have shown that in the case of areal mixing of H2O ice and dark components the associated band depth ratios (BDRs) of the individual H2O-ice absorptions can be reliably used to qualitatively infer variations in the H2O ice particle size and even to semi-quantitatively estimate particle size ranges on the satellites' surfaces when largely unaffected by hydrated non-ice materials or varying illumination conditions. This modeling approach implies H2O ice particle radii ranging between ~1 μm and ~1 mm on Ganymede and up to ~2 mm on Callisto Our analyses point to a continuous decrease of H2O - ice particle sizes toward the poles on both satellites. These variations may be caused by similar surface processes. The smooth latitudinal trend may be related to the surface temperatures and possible thermal migration of H2O vapor to colder region in higher latitudes (Spencer, 1987) and particle welding at lower latitudes. Larger particles in the equatorial region of Callisto compared to Ganymede could be due to its higher maximum surface temperature and a longer Callistoan day resulting in more extensive particle welding.

Sodium chloride on the surface of Europa.

The potential habitability of Europa's subsurface ocean depends on its chemical composition, which may be reflected in that of Europa's geologically young surface. Investigations using Galileo Near-Infrared Mapping Spectrometer data led to the prevailing view that Europa's endogenous units are rich in sulfate salts. However, recent ground-based infrared observations have suggested that, while regions experiencing sulfur radiolysis may contain sulfate salts, Europa's more pristine endogenous material may reflect a chloride-dominated composition. Chlorides have no identifying spectral features at infrared wavelengths, but develop distinct visible-wavelength absorptions under irradiation, like that experienced on the surface of Europa. Using spectra obtained with the Hubble Space Telescope, we present the detection of a 450-nm absorption indicative of irradiated sodium chloride on the surface. The feature correlates with geologically disrupted chaos terrain, suggesting an interior source. The presence of endogenous sodium chloride on the surface of Europa has important implications for our understanding of its subsurface chemistry.

The Global Distribution of Active Ionian Volcanoes and Implications for Tidal Heating Models

Abstract Tidal heating is the major source of heat in the outer solar system. Because of its strong tidal interaction with Jupiter and the other Galilean satellites, Io is incredibly volcanically active. We use the directly measured volcanic activity level of Io’s volcanoes as a proxy for surface heat flow and compare it to tidal heating model predictions. Volcanic activity is a better proxy for heat flow than simply the locations of volcanic constructs. We determine the volcanic activity level using three data sets: the Galileo Photopolarimeter Radiometer (PPR), Galileo Near-Infrared Mapping Spectrometer (NIMS), and New Horizons LEISA. We also present a systematic reanalysis of the Galileo NIMS observations to determine the 3.5 μm brightness of 51 active volcanoes. We find that potential differences in volcanic style between high and low latitudes make high-latitude observations unreliable for distinguishing between tidal heating models. Observations of Io’s polar areas, such as those by Juno, are necessary to unambiguously understand Io’s heat flow. However, all three of the data sets examined show a relative dearth of volcanic brightness near 180 W (anti-Jovian point) and the equator, and the only data set with good observations of the sub-Jovian point (LEISA) also shows a lack of volcanic brightness in that region. These observations are more consistent with the mantle-heating model than the asthenospheric-heating model. Furthermore, all three of the data sets are consistent with fourfold symmetry in longitude and peak heat flow at mid-latitudes, which best matches with the combined heating case of Tackley et al.

Stability of hydrated carbonates on Ceres

Deposits of carbonates have been observed and definitively identified by Dawn's Visible Near-Infrared Mapping Spectrometer (VIR), particularly in the faculae that lie within the central portion of Occator, Oxo, and Haulani craters, implying geologically recent cryo-volcanism or extrusion with sub-surface CO2 and H2O. Carbonate composition varies from primarily sodium at the Cerealia and Vinalia Faculae and at Oxo crater, where carbonate deposits are most abundant, to magnesium and calcium for most other bright regions. The formation of hydrated salts is expected from the aqueous alteration of silicates; however, VIR measurements of the faculae show no water signature, potentially the result of dehydration after exposure to Ceres’ surface conditions. We investigate the stability and decomposition pathway for hydrated sodium-carbonate, natron (Na2CO3.10H2O), grains in the laboratory under Ceres’ cryogenic, low-pressure environment by UV–vis–NIR reflectance spectroscopy and X-ray powder diffraction. H2O-loss begins simultaneously with vacuum-exposure, altering natron's spectral signature by attenuation of the water bands, enhancement of the carbonate features, and concurrent reduction of the NIR blue spectral slope. We find that the water absorption features in natron reduce below VIR's detection limit (< 2%) within a time scale of < 6 days at temperatures ≥ 200 K, eliminating hydrous sodium carbonate from Ceres’ surface mineralogy in the equatorial region and the mid-latitudes without continuous rehydration. A temperature-dependent systematic shift of the 1.9-µm band center to lower wavelengths is observed with vacumm-exposure time at 200 and 240 K. In Ceres’ polar-regions (∼120 K), natron retains water longer, depleting the 1.9-µm water band to < 2% within a few hundred years (∼300 years). No significant changes in the visible relative reflectance or spectral slope result from vacuum-exposure of hydrous or anhydrous sodium carbonate, which does not match the observed red-slope in the Framing Camera (FC) measurements for Occator Crater's faculae. In the UV, extended exposure to vacuum in all sodium carbonates examined here causes significant reddening due to an increase in short-range crystallographic defects and reduction of the conduction band energy; in addition, the development of new UV electronic transition features at ∼ 275 and 235 nm is observed in hydrous and anhydrous sodium-carbonates with vacuum-processing. Similar transitions in carbonates and organics may contribute to the unidentified 280 nm absorption feature on Ceres.

Composition and structure of fresh ammonia clouds on Jupiter based on quantitative analysis of Galileo/NIMS and New Horizons/LEISA spectra

Ammonia gas has long been assumed to be the main source of condensables for the upper cloud layer on Jupiter, but distinctive spectral features associated with ammonia have been seen only rarely. Since both ammonia and NH4SH absorb in the 3 µm region, and widespread absorption in the 3 µm region was present (Sromovsky and Fry, 2010), identification of the 2 µm absorption feature of NH3 provided an opportunity to clearly establish its presence in Jovian clouds. Baines et al. (2002) succeeded in finding in Near Infrared Mapping Spectrometer (NIMS) observations one feature that had both 2 µm and 3 µm absorption, and many which were known to have absorption at 2.73 µm. They named these Spectrally Identifiable Ammonia Clouds (SIACs). They also argued that these were fresh ammonia clouds that would eventually succumb to some process that would obscure their absorption features. Detection of many more of the 2 µm features was later achieved by New Horizon’s Linear Etalon Imaging Spectral Array (LEISA) instrument, which provided both the spatial and spectral resolution needed to identify these features. Here we report on the first quantitative modeling that uses NIMS spectra over a broad (1–5.2 µm) spectral range and LEISA spectra over a much narrower (1.25–2.5 µm) spectral range to constrain the cloud structure and composition of these rare cloud features and compare them to background clouds. We find that the absorption signature at 2 µm, which is well characterized in LEISA spectra, is relatively subtle and easily matched by model clouds containing spherical particles of ammonia ice with radii of 2–4 µm. The NIMS spectra, which cover both reflected sunlight as well as thermal emission regions are more difficult to model with cloud materials plausibly present in Jupiter’s atmosphere. The best signal/noise spectra obtained from NIMS provide a relatively sparse sampling of the spectrum, which does not establish the detailed shape of the 3 µm absorption region. NIMS SIAC spectra with much denser spectral sampling are limited by much higher noise levels that degrade the features that are key to identifying cloud composition. The structure which best matches the wide range NIMS SIAC spectra contains two overlapping NH3 clouds with a bi-modal size distribution over an optically thick NH4SH cloud. The bi-modal distribution may be a result of modeling non-spherical, possibly fractal aggregate, particles with spheres.

Analysis of IR-bright regions of Jupiter in JIRAM-Juno data: Methods and validation of algorithms

In this paper, we detail the retrieval methods developed for the analysis of the spectral data from the JIRAM experiment on board of the Juno NASA mission [1], operating in orbit around Jupiter since July 2016. Our focus is on the analysis of the thermal radiation in the 5 µm transparency window in regions of lesser cloud opacity (namely, hot-spots).Moving from the preliminary analysis presented in [2], a retrieval scheme has been developed and implemented as a complete end-to-end processing software. Performances in terms of fit quality and retrieval errors are discussed from tests on simulated spectra, while some example and issue from usage on actual Jupiter data are also discussed.Following the suggestion originally presented in [3] for the analysis of the data from the Near Infrared Mapping Spectrometer (NIMS) on board of the NASA Galileo spacecraft, the state vector to be retrieved has been drastically simplified on physically sounding basis, aiming mostly to distinguish between the 'deep' content of minor gaseous components (H2O, NH3, PH3) and their relative humidity or fractional scale height in the upper troposphere. The retrieval code is based on a Bayesian scheme [4], complemented by simulated annealing method for most problematic cases.The key parameters retrievable from JIRAM individual spectra are the NH3 and PH3 deep contents, the H2O vapor relative humidity as well as the total aerosol opacity.Limitations related to the approximations of forward model methods are also assessed quantitatively.

Surface composition of pull-apart bands in Argadnel Regio, Europa: Evidence of localized cryovolcanic resurfacing during basin formation

We combine Galileo Solid State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) data to investigate the composition of pull-apart bands in Europa's Argadnel Regio. Using spectral linear mixture modeling employing cryogenic laboratory reference spectra, we find that bands of intermediate age (“grey” bands) are compositionally distinct from bands that are stratigraphically younger (“dark” bands). The grey bands have higher abundances of larger ice grains and lower abundances of hydrated salts than the dark bands; both of these tendencies are statistically significant at the 1% level. The grey and dark bands have similar abundances of hexahydrite, a material which is relatively stable under irradiation; however, the derived abundances of frozen magnesium sulfate brine and of mirabilite, which are more susceptible to fragmentation by radiation, are significantly higher in the dark bands than in the grey bands. These results are consistent with a physical model in which the differences in composition and in ice grain sizes are linked to space weathering and radiolytic processing levels; the grey bands have presumably undergone higher levels of processing, due to being exposed on Europa's surface for a longer period of time. One prominent wedge-shaped band exhibits an anomalous albedo variation across its northern portion, appearing dark in its top third, and grey in its southernmost two-thirds. We find that the dark part of the band has a modeled composition that is in-family with other dark bands, while the grey portion has a modeled composition that is indistinguishable from other grey bands in the study area. Because these variations cannot easily be attributed to the band's formation mechanism (bands open sequentially along a central axis), we surmise that the northern part has been resurfaced, probably in response to the formation of a large topographic basin that cuts through the band. Faulting accompanying basin formation may provide conduits allowing transport to the surface of materials from Europa's interior. We hypothesize that the formation of the basin resulted in fresh cryovolcanic material being deposited across the northern portion of the band, effectively “resetting” its surface age. If, as has been suggested, the giant arcuate basins resulted from an episode of true polar wander, our study may help to more tightly constrain the age of that event within Europa's geologic column.

Radiation Noise Effects at Jupiter’s Moon Europa: In-Situ and Laboratory Measurements and Radiation Transport Calculations

There is great scientific interest of Jupiter’s moon Europa and in performing further spacecraft investigations. However, successful measurements at this satellite are difficult because transient charge pulses produced by Jupiter’s magnetospheric radiation can significantly degrade signals. In order to develop shielding and mitigation approaches for future Europa missions, we investigated the radiation noise produced in the Galileo spacecraft’s Near Infrared Mapping Spectrometer (NIMS) in the jovian magnetosphere. We then analyzed the radiation-transport characteristics of the NIMS shield and calculated the shielding efficiencies for thicker shields. Major findings are (1) NIMS data obtained at the orbital radius of Europa yield a rate of transient charge pulses (“hits”) of $1.5 \times 10^6~\hbox{cm}^{-2} ~\hbox{s}^{-1}$ behind 5 mm of Ta shielding, (2) radiation transport calculations indicate a decrease in the hit rate with increased shielding, achieving a factor of 10 reduction for 30 mm thick walls, (3) $\gamma $ -ray photons produced in the shield (and possibly the surrounding instrument and the spacecraft itself) are equally or more important in generating radiation noise, and (4) $\gamma$ -rays, with their mean-free-paths greater than the range of electrons with the same energy, are difficult to shield so heavier shielding is less beneficial than previously predicted.

PHASE ANGLE EFFECTS ON 3 μ m ABSORPTION BAND ON CERES: IMPLICATIONS FOR DAWN MISSION

Phase angle-induced spectral effects are important to characterize since they affect spectral band parameters such as band depth and band center, and therefore skew mineralogical interpretations of planetary bodies via reflectance spectroscopy. Dwarf planet (1) Ceres is the next target of NASA's Dawn mission, which is expected to arrive in March 2015. The visible and near-infrared mapping spectrometer (VIR) onboard Dawn has the spatial and spectral range to characterize the surface between 0.25-5.0 microns. Ceres has an absorption feature at 3.0 microns due to hydroxyl- and/or water-bearing minerals (e.g. Lebofsky et al. 1981, Rivkin et al. 2003). We analyzed phase angle-induced spectral effects on the 3-micron absorption band on Ceres using spectra measured with the long-wavelength cross-dispersed (LXD: 1.9-4.2 microns) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF). Ceres LXD spectra were measured at different phase angles ranging from 0.7o to 22o. We found that the band center slightly increases from 3.06 microns at lower phase angles (0.7o and 6o) to 3.07 microns at higher phase angles (11 o and 22o), the band depth decreases by ~20% from lower phase angles to higher phase angles, and the band area decreases by ~25% from lower phase angles to higher phase angles. Our results will have implications for constraining the abundance of OH on the surface of Ceres from VIR spectral data, which will be acquired by Dawn starting spring 2015.

Charting thermal emission variability at Amirani with the Galileo NIMS Io Thermal Emission Database (NITED)

We have examined the variability of thermal emission from lava flows at Amirani on Io, using measurements of radiant flux from detections by the Galileo Near Infrared Mapping Spectrometer (NIMS) between 1996 and 2001. Amirani is the longest currently active lava flow field in the Solar System and a persistent thermal source in every Galileo NIMS observation that covers its location. We have quantified the thermal emission from hot spots correlated with discrete areas of new lava flow emplacement at points along the length of the Amirani flow field in high spatial resolution NIMS observations in 2000 and 2001. Where discernible, the position of some effusive activity changes from orbit to orbit. We find the implied style of emplacement of lava at Amirani is consistent with pahoehoe-like flows. There is an estimated 50% decrease in discharge rate between 1997 and late 2001, perhaps linked to an outburst eruption in the Amirani vicinity in February 2001. The variability of thermal emission from Amirani is less than that seen at a number of other persistently active ionian volcanoes (such as Prometheus, Culann, and Loki Patera). All of these volcanoes exhibit large increases and decreases in the absolute magnitude of thermal emission (see Matson et al., 2006 [Loki Patera]; Davies, A.G. et al. [2006]. Icarus 184, 460–477 [Prometheus]; and Davies, A.G., Ennis, M.E. [2011]. Icarus 215, 401–416 [Culann]). At Amirani, as at these volcanoes, the thermal emission measured between 1996 and 2001 indicates a persistent, although variable, supply of magma to the surface. In high spatial resolution NIMS observations of hot spots obtained in 2000 and 2001 we estimate the emitted power from active lava flows associated with Amirani to be ∼170±30GW, which corresponds to total effusion rates (assuming a basaltic composition) from multiple points along the Amirani flow of 34–56m3/s.

CARBONIC ACID AS A RESERVE OF CARBON DIOXIDE ON ICY MOONS: THE FORMATION OF CARBON DIOXIDE (CO2) IN A POLAR ENVIRONMENT

Carbon dioxide (CO{sub 2}) has been detected on the surface of several icy moons of Jupiter and Saturn via observation of the ν{sub 3} band with the Near-Infrared Mapping Spectrometer on board the Galileo spacecraft and the Visible-Infrared Mapping Spectrometer on board the Cassini spacecraft. Interestingly, the CO{sub 2} band for several of these moons exhibits a blueshift along with a broader profile than that seen in laboratory studies and other astrophysical environments. As such, numerous attempts have been made in order to clarify this abnormal behavior; however, it currently lacks an acceptable physical or chemical explanation. We present a rather surprising result pertaining to the synthesis of carbon dioxide in a polar environment. Here, carbonic acid was synthesized in a water (H{sub 2}O)-carbon dioxide (CO{sub 2}) (1:5) ice mixture exposed to ionizing radiation in the form of 5 keV electrons. The irradiated ice mixture was then annealed, producing pure carbonic acid which was then subsequently irradiated, recycling water and carbon dioxide. However, the observed carbon dioxide ν{sub 3} band matches almost exactly with that observed on Callisto; subsequent temperature program desorption studies reveal that carbon dioxide synthesized under these conditions remains in solid form until 160 K, i.e., themore » sublimation temperature of water. Consequently, our results suggest that carbon dioxide on Callisto as well as other icy moons is indeed complexed with water rationalizing the shift in peak frequency, broad profile, and the solid state existence on these relatively warm moons.« less

Magnetospheric ion sputtering and water ice grain size at Europa

We present the first calculation of Europa's sputtering (ion erosion) rate as a function of position on Europa's surface. We find a global sputtering rate of 2×1027H2O s−1, some of which leaves the surface in the form of O2 and H2. The calculated O2 production rate is 1×1026O2 s−1, H2 production is twice that value. The total sputtering rate (including all species) peaks at the trailing hemisphere apex and decreases to about 1/3rd of the peak value at the leading hemisphere apex. O2 and H2 sputtering, by contrast, is confined almost entirely to the trailing hemisphere. Most sputtering is done by energetic sulfur ions (100s of keV to MeV), but most of the O2 and H2 production is done by cold oxygen ions (temperature ∼ 100 eV, total energy ∼ 500 eV). As a part of the sputtering rate calculation we compared experimental sputtering yields with analytic estimates. We found that the experimental data are well approximated by the expressions of Famá et al. for ions with energies less than 100keV (Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161), while the expressions from Johnson et al. fit the data best at higher energies (Johnson, R.E., Burger, M.H., Cassidy, T.A., Leblanc, F., Marconi, M., Smyth, W.H., 2009. Composition and Detection of Europa's Sputter-Induced Atmosphere, in: Pappalardo, R.T., McKinnon, W.B., Khurana, K.K. (Eds.), Europa. University of Arizona Press, Tucson.). We compare the calculated sputtering rate with estimates of water ice regolith grain size as estimated from Galileo Near-Infrared Mapping Spectrometer (NIMS) data, and find that they are strongly correlated as previously suggested by Clark et al. (Clark, R.N., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Implications for remote sensing of planetary surfaces. Icarus 56, 233–245.). The mechanism responsible for the sputtering rate/grain size link is uncertain. We also report a surface composition estimate using NIMS data from an area on the trailing hemisphere apex. We find a high abundance of sulfuric acid hydrate and radiation-resistant hydrated salts along with large water ice regolith grains, all of which are consistent with the high levels of magnetospheric bombardment at the trailing apex.

Exogenic controls on sulfuric acid hydrate production at the surface of Europa

External agents have heavily weathered the visible surface of Europa. Internal and external drivers competing to produce the surface we see include, but are not limited to: aqueous alteration of materials within the icy shell, initial emplacement of endogenic material by geologic activity, implantation of exogenic ions and neutrals from Jupiter's magnetosphere, alteration of surface chemistry by radiolysis and photolysis, impact gardening of upper surface layers, and redeposition of sputtered volatiles. Separating the influences of these processes is critical to understanding the surface and subsurface compositions at Europa. Recent investigations have applied cryogenic reflectance spectroscopy to Galileo Near-Infrared Mapping Spectrometer (NIMS) observations to derive abundances of surface materials including water ice, hydrated sulfuric acid, and hydrated sulfate salts. Here we compare derived sulfuric acid hydrate (H2SO4·nH2O) abundance with weathering patterns and intensities associated with charged particles from Jupiter's magnetosphere. We present models of electron energy, ion energy, and sulfur ion number flux as well as the total combined electron and ion energy flux at the surface to estimate the influence of these processes on surface concentrations, as a function of location. We found that correlations exist linking both electron energy flux (r∼0.75) and sulfur ion flux (r=0.93) with the observed abundance of sulfuric acid hydrate on Europa. Sulfuric acid hydrate production on Europa appears to be limited in some regions by a reduced availability of sulfur ions, and in others by insufficient levels of electron energy. The energy delivered by sulfur and other ions has a much less significant role. Surface deposits in regions of limited exogenic processing are likely to bear closest resemblance to oceanic composition. These results will assist future efforts to separate the relative influence of endogenic and exogenic sources in establishing the surface composition.