Europa's Surface Research Articles

Science Potential from a Europa Lander

The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

Implantation of multiply charged sulfur ions in water ice

Sulfuric acid has been suggested to be present on the surface of the jovian moon Europa where it is mixed with the dominant water ice. The formation mechanism of sulfuric acid is still under discussion.We present new experimental results on the implantation of Sq+ (q=7, 9, 11) ions at an energy range between 35 and 176keV in water ice at 80K. Previous results on 200keV S+ implantation in H2O at 80K have also been included in the data analysis. Experiments with multiply-charged ions at different energies are particularly relevant to simulate the complexity of the irradiation environment to which the surface of Europa is exposed being embedded in the jovian magnetosphere.The experiments were performed at the low energy ion beam facility ARIBE of GANIL in Caen (France). 35–176keVSq+ (q=7, 9, 11) ions were implanted in solid H2O layers which were frozen at 80K. Fourier Transform Infrared Spectrometry (FTIR) was used to analyze the sample in the 5000–600cm−1 (2–16.7μm) region with a spectral resolution of 1cm−1.The results of our experiments indicate that implantation produces hydrated sulfuric acid with yields that increase with ion energy, from 0.12moleculesion−1 for 35keV ions to 0.64moleculesion−1 for 200keV ions. We have also searched for the production of SO2 and H2S, but we were not able to find any evidence for their synthesis.We conclude that sulfur ion implantation is the dominant formation mechanism of hydrated sulfuric acid at Europa. The suggestion that the observed distribution of sulfuric acid on the surface is well correlated with the local flux of sulfur ions find a full explanation by present experimental data.

Does folding accommodate Europa's contractional strain? The effect of surface temperature on fold formation in ice lithospheres

On Europa, the mechanism by which contractional strains are accommodated remains a mystery. Subtle folds have been observed in the polar regions, but if folding is a dominant accommodation mechanism, ubiquitous large‐amplitude folds might be expected across Europa's surface. Here we use finite element modeling to show that fold growth rates are a strong function of surface temperature: warm temperatures result in lower rates, whereas cold temperatures result in higher rates. Combining diurnally averaged Europan surface temperatures derived from Galileo photopolarimeter‐radiometer data and new numerical simulations, we show that forming moderate‐amplitude folds requires roughly twice the contraction, or shortening, in Europa's equatorial latitudes relative to polar latitudes. Relatively large contractional strains in Europa's equatorial and midlatitudes can therefore be accommodated primarily through passive shortening without producing obvious folds. Lithospheric folding and/or passive shortening therefore remain plausible mechanisms for strain accommodation on Europa.

KECK II OBSERVATIONS OF HEMISPHERICAL DIFFERENCES IN H 2 O 2 ON EUROPA

We present results from Keck II observations of Europa over four consecutive nights using the near-infrared spectrograph (NIRSPEC). Spectra were collected in the 3.14--4.0 micron range, allowing detection and monitoring of the 3.5 micron feature due to hydrogen peroxide. Galileo Near-Infrared Spectrometer (NIMS) results first revealed hydrogen peroxide on Europa in the anti-jovian region of the leading hemisphere at an abundance of 0.13+/-0.07% by number abundance relative to water. We find comparable results for the two nights over which we observed the leading hemisphere. Significantly, we observed a small amount of hydrogen peroxide (~0.04%) during observations of Europa's anti- and sub-Jovian hemispheres. Almost no hydrogen peroxide was detected during observations of just the trailing hemisphere. We conclude that the Galileo observations likely represent the maximum hydrogen peroxide concentration, the exception potentially being the cold water ice regions of the poles, which are not readily observable from the ground. Our mapping of the peroxide abundance across Europa requires revisions to previous estimates for Europa's global surface abundance of oxidants and leads to a reduction in the total oxidant delivery expected for the sub-surface ocean, if exchange of surface material with the ocean occurs.

SALTS AND RADIATION PRODUCTS ON THE SURFACE OF EUROPA

The surface of Europa could contain the compositional imprint of a underlying interior ocean, but competing hypotheses differ over whether spectral observations from the Galileo spacecraft show the signature of ocean evaporates or simply surface radiation products unrelated to the interior. Using adaptive optics at the W.M. Keck Observatory, we have obtained spatially resolved spectra of most of the disk of Europa at a spectral resolution ~40 times higher than seen by the Galileo spacecraft. These spectra show a previously undetected distinct signature of magnesium sulfate salts on Europa, but the magnesium sulfate is confined to the trailing hemisphere and spatially correlated with the presence of radiation products like sulfuric acid and SO2. On the leading, less irradiated, hemisphere, our observations rule out the presence of many of the proposed sulfate salts, but do show the presence of distorted water ice bands. Based on the association of the potential MgSO4, detection on the trailing side with other radiation products, we conclude that MgSO4 is also a radiation product, rather than a constituent of a Europa ocean brine. Based on ocean chemistry models, we hypothesize that, prior to irradiation, magnesium is primarily in the form of MgCl2, and we predict that NaCl and KCl are even more abundant, and, in fact, dominate the non-ice component of the leading hemisphere. We propose observational tests of this new hypothesis.

Plasma ion composition measurements for Europa

Jupiter magnetospheric interactions and surface composition, both important to subsurface ocean detection for the Galilean icy moons Europa, Ganymede, and Callisto, can be measured using plasma ion mass spectrometry on either an orbiting spacecraft or one designed for multiple flybys of these moons. Detection of emergent oceanic materials at the Europa surface is more likely than at Ganymede and Callisto. A key challenge is to resolve potential intrinsic Europan materials from the space weathering patina of iogenic species implanted onto the sensible surface by magnetospheric interactions. Species-resolved measurements of pickup ion currents are also critical to extraction of oceanic induced magnetic fields from magnetospheric interaction background dominated by these currents. In general the chemical astrobiological potential of Europa should be determined through the combination of surface, ionospheric, and pickup ion composition measurements. The requisite Ion Mass Spectrometer (IMS) for these measurements would need to work in the high radiation environment of Jupiter's magnetosphere between the orbits of Europa and Ganymede, and beyond. A 3D hybrid model of the moon-magnetosphere interaction is also needed to construct a global model of the electric and magnetic fields, and the plasma environment, around Europa. Europa's ionosphere is probably usually dominated by hot pickup ions with 100–1000eV temperatures, excursions to a “classical” cold ionosphere likely being infrequent. A field aligned ionospheric wind driven by the electron polarization electric field should arise and be measurable.

A SEARCH FOR MAGNESIUM IN EUROPA'S ATMOSPHERE

Europa's tenuous atmosphere results from sputtering of the surface. The trace element composition of its atmosphere is therefore related to the composition of Europa's surface. Magnesium salts are often invoked to explain Galileo Near Infrared Mapping Spectrometer spectra of Europa's surface, thus magnesium may be present in Europa's atmosphere. We have searched for magnesium emission in Hubble Space Telescope Faint Object Spectrograph archival spectra of Europa's atmosphere. Magnesium was not detected and we calculate an upper limit on the magnesium column abundance. This upper limit indicates that either Europa's surface is depleted in magnesium relative to sodium and potassium, or magnesium is not sputtered as efficiently resulting in a relative depletion in its atmosphere.

Implantation of multiply charged carbon ions in water ice

Context. Several objects in the solar system like Europa, Ganymede, and Callisto have frozen surface (main component: H2O). The associated thickness is bigger than the penetration depth of the relevant projectile ions. Additionally, other species such as H2O2, SO2, and CO2 have been detected on these surface. The formation mechanisms of these molecules are still under discussion. Aims. We present new experimental results on the implantation of 13 C q+ (q = 2, 3) ions at an energy of 30 keV in water ice at low temperatures (15 and 80 K). Experiments with multiply-charged ions at energies of tens of keV are particularly relevant to simulating the complexity of the irradiation environment to which the surfaces of the icy moons in the outer solar system are exposed. Methods. The experiments were performed at the low-energy ion beam facility ARIBE of GANIL in Caen (France). 30 keV 13 C q+ (q = 2, 3) ions have been used to bombard solid H2O surface, which were frozen at 15 K and 80 K. Fourier transform infrared spectrometer (FTIR) was used to analyze the sample in the 5000‐600 cm 1 (2‐16.7 µm) region with a spectral resolution of 1 cm 1 . Results. The results of our experiments indicate that implantation produces 13 CO2 with yields in the range of 0.32‐ 0.57 moleculesion 1 . This yield seems to be independent of the temperature of the ices in the range studied. We have estimated the time scale necessary to accumulate by implantation of magnetospheric carbon ions the observed quantity of carbon dioxide on the surface of Europa, a Jovian moon. This time scale is of the order of 1.0‐1.3# 10 4 yrs, which is higher than that evaluated for carbon dioxide production by other relevant processes. Conclusions. We conclude that although a relevant quantity of CO2 can be formed by carbon ion implantation, this is not the dominant formation mechanism at Europa.

Characterization of Salting-Out Processes during CO2-Clathrate Formation Using Raman Spectroscopy: Planetological Application

ABSTRACT Clathrate hydrates are particular solids that planetologists study in detail because those solids may be present in several bodies of the solar system, such as Mars, comets, and the icy satellites. The solids are formed by solid H2O, like common water ice, but adopt open structures with cavities containing gas molecules. Clathrate hydrates are usually stable at relatively low temperature and high pressure, which are the typical conditions present inside these planetary objects. Their interest for astrobiology is that they represent potential sources of liquid water and gases when they decompose. The present work is focused on the crystallization of clathrates in Europa's (icy satellite of Jupiter) interior conditions. We postulate that clathrate hydrates may play an important role in its crust mineralogy and that it can explain some features of the satellite's surface due to their formation/destabilization. An in situ kinetic study by Raman Spectroscopy of the clathrate formation from salty solutions was performed in our laboratory. The chemical composition that we used mimics those obtained from Europa's surface during the Galileo mission. An effect of the salting-out process in the solution was monitored through the clathrate formational path. Our results demonstrate that this process may have geological consequences on Europa and confirm the suitability of Raman spectroscopy for planetary detection of clathrate hydrates and other ices.

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.

Radiation-induced radicals in hydrated magnesium sulfate

Abstract Radiation-induced free radicals in hydrated magnesium sulfate, which are thought to be present on the surface of Europa, one of the Jovian moons, have been studied by electron spin resonance (ESR). ESR signals of both atomic hydrogen (H·) at g = 2.0023 and sulfite radical ( SO 3 − ) at g = 2.0029 are observed in epsomite (MgSO 4 ·7H 2 O) and in quenched samples of magnesium sulfate solutions, after gamma-ray irradiation at 77 K. Atomic hydrogen disappears at temperatures above 90 K. The sulfite radical starts to decay above 190 K in the quenched sample of 10% magnesium sulfate solution, whereas in epsomite, it is stable even at 260 K. The sulfite radical accumulates at low ambient temperatures, and is a promising candidate for estimating the age of hydrated magnesium sulfate, especially in extraterrestrial environments.

Acidification of Europa's Subsurface Ocean as a Consequence of Oxidant Delivery

Oxidants are formed at the surface of Europa and may be delivered to the subsurface ocean, possibly in great quantities. Whether these substances would be available for biological metabolism is uncertain, because they may react with sulfides and other compounds to generate sulfuric and other acids. If this process has been active on Europa for much of its age, then not only would it rob the ocean of life-supporting oxidants but the subsurface ocean could have a pH of ~2.6, which is so acidic as to present an environmental challenge for life, unless organisms consume or sequester the oxidants fast enough to ameliorate the acidification.

Low-altitude, near-polar and near-circular orbits around Europa

The dynamics of orbits around planetary satellites, taking into account the gravitational attraction of a third-body and the non-uniform distribution of mass of the planetary satellite, is studied. The Hamiltonian considered is explicitly time-dependent. Conditions for frozen orbits are presented. Low-altitude, near-polar orbits, very desirable for scientific missions to study planetary satellites such as the Jupiter’s moon Europa, are analyzed. Lifetimes for these orbits are computed through the single and double averaged method. Comparison between the results obtained by the single and double averaged method is presented. The single-averaged model is more realistic, since it does not eliminate the term due to the equatorial ellipticity of the planetary satellite as done by the double-averaged problem. Considering the single-averaged method, we found unstable frozen orbits where the satellite does not impact with the surface of Europa for at least 200days. We present an approach using the unaveraged disturbing potential to analyze the effects of these terms in the amplitude of the eccentricity.

Formation of relief peculiarities on the surface of Europa

Formation of relief peculiarities on the surface of Europa

H 2O 2 production by high-energy electrons on icy satellites as a function of surface temperature and electron flux

Chemistry on the icy surface of Europa is heavily influenced by the incident energetic particle flux from the jovian magnetosphere. The majority (>75%) of this energy is in the form of high energy electrons (extending to >10 MeV). We have simulated the electron irradiation environment of Europa with a vacuum system containing a high-energy electron gun for irradiation of ice samples formed on a gold mirror cooled with a cryostat. Pure water films of ∼2.6 μm thickness were grown at 100 K and then either cooled (to 80 K), warmed (to 120 K) or left at 100 K and subsequently irradiated with 10 keV electrons. The production of hydrogen peroxide (H 2O 2) was monitored by observation of the 2850 cm −1 (3.5 μm) band. Equilibrium concentrations of H 2O 2, in units of percent by number H 2O 2 relative to water, were found to be 0.043% (80 K), 0.029% (100 K), and 0.0063% (120 K). These values are 33%, 22%, and 5%, respectively, that of the reported surface concentration on the leading hemisphere of Europa (Carlson, R.W., Anderson, M.S., Johnson, R.E., Smythe, W.D., Hendrix, A.R., Barth, C.A., et al. [1999]. Science 283(5410), 2062–2064) and less than the equilibrium concentrations formed by ion irradiation. In addition to the ice film temperature, the current of electrons was varied between different experiments to determine the production and destruction of H 2O 2 as a function of both electron flux and ice temperature. Variation in current was found to have little effect on the results other than accelerating arrival at radiolytic equilibrium.

Landforms of Europa and selection of landing sites

Three major features make Europa a unique scientific target for a lander-oriented interplanetary mission: (1) the knowledge of the composition of the surface of Europa is limited to interpretations of the spectral data, (2) a lander could provide unique new information about outer parts of the solar system, and (3) Europa may have a subsurface ocean that potentially may harbor life, the traces of which may occur on the surface and could be sampled directly by a lander. These characteristics of Europa bring the requirement of safe landing to the highest priority level because any successful landing on the surface of this moon will yield scientific results of fundamental importance. The safety requirements include four major components. (1) A landing site should preferentially be on the anti-Jovian hemisphere of Europa in order to facilitate the orbital maneuvers of the spacecraft. (2) A landing site should be on the leading hemisphere of Europa in order to extend the lifetime of a lander and sample pristine material of the planet. (3) Images with the highest possible resolution must be available for the selection of landing sites. (4) The terrain for landing must have morphology (relief) that minimizes the risk of landing and represents a target that is important from a scientific point of view. These components severely restrict the selection of regions for landing on the surface of Europa. After the photogeologic analysis of all Galileo images with a resolution of better than about 70 m/pixel taken for the leading hemisphere of Europa, we propose one primary and two secondary (backup) landing sites. The primary site (51.8°S, 177.2°W) is within a pull-apart zone affected by a small chaos. The first backup site (68.1°S, 196.7°W) is also inside of a pull-apart zone and is covered by images of the lower resolution (51.4 m/pixel). The second backup site (2.4°N, 181.1°W) is imaged by relatively low-resolution images (∼70 m/pixel) and corresponds to a cluster of small patches of dark and probably smooth plains that may represent landing targets of the highest scientific priority from the scientific point of view. The lack of the high-resolution images for this region prevents, however, its selection as the primary landing target.

Europa’s disk-resolved ultraviolet spectra: Relationships with plasma flux and surface terrains

The full set of high-resolution observations from the Galileo Ultraviolet Spectrometer (UVS) is analyzed to look for spectral trends across the surface of Europa. We provide the first disk-resolved map of the 280 nm SO 2 absorption feature and investigate its relationship with sulfur and electron flux distributions as well as with surface features and relative surface ages. Our results have implications for exogenic and endogenic sources. The large-scale pattern in SO 2 absorption band depth is again shown to be similar to the pattern of sulfur ion implantation, but with strong variations in band depth based on terrain. In particular, the young chaos units show stronger SO 2 absorption bands than expected from the average pattern of sulfur ion flux, suggesting a local source of SO 2 in those regions, or diapiric heating that leads to a sulfur-rich lag deposit. While the SO 2 absorption feature is confined to the trailing hemisphere, the near UV albedo (300–310 nm) has a global pattern with a minimum at the center of the trailing hemisphere and a maximum at the center of the leading hemisphere. The global nature of the albedo pattern is suggestive of an exogenic source, and several possibilities are discussed. Like the SO 2 absorption, the near UV albedo also has local variations that depend on terrain type and age.

Cosmic ion bombardment of the icy moons of Jupiter

A large number of experiments have been performed in many laboratories in the world with the aim to investigate the physico-chemical effects induced by fast ions irradiating astrophysical relevant materials. The laboratory in Catania (Italy) has given a contribution to some experimental works. In this paper I review the results of two class of experiments performed by the Catania group, namely implantation of reactive (H +, C +, N +, O + and S +) ions in ices and the ion irradiation induced synthesis of molecules at the interface between water ice and carbonaceous or sulfurous solid materials. The results, discussed in the light of some questions concerning the surfaces of the Galilean moons, contribute to understand whether minor molecular species (CO 2, SO 2, H 2SO 4, etc.) observed on those objects are endogenic i.e. native from the satellite or are produced by exogenic processes, such as ion implantation. The results indicate that: – C-ion implantation is not the dominant formation mechanism of CO 2 on Europa, Ganimede and Callisto. – Implantation of sulfur ions into water ice produces hydrated sulfuric acid with high efficiency such to give a very important contribution to the sulfur cycle on the surface of Europa and other satellites. – Implantation of protons into carbon dioxide produces some species containing the projectile (H 2CO 3, and O–H in poly-water). – Implantation of protons into sulfur dioxide produces SO 3, polymers, and O 3 but not H–S bonds. – Water ice has been deposited on refractory carbonaceous materials: a general finding is the formation of a noteworthy quantity of CO 2. We suggest that this is the primary mechanism to explain the presence of carbon dioxide on the surfaces of the Galilean satellites. – Water ice has been deposited on refractory sulfurous materials originating from SO 2 or H 2S irradiation. No evidence for an efficient synthesis of SO 2 has been found.

Constraints on Europa's surface properties from primary and secondary crater morphology

We investigate the near‐surface properties of Europa's ice shell by examining small crater morphology; we use both primary and secondary craters for our analysis. For primary craters, the simple‐to‐complex transition provides an estimate of 0.19–0.36 MPa for the cratering effective strength. Through the aggregate statistics of over 100 profiles, we find that secondary craters on Europa tend toward smaller depth‐to‐diameter (d/D) ratios than primary craters, consistent with observations on other cratered surfaces (the Moon and Mars). In addition, we find that adjacent secondaries tend to be more shallow than distant secondaries, also consistent with trends seen on the Moon and Mars. The presence of numerous and far‐flung secondaries requires a solid, competent surface for spallation that is inconsistent with weak ice. Although the effective strength evaluated by crater scaling laws does not have a direct quantitative relationship to lab‐based measurements of shear or tensile strength, comparison to other observationally derived effective strengths and geologic materials suggests that the mechanical properties of Europa's surface ice should be consistent with terrestrial measurements of cold ice.