Strategy for the Post-Galileo Exploration of Europa
In this chapter COMPLEX provides additional discussion of the outstanding scientific issues regarding Europa that are outlined in the final section of Chapter 2 and, in particular, details the measurements that are required to address these issues. Although the geological and geophysical topics are discussed separately for simplicity, information elucidating them must be integrated to obtain an overall understanding of the nature of Europa and its history through time, the possibility for liquid water, and the potential for life.
GLOBAL CHARACTERIZATION OF GEOLOGY AND SURFACE COMPOSITION
The complexity of the europan surface was determined primarily from reconnaissance imaging provided by Voyager and Galileo camera systems and enhanced by other remote-sensing data, including those from the Near-Infrared Mapping Spectrometer (NIMS), which provided data on surface compositions. These data, however, covered only limited parts of the europan surface and are inadequate for characterization on global scales. Moreover, regional and local analyses are very limited. Characterizations on a wide range of scales are required not only to enable a more complete understanding of Europa's complex history, but also to provide critical information for potential landing operations, such as site selection and the global and regional scientific context in which to understand lander results.
Galileo results suggest that the general geology of Europa can be characterized with imaging data of a few hundred meters per pixel covering a substantial fraction of the surface (note that as indicated in Table 3.1, only a very small fraction of the surface was imaged at this resolution during GEM). These data enable the general terrains to be mapped, the major structural features to be identified, and the stratigraphic relationships to be determined, from which surface histories can be derived.
Information on surface compositions should be determined from orbital observations conducted at spectral resolutions of 10 to 15 nm in the near infrared with contiguous wavelength sampling of spatial locations. Significant uncertainty was introduced by NIMS's discontinuous spectral coverage at a given spatial location. NIMS's design is such that it acquires data in only 17 spectral channels at a given time. As a result, its grating must be moved between 6 and 24 times to obtain data spanning its full spectral range of 0.7 to 5.2 microns. Thus, pointing uncertainties and the motion of the spacecraft between observations can cause the spatial mislocation of adjacent spectral channels. This misregistration can cause apparent spectral patterns when observations are taken as the spacecraft is moving over terrain with a rapidly varying albedo.
TABLE 3.1 Galileo's Imaging Coverage of Europa's Surface
If all wavelengths are observed simultaneously, then the compositional information is free of such artifacts. Indeed, reflectance spectroscopic determination of surface composition works remarkably well for ices. The widely spaced and relatively deep absorption bands of different ices make unique identification of ices straightforward. Band shapes and depths allow determination of relative abundances with precisions and accuracy approaching 5%. This type of information, obtained on a global scale at a spatial resolution of 1 km, will allow models of surface origins to be tested and will help resolve the spectral contributions from water ice of varying textures and clarity in addition to the contributions from non-ice components.
In addition to conducting low-resolution global imaging and compositional mapping, selected regions should be targeted for coverage at higher spatial resolutions. For example, the Galileo Europa Mission (GEM) identified several regions exhibiting terrains that have been disrupted, presumably by internal activity, and that have a paucity of superposed impact craters, indicative of relative youth. These areas should be imaged at resolutions better than 50 m/pixel (with compositional mapping at 300 m/pixel) to determine if there have been changes on the surface that would be indicative of ongoing activity. Imaging at similar resolution is recommended for sites identified by GEM as being of high interest for exploration by future landers.
Orbital imaging should also be used to search for active eruptions such as geysers. Although GEM has so far failed to reveal current activity, viewing and lighting geometries precluded this type of observation on all but one brief encounter. Current estimates of the height to which geyser plumes would rise suggest that surface features produced by them would be about 15 to 20 km across; thus, images should have resolutions of 1 km or better for their detection.
GLOBAL MAPPING OF TOPOGRAPHY AND GRAVITY
Topography and gravity are basic geophysical data sets that provide information on the internal structure and dynamics of planetary bodies. Flybys of Europa during the nominal Galileo mission and GEM will yield information on the lowest-degree and lowest-order spherical harmonic contributions to the gravity field. The degree-two gravity field has been used to infer the moment of inertia of Europa and its layered internal structure.1,2 Only limited data exist on the very subdued topography of Europa. Its average shape, inferred from Galileo limb profiles, is not very different from that of a sphere and, given the precision of current measurements (>500 m), is indistinguishable from an object in hydrostatic rotational and tidal equilibrium (P. Thomas, Cornell University, private communication, 1999). A more precise determination of the radii of the three principal axes of the ellipsoid defining Europa's shape (accurate to approximately 100 m) would provide a crucial verification of the hydrostatic equilibrium assumed in the interpretation of the second-degree spherical harmonic gravity data. It would also provide an independent measure of CIMR2.
To proceed further in the exploration of Europa's interior after the completion of GEM requires determination of both the gravity field and topography of the satellite over the entire surface. This can be accomplished from an
orbiting spacecraft, for example, by radio tracking and laser altimetry. The relationship between gravity and topography and their connection to geologic features can be exploited to provide information on internal structure and dynamics and surface tectonics. Most importantly, it will enable better constraints on the thickness of the outer water ice-liquid shell and determination of variations in this thickness. The nonhydrostatic contributions to the gravity field will be characterized and internal mass anomalies identified. Modes of compensation for surface loads can be determined, thereby constraining the rheology of the ice shell.
The periodic distortion of Europa as it revolves around Jupiter produces variations in Europa's shape and gravitational field with a period equal to Europa's period of orbital revolution (3.55 days). The distortion of Europa is caused by the forced eccentricity of its orbit and the consequent variation in the tidal force from Jupiter with Europa's orbital position. Also contributing is the small periodic motion in the position of the sub-jovian point on Europa's surface. Europa's periodic changes in shape redistribute its mass and result in periodic changes in the gravitational field. These periodic variations in Europa's shape and gravity field are superimposed on the permanent and much larger ellipsoidal shape and degree-two gravitational field that result from Europa's rotation and Jupiter's tidal force, already measured by Galileo.
Measurements of the time-varying shape and gravity field of Europa can readily be taken by an orbiting spacecraft with a minimum lifetime of a few tens of europan orbital periods. If Europa has a global liquid-water ocean, the surface of its icy shell will rise and fall by about 30 m during a revolution around Jupiter. If there is no ocean, the periodic displacement of the surface will be only a few meters. The tidal response of a patchy shell will be intermediate between these two limits, with its exact amplitude determined by the degree to which the icy shell is decoupled from Europa's interior.
Determination of the topography will make it possible to distinguish easily between these possibilities. Radio tracking of the orbiting spacecraft could measure periodic changes in the gravitational field. The detection of the periodic variations in the gravitational field of the larger mass redistribution that would occur if Europa has an ocean would also readily determine if indeed there was an ocean. Comparison of the periodic changes in the topography and gravity could provide information on the ice thickness and rheology.
MAPPING OF ICE THICKNESS AND INTERNAL STRUCTURE OF THE ICY SHELL
In explorations of Europa, spatial patterns of ice thickness and the internal structure of the ice shell are first-order scientific objectives. Thickness patterns reveal information about present dynamics, the origin of surface structures, and the relationship of the ice cover to the underlying ocean and/or the ocean floor. The internal structure of the shell also may hold clues to past dynamics as well as provide information on the geologic evolution of the shell since its formation.
The relatively simple dielectric behavior of pure ice means that high-frequency radar waves penetrate great thicknesses of cold ice with relatively little attenuation, allowing for the application of radar technologies to subsurface exploration. Geophysical applications of radar technology for subsurface exploration have been demonstrated on Earth over the past 30 years. Although there is a wealth of experience in the use of ground-and airborne-radar systems of this type, the technique has not yet been successfully used by Earth-orbiting spacecraft, let alone a spacecraft orbiting another planetary body.
Sounding radars with average powers of less than 200 watts have penetrated deep ice on the Greenland ice sheet.3 Indeed, the ice in some locations studied is greater than 3000 m thick. Earlier observations conducted in Antarctica with more powerful but less sophisticated radars successfully sounded ice approaching 5000 m in thickness.4 These techniques have been used to determine the locations of subsurface structures to an accuracy of 10 m, as has been verified experimentally with boreholes drilled through the ice to bedrock.5
The reason sounding-radar techniques work so well is that the wave velocity (or, equivalently, dielectric constant) depends most strongly on the density of the ice sheet, and there are well-established mixing formulas for estimating velocity given an ice density. For most parts of Greenland and Antarctica, important density variations are confined to the upper 100 or so meters of ice, and the shape of the densification curve is well understood. Consequently, small uncertainties in density with depth tend to be unimportant in the determination of ice thickness from radar data.
The situation is dramatically different for sea ice, which in general is a mixture of ice, brine, air, and precipitated salts. Brine pockets within the ice are strong scatterers and attenuators that greatly reduce the depth of penetration into ice even at radio frequencies. There can be strong vertical variations in the dielectric constant as well. These change seasonally as thermodynamic forcings cause brine to migrate within the icy matrix. Consequently, the inversion problem is considerably more difficult,6 even with radar systems sufficiently powerful to penetrate sea ice, which is typically only several meters thick.
Radar technology offers the unique potential for detailed and direct mapping of Europa's ice shell and its internal structure. Information on the phase and amplitude of radar echoes from the bottom of the shell may also reveal something about the interface, for example, if the ice rests on water. Nevertheless, the potential of radar has to be tempered against the possibly complicated, three-dimensional variations of the dielectric constant in Europa's ice shell. For example, radar absorption through materials that might compose portions of the shell ranges from 10-5 dB/m for pure ice at 200 K to 1 dB/m for briny ice (Figure 3.1). Recent calculations suggest that available constraints on the properties of Europa's ice place a limit of about 10 km on the depth to which an ocean might be detectable by an orbiting radar system.7 Of course, the actual performance may be better or worse depending on the true temperature and chemical composition of the icy shell.
CHARACTERIZATION OF DEEP INTERIOR STRUCTURE AND DYNAMICAL PROCESSES
Measurements of the topography, gravity field, and magnetic field of Europa will aid efforts to characterize Europa's deep internal structure and dynamics. It is important to determine if Europa has a magnetic field, which will indicate whether convection and dynamo action are occurring in a liquid part of Europa's core. Although Galileo's magnetometer has been able to detect Ganymede's magnetic field,8 it has not been able to detect an intrinsic europan magnetic field.9 A small field, however, could exist but remain undetectable in the presence of larger magnetic-field perturbations due to induction effects in Europa and plasma effects around Europa.
A magnetometer and a plasma detector on a spacecraft orbiting Europa would provide global data on the charged-particle populations and magnetic field over long periods of time and be able to distinguish a permanent intrinsic field from time-varying plasma and induction fields. New, lightweight plasma detectors, such as the Plasma Experiment for Planetary Exploration (PEPE) instrument now operating on Deep Space 1, are able to make good measurements while placing fewer demands on spacecraft mass or power resources than past instruments. Additional data on the induction field would also help to characterize the highly electrically conducting near-surface layer, perhaps a liquid-water ocean, in terms of depth, thickness, and electrical conductivity.
If it turns out that Europa does not have an intrinsic magnetic field, it will be possible to conclude only that there is no convection or dynamo activity in its core. Whether its core is liquid or solid will still be uncertain.
More accurate and complete determinations of Europa's gravity, global shape, and topography will enable refinement of interior structural models; tests of hydrostaticity; and inferences about ice thickness and variations thereof, topography on the water-rock boundary, and mechanical properties of the ice.
DETERMINATION OF THE GEOCHEMICAL ENVIRONMENT OF THE SURFACE AND POSSIBLE OCEAN
The prospect of skipping a systematic geochemical assessment of Europa and discovering some form of life is exciting but has a low probability of success. It is more likely that many geochemical properties of the europan environment will have to be characterized before the probability of the origin and development of life there can be sensibly assessed. Among the properties of interest are the presence and concentrations of chemicals that might serve as nutrients or as poisons, the energy sources available that might support life, the present and past redox states, organic materials that might be residues from living organisms or prebiotic processes, or the characteristic times for physical processes, both in sequence and duration. In this section COMPLEX takes a ''top-down" approach to the study of europan geochemistry, beginning with the atmosphere.
The Atmosphere and Ionosphere
Europa has a thin neutral atmosphere and an ionosphere derived from it. The current state of knowledge of Europa's neutral atmosphere and its embedded ionosphere is still rudimentary, however. The vertical structure and the horizontal patterns (latitudinal and longitudinal morphologies) are yet to be measured. The day-to-day variability of both the neutral atmosphere and the ionosphere and their responses to the stresses caused by electrodynamic interactions with the magnetosphere in which they reside are essentially unknown. Atmospheric species cannot yet be accurately sorted into endogenous and exogenous components. Most of the atmosphere consists of chemical species produced by sputtering of the materials that are found in the surface ice, but the compositions of those materials are surely changed as a consequence of the chemical reactions induced during their ejection into the atmosphere. Certainly, without the strong interaction of the bombarding plasma, the atmosphere would be much more meager than it is.
The atmosphere of Europa informs us of the presence of molecular oxygen (O2) and molecular hydrogen (H2) and perhaps of their rates of formation. The O2 and H2 are produced by plasma irradiation of water ice, and it is likely that some of these gases do not escape from the ice but remain buried in it (as is observed, for example, on Ganymede). In principle, if the ice convects more rapidly than those species in the ice are destroyed, they might be transported downward to the water-ice interface. There, the O2 could serve either as an energy source or as a poison for possible organisms.
Also, because ion sputtering is an important process contributing to the atmosphere, atoms and molecules of all types that are present in the ice are released into the atmosphere. For example, ground-based spectroscopy in 1995 revealed the presence of sodium (Na) in the atmosphere. Additional detailed sampling of the atmosphere can provide information about these minor and even trace constituents of the ice. Key element ratios may be obtainable from analysis of atmospheric species. Thus, if the atmosphere is sampled in detail, so that its trace constituents are measured, the composition of the surface materials can be determined.
The atmosphere also seems an especially appealing location to search for organic substances. The concentrations of some organic species may be high enough in the atmosphere for ready observation. Fragments of complex organic molecules may appear as sputtering fragments, analogous to the fragments used to identify organic compounds by analytical mass spectrometry. Other molecules may be intrinsically volatile, such as methane, perhaps released from clathrates. Carbon dioxide might be present from oxidation of organic species or from destruction of organic acids or other carbonyl groups as a consequence of particle or ultraviolet radiation.
Although some clues to the makeup of Europa's neutral and ionized gas can be achieved from ground-based telescopes and their counterparts in low Earth orbit (e.g., the Hubble Space Telescope), major progress will come only from in situ observations from an orbiter, a suite of landed instruments, or both.
The discovery of Europa's atmosphere/ionosphere offers a significant opportunity for investigations of the physical and chemical conditions on Europa's surface. This results from the fact that Europa's thermal gases and plasmas are derived directly from the surface. The agent responsible for their release is the harsh, ever-present magnetospheric charged-particle populations at Europa's location (~9.5 Rj) in the jovian magnetosphere. A systematic study of the cause-and-effect cycles in this relatively isolated surface-bounded exosphere represents both a remarkable opportunity and a significant technical challenge to our understanding of surface-environment interactions as a tool for solar system exploration. Several sophisticated techniques can be brought to such studies.
Both remote and in situ methods can be used in various portions of the electromagnetic spectrum to identify neutral and ionized species, as well as their relative abundances, in the europan system. Numerical modeling studies show that the radial extent of a detected species depends primarily on its ejection speed from the surface. This, in turn, depends on surface structure and on the ability of incident charged particles to liberate gases. Since the fluxes above Europa can be measured in space and the sputtering process studied in the laboratory, the characterization of atmospheric constituents will be a robust way to address the details of surface properties and their global morphologies.
Observations should be made in the ultraviolet and in the infrared to document the abundances of as many species as possible. In the visible, observations of the spatial distribution of sodium and, potentially, potassium are needed to determine if they are produced locally or transported to the surface from Io. COMPLEX notes that Earth-based observations of Europa's atmosphere prior to and during the lifetime of an orbiter will be important for establishing a baseline from which any unusual conditions during the mission might be recognized.
Both neutral mass and ionized mass spectrometers operating in low orbit above Europa can be used to observe the composition and global distribution of gases released from the surface. The identification by mass offers information at a single (orbital) height that is complementary to information obtained by optical methods above and below that height. It offers potentially unique capabilities for the detection of entire organic molecules at low altitudes. Neutral and ion mass spectrometers with a resolution of 1 amu will be required. Such instruments have a long history of successful use on sounding rockets, Earth orbiters, and planetary spacecraft such as the Pioneer Venus orbiter; a neutral mass spectrometer is currently on its way to Mars aboard Japan's Nozomi spacecraft.
Because the picked-up plasma in the vicinity of Europa is derived from its surface, measurements of plasma and field conditions in the vicinity of Europa also provide valuable clues about the composition of the surface of Europa (with careful modeling of the interaction and sputtering processes). In addition to the charged material, a large fraction of the sputtered material in the vicinity of Europa would be neutral, and studies of it using a neutral mass spectrometer would provide further information on the composition of Europa's surface. However, to derive relative abundances of surface materials from the measurements made in space, a good determination of the energy and fluxes of the bombarding plasma would be required. Thus, a simple, lightweight, charged-particle instrument is needed to address this question.
Energetic Particles and Fields
The measurements of the electromagnetic induction response from Europa using a direct-current magnetometer on an orbiter would provide indirect evidence about the possibility of the existence of a salty ocean within Europa. Because of the limited coverage of Europa from Galileo, investigators have so far used only the 11-hour periodicity in the background magnetic field as the basic forcing signal for sounding. However, other short-and long-period waves are present in any background environment. Shorter-period waves would be affected by the ionosphere of Europa, because their penetration skin depth is smaller. On an orbiter that makes measurements continuously over several months, such waves could be used to study the properties of the ionosphere. Longer-period waves would sound Europa's interior to large depths directly. One source of longer-period fluctuations in the background field is the small dawn/dusk asymmetry in the magnetic field of Jupiter even at the location of Europa's orbit. As Europa and the orbiting spacecraft orbit around Jupiter (with a period of ~ 3.55 days), they would sample a changing background. Magnetic reconnection between the interplanetary magnetic field and the field of Jupiter also would create longer-period fluctuations in the background field. Similar very-small-amplitude fluctuations have been used to study the variations of electrical conductivity within Earth's mantle. As mentioned above, distinguishing between different sources of magnetic signatures (intrinsic field, induction, or plasma effects) requires measurement of the plasma environment as well as the magnetic field.
In addition to water ice, a variety of other materials may exist on Europa's surface.10 There is spectral evidence for some of these materials, but most of them have not been identified to a high level of confidence, nor have their exact nature and origin (if they are indeed present) been determined. Some materials are exogenous in origin, at least in part. These include the chemical element sulfur, presumably implanted from the plasma surrounding Europa and now apparently present as SO2.11,12 Although Europa probably has abundant sulfur within its rocky interior, this material is likely not accessible to the surface. The most plausible source of the sulfur is Io, whence it is erupted by this satellite's volcanoes and, having been carried by Jupiter's magnetosphere, is implanted onto the surface of Europa. Other candidate materials are those from comets, interplanetary dust, and perhaps other objects that have bombarded the europan surface. These include minerals (silicates, sulfides, iron and iron alloys, and minor minerals) and carbonaceous materials; none of these has yet been observed. Europa appears to have been resurfaced continually with water from beneath, either as liquid or ice. If as liquid it most likely would be briny, and salts spilled onto the surface would separate from the water when the brines froze. Evidence for such salts in the form of distorted water-absorption features in reflectance spectra has been obtained in some areas of Europa. This distortion has been interpreted as water of hydration, and it resembles that seen in laboratory spectra of the salts hexahedrite (MgSO4• 6H2O), epsomite (MgSO4• 7H2O), and natron (Na2CO3• 10H2O).13
Although materials present at depth within Europa are of great interest to the question of life, it is anticipated here that near-term studies of Europa will focus on the surface. Determination of deeper properties will depend on processes that might expose once-deeper ice at the surface. Such processes include tectonic and solid-state convective motion, or impact cratering. Upwelling of water from the interior of Europa similarly could bring material from depth to the surface. The salts proposed to explain the distorted water spectra are examples of such material.
Sampling and examination of ice from a depth of at least several centimeters below the surface will likely be required to observe properties of ice undisturbed by the continual bombardment by magnetospheric ions. Some of these bombarding ions, such as sulfur, are implanted. Other ions break up water molecules or sputter them into the atmosphere. Many of these molecules re-condense onto the surface, changing its texture. As a precursor to detailed study, certain geological and geophysical properties of the ice need to be characterized to provide context for interpretation of geochemical properties of the ice and the possibility of life. Lateral and vertical temperature profiles and the rate of interior heating, if known, would constrain models of the interior composition of the aqueous outer shell and the distribution of heat-producing, radioactive elements below that crust, which in turn
constrains models of silicate differentiation. Determinations of some of these physical properties may require geochemical measurements. In addition to hydrated minerals crystallized from the ice, there may be gases and solids dissolved within the ice and solids occluded within the ice, all possibly brought to the surface from depth. Determining the mineralogy and chemical composition of the materials associated with the ice will help distinguish between those of exterior and interior origin. Ratios of chemical elements of materials of interior origin will enable the geochemist to infer the nature of the sub-ice interior of Europa. Conceivably, the ice may contain fine rock flour originating from any one of a number of geological processes, including water-rock chemical weathering and glacier-like scraping if the ice shell touches the surface of the rocky mantle.
The history of past motions and subsurface environments of the ice may be recorded in the mineralogical properties of the ice. Textural properties such as grain size distributions, preferred grain orientations, and evidence of crystal strain or shock features, breakage, and annealing may provide this information. These properties can be used to search for ice that has not been badly deformed, including, perhaps, ice that suffered relatively little stress during transport from depth. Surface ice that has undergone convection or was extruded onto the surface as a slushy melt may have brought material up from substantial depths. This material could include chemical species that had been dissolved in the ocean, debris that had become entrained in the ice, or even silicates from the rocky interior. Such ice would, therefore, be rich in chemical information about the interior of Europa. Substantial recrystallization may have occurred during motion of the ice as it moved from depth toward the surface (if it did), or through surface metamorphism caused by tectonic movement of the crust. Such alteration of the original ice grains and textures may erase pressure and temperature conditions from the initial formation of the ice but should not substantially alter the abundances or character of entrained chemical or mineralogical species.
The number of chemical elements required by living organisms as we know them is large. Most are important only as trace constituents, however. The principal building blocks of cells are carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, calcium, iron, and potassium. Determining the concentrations, concentration ratios, and chemical forms of these and other elements in the ice will provide insights into the sources of these elements in the ice and their transport mechanisms. It may then be possible to say whether these elements are available within Europa's interior. Surface concentration ratios for many elements can be used to predict concentration ratios in the europan ocean.
The presence, variety, and concentrations of organic substances are of obvious interest to questions of life. Organic materials are continually supplied to the europan surface in cometary material. Conceivably, there could be a mechanism by which any of this material can reach the ice-ocean interface where it could serve as nutrients; alternatively, sputtering may remove such material before it enters the subsurface. Residual carbonaceous material from the accretion of Europa may be present in the interior, although its nature has likely been altered by metamorphism anticipated at high temperatures (see below). 14,15 Of paramount interest would be forms of organic material that might be indicative of life and those that are precursor materials of life.
Chemical substances that originate beneath the surface of Europa may be products of rock-water interactions, discussed below. Others may be in the form of dust or fragments of rock produced by a variety of geologic processes. Convection of the ice might deliver this material to the surface, or the ice may deliver the material to the ocean, where part of it might remain suspended for long periods as colloidal particles. The mineralogical composition of this material would provide insight into the rocky mantle. Measurements of concentration ratios of a few well-chosen elements in the material would provide general information about the rocky mantle, including probable abundances of elements such as potassium and phosphorus that are essential for life on Earth. Isotopic properties of the material can constrain time scales for a variety of events, including europan igneous differentiation and ice movement.
Charged-particle fluxes are high at the europan surface. These charged particles cause a variety of nuclear reactions within the uppermost part of the ice. In addition to the products of such radiation chemistry mentioned above, hydrogen, oxygen, and hydrogen peroxide are produced within the uppermost ice. Oxidizing and reducing agents are thus produced, and these, as well as highly reactive species such as free atoms and free radicals such as OH, can interact with organic matter trapped within the ice. The most energetic charged particles cause spallation reactions and generate a flux of neutrons within the ice. Products of these reactions would include isotopes such as tritium (3H) and 10Be; profiles of these radioisotopes versus depth would be useful in determining deposition,
erosion, or burial rates. Neutron energies would moderate rapidly and the thermalized neutrons would be captured. Although most of the capture would be by hydrogen, other components of the ice would be activated and, if abundant enough, could provide additional time markers for processes such as erosion and convection in the ice.
Hydrogen and oxygen isotopic ratios will yield crucial information about the origin of the ice and the processes that have brought it to its current state. Measurement of isotopes of other light elements such as carbon and nitrogen also can lead to understanding of the sources of compounds containing them and the properties of these sources and constraints on their origins.
The Possible Europan Ocean, Present and Past
If the putative europan ocean can be accessed directly, numerous characterizing measurements can be made. Some bulk characteristics such as density, depth, temperature profile, and convection pattern and rates should be determined. Simple geochemical measurements include conductivity, ionic strength, and pH and the mechanism of its control. Identification of the principal dissolved salts and measurement of their concentrations will help to characterize the water-rock reaction, as will the identity and concentrations of dissolved gases. Concentrations of redox-sensitive elements will reveal the extent to which redox equilibrium has been attained. Lack of redox equilibrium is required for energy to be available to drive metabolism in chemosynthetic life. Earth's ocean, which harbors life and may have been the site of its origin, is buffered at a slightly alkaline pH by dissolved HCO3- and it is sufficiently oxidizing to produce Fe3+. This enables precipitation of hydrated oxyhydroxides of Fe3+, which in turn scavenge the ocean of many trace ions. Measurements of the pH of europan ocean water, together with the other geochemical measurements, would reveal much about the water-rock chemical reactions and the likely composition of the rock. Of course, we do not know how the factors that control the composition and properties of a europan ocean would relate to those that control Earth's oceans, due both to the very different chemical environment at Europa and to the tremendous uncertainty in current knowledge of that environment. Therefore, it is prudent to be prepared for a much wider range of possible properties.
The clarity of the ocean and the content and nature of suspended solids should be determined. Organic substances are of particular interest, both as possible life-supporting chemicals and as precursors to or residues of living organisms (or, more optimistically, the organisms themselves). Another possible suspended solid, rock flour, may have been converted to phyllosilicate minerals such as clays. Such material may be the principal form of sediment at the bottom of a europan ocean. A more exciting prospect is that biological precipitates may have formed. The most promising method for early detection of such material may be its observation in ice of deep origin that has carried the material to the surface.
Europa's Biological Potential
For biological systems to occur on Europa or to have occurred in the past, Europa would have to have undergone extensive geochemical differentiation. The aqueous outer shell (whether it be predominantly solid ice or liquid water) indicates that it probably did. Silicate materials in contact with water at some point in its history would have a chance to react with water (or may even have done so prior to accretion). At some point there would have been sedimentation of silicates and Fe oxides from fully melted and slushy regions, perhaps producing a mineral assemblage similar to that of carbonaceous chondrites. Hydrated phyllosilicates, Fe3O4, and soluble salts (predominantly magnesium sulfate) may have been the principal chemical products. 16 The presence of water-rock interactions thus seems plausible on Europa. Circulation of water through magnetically heated rock in the form of hydrothermal systems could have provided access to abundant energy, and may still.17
Most of the chemical constituents crucial for life are relatively low in abundance in solid planets. On Earth, life is possible because these elements are strongly concentrated into the crust. We can reasonably believe that, on Europa, the key elements will have been extracted out of the rocky mantle along with the water and are thus either dissolved in the water or are available at the water-rock boundary in a manner that should make them accessible. However, we do not truly know the chemical composition of Europa's interior, how thorough its igneous
differentiation may have been, or what elements of biological interest may have been lost to space as that differentiation took place.
Chemosynthesis may have occurred either at the present or in the past. Terrestrial chemosynthetic organisms take advantage of sluggish oxidation-reduction reactions as energy sources. Many redox reactions remain far from equilibrium owing to kinetic constraints, and life has evolved many ways of taking advantage of redox disequilibria involving iron, sulfur, carbon, nitrogen, manganese, arsenic, uranium, and other redox-sensitive elements. Redox reactions may also supply the chemical energy that could drive organic synthesis or the processing of organic compounds into primitive versions of biomolecules.
One test of whether Europa can support life is to identify whether there are sources of chemical energy available that are sufficient to drive metabolism. Measurements of the nature and abundance of chemical species within the water, the extent of any redox disequilibrium, and the abundance, if any, of organic molecules will help to determine the biological potential within the ocean.
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