Since its discovery in 1610, Europa — one of Jupiter's four large moons — has been an object of interest to astronomers and planetary scientists. Much of this interest stems from observations made by NASA's Voyager and Galileo spacecraft and from Earth-based telescopes indicating that Europa's surface is quite young, with very little evidence of cratering, and made principally of water ice.
More recently, theoretical models of the jovian system and Europa have suggested that tidal heating may have resulted in the existence of liquid water, and perhaps an ocean, beneath Europa's surface. NASA's ongoing Galileo mission has profoundly expanded our understanding of Europa and the dynamics of the jovian system, and may allow us to constrain theoretical models of Europa's subsurface structure.
Meanwhile, since the time of the Voyagers, there has been a revolution in our understanding of the limits of life on Earth. Life has been detected thriving in environments previously thought to be untenable — around hydrothermal vent systems on the seafloor, deep underground in basaltic rocks, and within polar ice. Elsewhere in the solar system, including on Europa, environments thought to be compatible with life as we know it on Earth are now considered possible, or even probable. Spacecraft missions are being planned that may be capable of proving their existence.
Against this background, the Space Studies Board charged its Committee on Planetary and Lunar Exploration (COMPLEX) to perform a comprehensive study to assess current knowledge about Europa, outline a strategy for future spacecraft missions to Europa, and identify opportunities for complementary Earth-based studies of Europa. (See the preface for a full statement of the charge.)
Perhaps the most exciting aspect of Europa revealed by recent studies is the possible existence of liquid water beneath a surface covering of ice. Although no unique evidence for such an ocean exists yet, very intriguing indications have been seen from spacecraft.
Europa's reflectance characteristics indicate that its surface is almost-pure water ice. Local-and global-scale ice tectonics dominates the geology, with a very large number of cracks criss-crossing Europa's surface. Seen at kilometer-scale resolution from the Galileo spacecraft, large ''rafts" of ice appear to have broken up and moved with respect to each other; the appearance is similar to that of sea ice on Earth. Contaminants have been detected within the ice, including sulfur dioxide (SO2) frost, hydrogen peroxide (H2O2), and a variety of salts. The salts, in
particular, may provide additional evidence for a global ocean, as they are easily dissolved in and transported by water. The presence of hydrogen peroxide suggests that Europa's surface chemistry is dominated by radiolysis.
Gravity measurements obtained from the tracking of Galileo indicate that Europa's interior is differentiated. The outermost layer is predominantly water and/or water ice and is perhaps 100 km thick. Below the water exists a "rocky" interior, which also has differentiated into a dense core and a less-dense mantle; these are thought to be analogous to the iron core and silicate mantle of the terrestrial planets. Europa's magnetic "signature" indicates the presence of a conducting layer near the satellite's surface, most likely owing to water containing dissolved salts.
Europa also has a thin atmosphere, likely composed primarily of materials ejected from its surface. To date, molecular oxygen and atomic sodium have been identified, although other species are expected to exist. These species are thought to have been emplaced into the atmosphere as a result of the collisions of highly energetic particles from the jovian magnetosphere; some of the sodium, however, may come from Io, where it is ejected by similar processes. The gases reside in an extended atmosphere until they are ionized by solar ultraviolet light or magnetospheric electrons and picked up by Jupiter's magnetic field.
As a result of the likely existence of liquid water, at least on a transient or intermittent basis, Europa has the potential for life to exist below its surface. The other requirements for life — access to the biogenic elements and to a source of energy — are present at the water-rock boundary at the bottom of the water layer. While no evidence for life exists, the potential for life makes Europa an exciting target for additional exploration following the completion of the Galileo mission.
OUTSTANDING QUESTIONS AND ISSUES
At our current level of understanding, then, the outstanding questions and issues to be addressed for Europa include the following:
Is there liquid water on Europa today, or was there liquid water in the geologically recent past?
Are the ice rafts seen in Galileo's images of Europa the result of movement atop liquid water or through a warm, soft (but not necessarily melted) ice?
What is the composition of the deep interior of Europa, below the water/ice layer?
What is the composition of the non-ice component of the surface materials (such as the salts)?
What is the nature of the ice-tectonic processes that have affected the surface?
What is the composition of the atmosphere and of the ionosphere?
What are the characteristics of the surface radiation environment and what are the implications for organic/biotic chemistry?
What is the abundance of geochemical sources of energy that could support life?
The outstanding questions and issues for Europa can be addressed through a series of spacecraft missions that, together, can contribute to an integrated understanding of the nature of Europa, the possibility that liquid water exists there, and the potential for life. In particular, important measurements will include:
Measuring Europa's global topography and gravity, and determining how Europa's shape changes as it orbits Jupiter;
Characterizing Europa's geology and surface composition on a global scale;
Mapping the thickness of Europa's ice shell and determining the interior structure;
Distinguishing between any intrinsic europan magnetic field and induction and/or plasma effects; and
Sampling the geochemical environment of Europa's surface and possible ocean.
CONCLUSIONS AND RECOMMENDATIONS
Priority Status of Europa Exploration
With the likelihood that it has vast quantities of liquid water beneath its icy surface, Europa is one of the places in our solar system with the greatest potential for the existence of life. Along with Mars, it appears to possess all of the environmental conditions necessary to support the origin and the continued existence of biota. As a result, finding evidence that might indicate whether life had existed on either Mars or Europa would help us to understand whether our theories for the origin of life on Earth are correct and would help us to understand whether life might be widespread outside our solar system.
Thus, COMPLEX concludes that Europa is an exciting object for additional study following the completion of the Galileo mission. It offers the potential for major new discoveries in planetary geology and geophysics, planetary atmospheres, and, possibly, studies of extraterrestrial life. In light of these possibilities and the equal priority given to the exploration of Mars and the Jupiter system by COMPLEX's Integrated Strategy,1COMPLEX feels justified in assigning the future exploration of Europa a priority equal to that for the future exploration of Mars. This equality must, however, be tempered by the uncertainty as to whether liquid water is actually present and the technological challenges posed by the exploration of Europa.
The two highest-priority overall science goals identified by COMPLEX for exploration of Europa reflect the emphasis on the potential for life as a major driver in Europa's exploration:
Determining whether liquid water has existed in substantial amounts subsequent to the period of planetary formation and differentiation, whether it exists now, and whether any liquid water that is present is globally or locally distributed; and
Understanding the chemical evolution that has occurred within the liquid-water environment and the potential for an origin of life and for its possible continuation on Europa.
The Need for a Systematic Program of Exploration
COMPLEX recognizes the frustration that will inevitably result from following a well-conceived strategy for conducting a thorough and detailed investigation of the potential for life on Europa that likely will take one or two decades to carry out. With the excitement today about searching for life elsewhere, it is tempting to advocate a spacecraft mission that will immediately search for europan life or return samples of surface ice to Earth for such analyses. However, the history of space exploration suggests that a phased approach, in which the results of one mission provide the scientific foundation for the next incremental advance, is more productive in the long term.
We need only look to the history of the search for life on Mars to see the wisdom of an incremental approach. Although the Viking missions seemed very well conceived in 1970, they look naive today in the light of current understanding of the martian environment, and of the diversity of life on Earth and its ability to survive in extreme conditions. As a result, Viking did not sample the most appropriate environments in its search for extant life on Mars. The results from the Viking biology experiments, though, have provided a remarkable foundation for understanding of martian geochemistry that is playing a key role in knowing how and where to look for life on Mars today.
In a similar vein, the absence of identifiable surface environments that might support life or contain evidence of life on Europa and our complete lack of understanding of the chemical environment of the icy surface layer, the liquid water layer that may or may not underlie it, and the rocky interior of Europa suggest that a detailed exploration of the satellite will provide the best opportunity to answer these exciting questions. In other words, understanding the history of the satellite and the potential for life requires a detailed investigation into the geochemistry of the surface and subsurface ice or water, and of possible organic molecules or biological activity. Measurements of the atmosphere, ionosphere, the rocky interior, and the ice-or water-rock interface will also be important.
Therefore, COMPLEX recommends that Europa be explored within the framework of a well-conceived and planned strategy designed to create a scientific base of information that is sufficient to provide a global context for interpreting data pertaining to the possible presence of life on Europa. A comprehensive understanding of the geology, geochemistry, and geophysics of Europa, and of the nature of its atmosphere, is not strictly necessary in order to determine if liquid water is present. Knowledge of these is necessary, however, to assess the potential for life, to determine whether life is present, and to understand the chemical evolution of Europa.
COMPLEX concludes that, should it turn out that liquid water is not present on Europa and has not been present in geologically recent times, the strong evidence for comparatively recent or ongoing geologic activity still makes it an appropriate target for exploration. However, the priority accorded Europa in the solar system exploration program and the sequence of exploration activities would have to be reassessed at that time.
Europa and the Search for Life in the Solar System
The search for extinct or extant life on Mars, and the geophysical and geochemical analyses that are a fundamental part of the search, will provide substantial new insights into the environments in which life might exist and the precursor and resulting molecules that might obtain. Similarly, the search for life in extreme environments on Earth is providing key new insights into the potential for life elsewhere in the universe. In both cases, the new results need to be integrated into the ongoing Europa program to ensure a solid basis for investigation and analysis.
Thus, COMPLEX recommends that the search for evidence of present or past life on Europa, or for evidence of chemical evolution that has the potential to lead to life, should be coordinated with other aspects of the search for possible abodes of life in the solar system.
Elements of a Comprehensive Exploration Program
A comprehensive exploration of Europa that can address the major scientific goals will require a combination of spacecraft missions, ground-based telescopic observations, technology development, and supporting research and analysis. The scientific priorities for exploring Europa should proceed from the global to the local scale in searching for liquid water, determining the composition of the surface and near-surface ice, and exploring any pockets of liquid or oceans that may be discovered. The set of subsequent spacecraft missions to Europa that follows from this, then, likely should proceed from a polar orbiter, to landed experiments, to subsurface devices that can penetrate to depths necessary to reach liquid water. COMPLEX recognizes that implementation of such an ambitious sequence of spacecraft, with each being able to take advantage of results from the earlier missions, may require decades.
COMPLEX recommends that a staged series of missions be utilized to explore Europa, with the scientific focus of the first mission being to determine whether liquid water exists at the present epoch or has existed relatively recently. If liquid water is present, the focus of follow-on missions should be to characterize surface materials and to access and study the liquid water.
Priorities for the Initial Europa Mission
COMPLEX recommends that the primary goals for the first Europa mission should be determining whether a global ocean of liquid water exists beneath the icy surface, determining if possible the spatial and geographical extent of liquid water, determining the bulk composition of the surface material, and charac-
terizing the global geologic history and the nature of any ongoing surface and atmospheric processes. These science objectives can best be met by observations from polar or near-polar orbiting spacecraft.
Specific measurement objectives include, in priority order:
Obtaining measurements of the time variations of Europa's global topography and gravity field over a period of several tens of orbits of Europa around Jupiter, with a precision and accuracy of ±2 meters to uniquely distinguish between tidal distortions of several meters (expected for a completely solid ice cover) and several tens of meters (expected if a global layer of liquid is present). The results of these efforts will allow a unique conclusion regarding the present-day existence of a global liquid-water layer;
Imaging Europa's surface, with resolution of at least 300 m/pixel for global coverage and with higher resolution (< 50 m/pixel) for selected regions, to understand the global geologic history and identify regions where liquid water may be readily accessed;
Performing radar sounding of Europa's subsurface structure to a depth of 5 to 10 km, to identify possible regions where liquid water might exist close to the surface. If the ice is less than 5 to 10 km thick, use of ice-penetrating radar may allow determination of the vertical extent of the surface ice layer (and possibly a direct detection of any underlying liquid water), as well as the local structure of the ice;
Mapping the near-infrared reflectance spectrum of Europa's surface materials globally at kilometer-scale resolution, supplemented by 300-m resolution in selected areas, and using the results to identify the bulk composition of the surface materials, their abundances, and their spatial distributions. A spectral resolution of 10 to 15 nm will be required;
Measuring the magnetic field to a precision of 0.5 nT under a variety of different background conditions (i.e., at different jovian longitudes), combined with coordinated measurements of the plasma environment, to determine whether there is an intrinsic magnetic field and what the properties of either the intrinsic or induced field are. Such measurements may provide important information about the structure of and dynamical processes operating in Europa's deep interior; and
Determining the composition and properties of the atmosphere using both in situ and remote-sensing experiments.
Priorities for Follow-on Europa Missions
Following the systematic orbital characterization of Europa, the focus of follow-on missions should shift to studies of the nature of Europa's surface materials and the means to access and study any liquid water present. Therefore, COMPLEX recommends that:
The science objectives for follow-on experiments designed to elucidate the properties of Europa's surface materials should include in situ determination of the composition of the ice and of any non-ice surface components, including the bulk material, trace elements, isotopes, and mineralogy; analyses of any organic molecules at or near the surface, and identification of endogenic or exogenic sources; determination of the composition and properties of the atmosphere and of any materials sputtered from the surface; and estimation of the absolute ages of surface materials. These science goals probably can best be met using a landed package of instruments on Europa's surface.
If subsurface liquid water is detected and found to be accessible with an instrumented probe, the science objectives of subsequent missions should include determination of the physical and chemical properties of the water, including salinity, acidity, pressure and temperature profiles within the water, abundances and chemical gradients in key redox compounds, and existence and abundances of organic materials; determination of the composition and abundance of suspended particles; exploration of the properties at the water-ice interface; and a search for extant life in the water.
Earth-based Studies, Technology Development, and Other Issues
Much additional laboratory and theoretical work, together with field studies and associated technical developments, is required in a program designed to pursue the exploration of Europa. As a result, COMPLEX recommends that:
A vigorous program of laboratory measurements and supporting theoretical analyses be carried out, to encompass the nature of materials at temperatures, pressures, and irradiation conditions likely to be found on Europa.
NASA support a program of theoretical analysis of the geophysical and geochemical environment at Europa, including the nature of the interior, surface, atmosphere, and magnetospheric interactions.
New large telescopes and instrumentation that are being developed incorporate, from the beginning of the design stages, the ability to observe relatively bright targets moving with respect to the background stars, and that these capabilities be implemented in a timely manner. For new ground-and space-based facilities, a non-sidereal tracking capability with an accuracy analogous to that of the Hubble Space Telescope would be appropriate.
Low-mass, radiation-hardened instruments be developed for use on orbiting and surface spacecraft.
Devices that can penetrate through any surface ice and explore the subsurface ice and possible liquid water ocean on Europa be developed, on a schedule that will allow them to be launched on possible spacecraft missions a decade from now.
Appropriate diagnostic remote tests and instrumentation for determining the physical and chemical properties of a sub-ice ocean and for detecting the presence or potential for life be developed.
NASA continue its collaborative efforts with other government agencies to explore sub-ice freshwater lakes (such as Antarctica's Lake Vostok) and sub-ice-shelf ocean environments as a means of understanding scientific, technological, and operational issues associated with the exploration of isolated environments.
Peer review be used to select Earth-analog programs and investigators to ensure a significant and appropriate level of participation by all of the relevant scientific communities.
NASA, to avoid "reinventing the wheel," should look to other federal agencies to deal with some of the scientific and technological issues and develop mechanisms for cooperating with governments of other countries in exploring Earth analogs.
Appropriate planetary protection measures be determined and implemented on all relevant spacecraft missions.*
1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, pages 8 and 191.