The chemical nature of Europa surface material and the relation to a subsurface ocean

The surface composition of Europa is of special interest due to the information it might provide regarding the presence of a subsurface ocean. One source of this information is the infrared reflectance spectrum. Certain surface regions of Europa exhibit distorted H₂O vibrational overtone bands in the 1.5 and 2.0 μm region, as measured by the Galileo mission Near Infrared Mapping Spectrometer (NIMS). These bands are clearly the result of highly concentrated solvated contaminants. However, two interpretations of their identity have been presented. One emphasizes hydrated salt minerals and the other sulfuric acid, although each does not specifically rule out some of the other. It has been pointed out that accurate chemical identification of the surface composition must depend on integrating spectral data with geochemical models, and information on the tenuous atmosphere sputtered from the surface. It is also extremely important to apply detailed chemistry when interpreting the spectral data, including knowledge of mineral dissolution chemistry and the subsequent optical signatures of ion solvation in low-temperature ice. We present studies of flash frozen acid and salt mixtures as Europa surface analogs and demonstrate that solvated protons, metal cations and inorganic anions all influence the spectra and must all, collectively, be considered when assigning Europa spectral features. These laboratory data show best correlation with NIMS Europa spectra for multi-component mixtures of sodium and magnesium bearing sulfate salts mixed with sulfuric acid. The data provide a concentration upper bound of 50-mol% for MgSO₄ and 40-mol% for Na₂SO₄. This newly reported higher sodium and proton content is consistent with low-temperature aqueous differentiation and hydrothermal processing of carbonaceous chondrite-forming materials during the formation and early evolution of Europa.     

Hydrated minerals on Europa’s surface: An improved look from the Galileo NIMS investigation

The surface composition of Europa is of great importance for understanding both the internal evolution of Europa and its putative ocean. The Near Infrared Mapping Spectrometer (NIMS) investigation on Galileo observed Europa and the other Galilean satellites from 0.7 to 5.2 μm with spatial resolution down to a few kilometers during flybys by the spacecraft as it orbited Jupiter. These data have been analyzed and results published over the life of the Galileo mission and afterward. One result was the discovery of hydrated minerals at some locations on Europa and Ganymede. The data are noisy, especially for Europa, due to radiation affecting the NIMS electronics and detectors, and other artifacts are also present. The NIMS data are now being reprocessed using the accumulated knowledge gained over the entire missions to remove noise spikes and compensate for some other defects in the data. We are analyzing these reprocessed data in an attempt to defined better the nature of the hydrate spectral features and improve their interpretation. We report here on analyses of two NIMS reprocessed observations for the 0.7-3-μm region. A revised hydrate spectrum is calculated and mapped in detail across two lineaments. The spectrum shows the expected distorted water features but little or no spectral structure in these features. A narrow, weak spectral feature appears at 1.344 μm, which is weakly correlated with lower albedo. Several other weak features may be present but are difficult to confirm in these limited data sets. The hydrate signature shows the greatest strength within and toward the center of the lineaments, confirming and strengthening the association of the hydrate with these endogenic features. This trend may indicate that the material in the lineaments is youngest toward the center and has more water frost coverage toward the edge. A small, visually dark, circular feature has a spectrum that shows both hydrate and crystalline water ice features and perhaps contains a hydrate different in spectral characteristics and perhaps composition than found in the lineament.     

Europa's neutral cloud: morphology and comparisons to Io

Ground based observations of sodium escaping from Europa suggest the presence of an extended cloud of neutrals orbiting Jupiter. Using a Monte Carlo model we show that the large scale morphology differs from the sodium cloud at Io. At Europa, the trailing cloud is brighter and more extended than the leading cloud. We then use our results to consider the morphology of Europa's oxygen cloud.     

Europa’s ridged plains and smooth low albedo plains: Distinctive compositions and compositional gradients at the leading side-trailing side boundary

This investigation uses linear mixture modeling employing cryogenic laboratory reference spectra to estimate surface compositions and water ice grain sizes of Europa’s ridged plains and smooth low albedo plains. Near-infrared spectra for 23 exposures of ridged plains materials are analyzed along with 11 spectra representing low albedo plains. Modeling indicates that these geologic units differ both in the relative abundance of non-ice hydrated species and in the abundance and grain sizes of water ice. The background ridged plains in our study area appear to consist predominantly of water ice (∼46%) with approximately equal amounts (on average) of hydrated sulfuric acid (∼27%) and hydrated salts (∼27%). The solutions for the smooth low albedo plains are dominated by hydrated salts (∼62%), with a relatively low mean abundance of water ice (∼10%), and an abundance of hydrated sulfuric acid similar to that found in ridged plains (∼27%). The model yields larger water ice grain sizes (100 μm versus 50-75 μm) in the ridged plains. The 1.5-μm water ice absorption band minimum is found at shorter wavelengths in the low albedo plains deposits than in the ridged plains (1.498 ± .003 μm versus 1.504 ± .001 μm). The 2.0-μm band minimum in the low albedo plains exhibits a somewhat larger blueshift (1.964 ± .006 μm versus 1.983 ± .006 μm for the ridged plains).The study area spans longitudes from 168° to 185°W, which includes Europa’s leading side-trailing side boundary. A well-defined spatial gradient of sulfuric acid hydrate abundance is found for both geologic units, with concentrations increasing in the direction of the trailing side apex. We associate this distribution with the exogenic effects of magnetospheric charged particle bombardment and associated chemical processing of surface materials (the radiolytic sulfur cycle). However, one family of low albedo plains exposures exhibits sulfuric acid hydrate abundances up to 33% lower than found for adjacent exposures, suggesting that these materials have undergone less processing, thus implying that these deposits may have been emplaced more recently.Modeling identifies high abundances (to 30%) of magnesium sulfate brines in the low albedo plains exposures. Our investigation marks the first spectroscopic identification of MgSO₄ brine on Europa. We also find significantly higher abundances of sodium-bearing species (bloedite and mirabilite) in the low albedo plains. The results illuminate the role of radiolytic processes in modifying the surface composition of Europa, and may provide new constraints for models of the composition of Europa’s putative subsurface ocean.     

Iapetus dark and bright material:: giving compositional interpretation some latitude

Narrowband reflectance spectra (0.53-1.0 μm) of Iapetus' leading and trailing sides were obtained in 2000 to test the presence of an absorption feature located near 0.67 μm seen in reflectance spectra of Iapetus' dark material and Hyperion's surface material. No feature was observed. The difference in reflectance across the UV/VIS/NIR spectral region, and the dependence of the presence or absence of this absorption feature on angular separation from the apex of Iapetus in its orbit, phase angle, and heliocentric distance (affecting temperature), were examined. A trend of increased reddening, and the presence of the absorption feature, correlate with an angular separation from the apex of ? approximately 10°. Spectral information is lost when the contribution of the bright water ice signal to the reflectance spectrum increases sufficiently. In order to optimize compositional studies of Iapetus, we encourage future ground-based and space-based spectral observations to maximize the concentration of dark material in the instrumental field of view.     

Putative ice flows on Europa: Geometric patterns and relation to topography collectively constrain material properties and effusion rates

Europa's surface exhibits numerous small dome-like and lobate features, some of which have been attributed to fluid emplacement of ice or slush on the surface. We perform numerical simulations of non-Newtonian flows to assess the physical conditions required for these features to result from viscous flows. Our simulations indicate that the morphology of an ice flow on Europa will be, at least partially, influenced by pre-existing topography unless the thickness of the flow exceeds that of the underlying topography by at least an order of magnitude. Three classes of features can be identified on Europa. First, some (possibly most) putative flow-like features exhibit no influence from the pre-existing topography such as ridges, although their thicknesses are generally on the same order as those of ridges. Therefore, flow processes probably cannot explain the formation of these features. Second, some observed features show modest influence from the underlying topography. These might be explained by ice flows with wide ranges of parameters (ice temperatures >230 K, effusion rates >10⁷ m³ year⁻¹, and a wide range of grain sizes), although surface uplift (e.g., by diapirism) and in situ disaggregation provide an equally compelling explanation. Third, several observed features are completely confined by pre-existing topographic structures on at least one side; these are the best known candidates for flow features on Europa. If these features resulted from solid-ice flows, then temperatures >260 K and grain sizes <2 μm are required. Such small grain sizes seem unlikely; low-viscosity flows such as ice slurries or brines provide a better explanation for these features. Our results provide theoretical support for the view that many of Europa's lobate features have not resulted from solid-ice flows.     

Resurfacing history of Europa from pole-to-pole geological mapping

We produced geologic maps from two regional mosaics of Galileo images across the leading and trailing hemispheres of Europa in order to investigate the temporal distribution of units in the visible geologic record. Five principal terrain types were identified (plains, bands, ridges, chaos, and crater materials), which are interpreted to result from (1) tectonic fracturing and lineament building, (2) cryovolcanic reworking of surface units, with possible emplacement of sub-surface materials, and (3) impact cratering. The geologic histories of both mapped areas are essentially similar and reflect some common trends: Tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments, the opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges and ridge complexes along prominent fractures in the ice. It also appears that lineaments are narrower and more widely spaced with time. The lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation. In contrast, the degree of cryovolcanic resurfacing appears to increase with time, as chaos formation dominates the later parts of the geologic record. These trends, and the transition from tectonic- to cryovolcanic-dominated resurfacing could be attributed to the gradual thickening of Europa's cryosphere during the visible geologic history, that comprises the last 2% or 30-80 Myr of Europa's history: An originally thin, brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes; the degree of fracturing and plate displacements decreased with time in a thickening shell, and lineaments became narrower and more widely spaced; formation of chaos regions could have occurred where the thickness threshold for solid-state convection was exceeded, and can be aided by preferential tidal heating of more ductile ice. In a long-term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite, or cyclic or irregular episodes of tectonic and cryovolcanic activity.     

A search for temporal variability in the surface chemistry of the icy Galilean satellites

The purpose of this study was to determine if any temporal variability in the broadband UV spectral properties of the icy Galilean satellites exists, and if so, to characterize its spatial distribution as a function of longitude in order to attempt to correlate any temporal changes with satellite surface interactions with the space environment. The temporal time period examined is between 1978-1984 (referred to as the 1980s data) and those from 1995-1996 (referred to as the 1990s data). The plausible temporal characteristics detected appear to vary from one satellite to the next. For Europa possible temporal variations are concentrated on the leading, anti-jovian quadrant. Example broadband UV spectra show Europa's spectral slope decreases (and darkens) with time on the leading and anti-jovian hemispheres, but remains essentially constant with time on the trailing hemisphere. The data quality does not support any definitive temporal changes for Ganymede. Possible temporal changes seen in the Callisto data set are concentrated on the jovian hemisphere. Example broadband UV spectra for Callisto show no definitive change in slope with time. The hypothesis is that these temporal differences in UV spectral properties are caused by variations in the surface ice chemistry due to temporal variability in the space environment. It is postulated that the UV spectral changes suggested for Europa may be linked to changes in H₂O₂ concentrations, whereas the changes on Callisto may be linked to variability in SO₂ concentration.     

Revisiting the thermal inertia of Iapetus: Clues to the thickness of the dark material

The energy balance at the surface of an airless planetary body is strongly influenced by the bolometric Bond albedo and the surface thermal inertia. Both of these values may be calculated through the application of a thermal model to measured surface temperatures. The accuracy of either, though, increases if the value of the other is better constrained. In this study, we used the improved global bolometric Bond albedo map of Iapetus derived from Cassini VIMS and ISS and Voyager ISS data in conjunction with Cassini CIRS temperature data to reevaluate surface thermal inertia across Iapetus. Results showed the thermal inertia of the dark terrain varies between 11 and 14.8 J m⁻² K⁻¹ s^−1/2 while the light material varies between 15 and 25 J m⁻² K⁻¹ s^−1/2. Using an approximation to the thermal properties of the dark overburden derived from our thermal inertia results, we can implement our thermal model to provide estimates on the dark material thickness, which was found to lie between 7 cm and 16 cm. In order to develop an accurate global thermal model, a weighted function that approximates the surface thermal inertia across Iapetus was developed and verified via our measurements. The global bolometric Bond albedo map, surface thermal inertia map, and the thermal model are then used to synthesize global temperature maps that may be used to study the stability of volatiles.     

Ground-based photometric observations of Jupiter's inner satellites Thebe, Amalthea, and Metis at small phase angles

We present the results of photometric measurements of the inner jovian satellites Thebe, Amalthea and Metis based on extensive optical observations taken from October 1999 to January 2002. The observations were made in the phase angle range from 8.1° to 0.3°. The Two-Channel Focal Reducer of the Max-Planck Institute for Aeronomy attached to the 2-m RCC telescope at Terskol Observatory (Pik Terskol, Northern Caucasus) was used in coronagraph mode. The observations were performed at a wavelength of 0.887 μm. Mean observational uncertainties corresponding to 1σ rms errors were 3% for the leading and trailing sides of Amalthea, 7 and 9% for the leading and trailing sides of Thebe and 9% for the leading side of Metis after taking into account the longitude brightness variations. Photometric data calibrated on an absolute scale were used to evaluate the near-opposition behavior of satellite brightness. All three satellites exhibit significant opposition brightening, but the strength of this effect, measured as the ratios of intensities at α₁=1.6° and α₂=6.7° does not vary significantly among these satellites. In order to measure the opposition surge parameters the empirical law proposed by Karkoschka and Hapke's model were used. The parameters of the satellite opposition effects are presented and discussed. The values of geometric albedos calculated with best-fit Hapke parameters are 0.096, 0.157, and 0.24 for Thebe, Amalthea, and Metis respectively. We found that the average leading/trailing ratios of surface reflectance at the measured phase angles are 1.53±0.05, 1.25±0.04, 1.04±0.08 for Amalthea, Thebe, and Metis.     

Mass anomalies on Ganymede

Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions—a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77° N, 333° W), (36° N, 0° W), (33° N, 130° W), and (7° N, 194° W). If the mass anomalies are located at the deep ice-rock interface 800 km below the surface, the regions' centers are approximately (65° N, 17° W), (32° N, 30° W), (37° N, 175° W), and (15° N, 211° W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10¹⁷ kg. Mass anomalies at the ice-rock interface are on average no more than an order of magnitude larger (10¹⁸ kg). There are two positive and two negative mass anomalies in both the surface and ice-rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice-rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice-rock interface.     

The origin of (90) Antiope from component-resolved near-infrared spectroscopy

The origin of the similarly-sized binary Asteroid (90) Antiope remains an unsolved puzzle. To constrain the origin of this unique double system, we recorded individual spectra of the components using SPIFFI, a near-infrared integral field spectrograph fed by SINFONI, an adaptive optics module available on VLT-UT4. Using our previously published orbital model, we requested telescope time when the separation of the components of (90) Antiope was larger than 0.087″, to minimize the contamination between components, during the February 2009 opposition. Several multi-spectral data-cubes in J band (SNR = 40) and H + K band (SNR = 100) were recorded in three epochs and revealed the two components of (90) Antiope. After developing a specific photometric extraction method and running an error analysis by Monte-Carlo simulations, we successfully extracted reliable spectra of both components from 1.1 to 2.4 μm taken on the night of February 21, 2009. These spectra do not display any significant absorption features due to mafic mineral, ices, or organics, and their slopes are in agreement with both components being C- or Cb-type asteroids. Their constant flux ratio indicates that both components’ surface reflectances are quite similar, with a 1-sigma variation of 7%. By comparison with 2MASS J, H, K color distribution of observed Themis family members, we conclude that both bodies were most likely formed at the same time and from the same material. The similarly-sized system could indeed be the result of the breakup of a rubble-pile proto-Antiope into two equal-sized bodies, but other scenarios of formation implying a common origin should also be considered.     

Galileo PPR observations of Europa: Hotspot detection limits and surface thermal properties

The Galileo photopolarimeter–radiometer (PPR) made over 100 observations of Europa’s surface temperature. We have used these data to constrain a diurnal thermal model and, thus, map the thermal inertia and bolometric albedo over 20% of the surface. We find an increased thermal inertia at mid-latitudes that is widespread in longitude and does not appear to correlate with geology, albedo, or other observables. Our derived thermophysical properties can be used to predict volatile stability across the surface over the course of a day and in planning of infrared instruments on future missions. Furthermore, while observations in the thermal infrared can and have been used to find endogenic activity, no such activity was detected at Europa. We have calculated the detection limits of these PPR observations and find that 100 km² hotspots with temperatures of 116–1200 K could exist undetected on the surface, depending on the location.     

Iapetus surface variability revealed from statistical clustering of a VIMS mosaic: The distribution of CO2

We present a detailed study of an Iapetus mosaic of VIMS data with high spatial resolution (0.5 × 0.5° or ∼6.4 km/pixel). The spectra were taken in August 2007 and provide the highest VIMS spatial resolution data for this object during Cassini’s primary mission. We analyze this set of data using a statistical clustering approach to reduce the analysis of a large number of data (∼10⁴ spectra from 0.35 to 5.10 μm) to the study of seven representative groups accounting for 99.6% of the surface covered by the original sample. We analyze the spectral absorption bands in the spectra of the different clusters indicative of different composition over the observed surface. We find coherence between the distribution of the clusters and the geographical features on the surface. We give special attention to the study of the water ice and CO₂ bands. We find that CO₂ is widespread over the entire surface being studied, including the bright and dark areas on Iapetus’ surface, and is probably trapped at the molecular level with other materials. The strength of the CO₂ band in the areas where both, H₂O- and carbon-bearing materials exist, gives support to the hypothesis that this volatile is formed on the surface of Iapetus as a product of irradiation of these two components. Finally, we also compare the Iapetus CO₂ with that on other satellites confirming, that there are evident differences on the center, depth and width of the band on Iapetus and Phoebe, where CO₂ has been suggested to be endogenous.     

Io: Heat flow from dark paterae

Dark paterae on the jovian satellite Io are evidence of recent volcanic activity. Some paterae appear to be entirely filled with dark volcanic material, while others have only partially darkened floors. Dark paterae have area and heat flow longitudinal distributions that are bimodal as well as anti-correlated with the longitudinal distribution of mountains on Io at a global scale. As part of our study of Io’s total heat flow, we have examined the darkest paterae and quantified their thermal emission in order to assess their contribution. This is the first time that the areas of the dark material in these paterae have been measured with such precision and correlated with their thermal emission. Dark paterae yield a significantly larger contribution to Io’s heat flow than dark volcanic fields. Dark paterae (including Loki Patera) yield at least ∼4 × 10¹³ W or ∼40% of Io’s total heat flow. In comparison, dark flow fields yield ∼10¹³ W or ∼10% of Io’s total heat flow. Of the total heat loss from dark paterae, Loki Patera alone yields ∼10¹³ W or ∼10% of Io’s total thermal emission.     

Near-infrared spectroscopy of Himalia

We present the first spectrum through the L band of an irregular satellite from the outer Solar System. Spectra of Himalia (JVI) were obtained with the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. The Himalia spectrum is essentially featureless, showing a slight red slope and the suggestion of an absorption at 3 μm that might indicate the presence of water in some form. Better measurements of the spectrum of Himalia, particularly in the region of the apparent 3-μm band, could help determine whether water is present, and if so, in what form.     

Io: Heat flow from dark volcanic fields

Dark flow fields on the jovian satellite Io are evidence of current or recent volcanic activity. We have examined the darkest volcanic fields and quantified their thermal emission in order to assess their contribution to Io’s total heat flow. Loki Patera, the largest single source of heat flow on Io, is a convenient point of reference. We find that dark volcanic fields are more common in the hemisphere opposite Loki Patera and this large scale concentration is manifested as a maximum in the longitudinal distribution (near ∼200 °W), consistent with USGS global geologic mapping results. In spite of their relatively cool temperatures, dark volcanic fields contribute almost as much to Io’s heat flow as Loki Patera itself because of their larger areal extent. As a group, dark volcanic fields provide an asymmetric component of ∼5% of Io’s global heat flow or ∼5 × 10¹² W.     

Effects of plasticity on convection in an ice shell: Implications for Europa

Europa's surface exhibits numerous pits, uplifts, and disrupted chaos terrains that have been suggested to result from convection in the ice shell. To test this hypothesis, we present numerical simulations of convection in an ice shell including the effects of plasticity, which provides a simple continuum representation for brittle or semibrittle deformation along discrete fractures. Plastic deformation occurs when stresses reach a specified yield stress; at lower stresses, the fluid flow follows a Newtonian, temperature-dependent viscosity. Four distinct modes of behavior can occur. For yield stresses exceeding ∼1 bar, plastic effects are negligible and stagnant-lid convection, with no surface motion and minimal topography, results. At intermediate yield stresses, a stagnant lid forms but deforms plastically, leading to surface velocities up to several millimeters per year. Slightly smaller yield stresses allow episodic, catastrophic overturns of the upper conductive lid, with (transient) stagnant lids forming in between overturn events. The smallest yield stresses allow continual recycling of the upper lid, with simultaneous, gradual ascent of warm ice to the surface and descent of cold, near-surface ice into the interior. The exact yield stresses over which each regime occurs depend on the ice-shell thickness, melting-temperature viscosity, and activation energy for viscous creep. To form hummocky matrix and translate chaos plates by several kilometers, substantial surface strain must accompany chaos formation, and the three plasticity-dominated convection modes described here can provide such deformation. Our simulations suggest that, if yield stresses of ∼0.2-1 bar are relevant to Europa, then convection in Europa's ice shell can produce chaos-like structures at the surface. However, our simulations have difficulty explaining Europa's numerous pits and uplifts. When plasticity forces the upper lid to participate in the convection, dynamic topography of ∼50-100-m amplitude results, but the topographic structures generally have diameters of 30-100 km, an order of magnitude wider than typical pits and uplifts. None of our simulations produced isolated pits or uplifts of any diameter.     

The roughness of the dark side of Iapetus from the 2004 to 2005 flyby

The roughness of a planetary surface offers clues to its past geologic history. We apply a surface roughness model developed by Buratti and Veverka (Buratti, B.J., Veverka, J. [1985]. Icarus 64, 320-328) to Cassini ISS data from the January 1st, 2005 flyby of Iapetus. This model uses the observed scattering behavior to provide a depth to radius factor q quantifying the size of idealized craters on the surface. Our findings indicate that the surface on the dark side is significantly smoother than the surfaces of other icy low-albedo saturnian satellites. We have found that the average depth to radius on the leading (dark) side is 0.084, corresponding to a Hapke mean slope angle of 6°. As compared to the 13-33° Hapke mean slope angle of other icy satellites (Buratti, B.J., and 10 colleagues [2008]. Icarus 193, 309-322), our results present a clearly different picture for the leading surface of Iapetus, suggesting that the dark deposit contributes to the decrease in macroscopic surface roughness of the leading side. Attempts were made to obtain an average depth to radius value for the trailing (bright) side; however the scans of the bright side from this flyby exhibited large variations in albedo, resulting in results that were physically unrealistic.     

Production of noble gases near the surface of Europa and the prospects for in situ chronology

The age of the surface of Europa is probably tens of Myr or less, but is poorly constrained. Two different geochronology schemes could potentially be applied to near-surface samples to provide far more precise ages. First, the surface salts apparently contain enough potassium to make potassium-argon dating feasible. Second, the bombardment of the surface with both galactic cosmic rays and protons trapped within the jovian magnetosphere will cause nuclear reactions that can lead to measurable buildups of cosmogenic noble gases, which can be used to determine both cosmic-ray exposure ages and erosion, deposition, or mixing rates for surface modification. The major differences between Europa's salt-rich ice and the rocks (meteorites, lunar samples and terrestrial rocks) in which cosmogenic noble gases are normally measured are that the abundance of target elements for nuclear reactions creating neon and argon are lower (because of the high water content), but neutron-capture reactions, particularly ³⁵Cl(n,γ)³⁶Cl(β⁻)³⁶Ar, are enhanced because of the thermalizing effects of the water. As well as presenting calculations of noble gas production near the surface of Europa, we also show that the measurements required to determine ages are within reach of technology in the near future, if an instrument can be landed on the surface of Europa.