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 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 presents the opportunity for major new discoveries in planetary geology and geophysics, and planetary atmospheres. In addition, Europa offers the potential for 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,1 COMPLEX 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.
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.
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 understanding the limits of life on Earth and searching for
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A Science Strategy for the Exploration of Europa 6 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 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 presents the opportunity for major new discoveries in planetary geology and geophysics, and planetary atmospheres. In addition, Europa offers the potential for 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,1 COMPLEX 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. 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 understanding the limits of life on Earth and searching for
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A Science Strategy for the Exploration of Europa life elsewhere, it is tempting to try to initiate 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, however, to see the difficulty of crafting such an approach. While 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 environments. 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 an 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. Thus, 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, and to determine whether life is present. 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. 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 or oceans of liquid that might be discovered. The set of 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
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A Science Strategy for the Exploration of Europa 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 characterizing 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 body) 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 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. COMPLEX recommends that the science objectives for follow-on experiments designed to elucidate the properties of Europa's surface materials 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, COMPLEX recommends that the science objectives of subsequent missions include determination of the physical and chemical properties of the water, including salinity, acidity, pressure and temperature profiles within the
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A Science Strategy for the Exploration of Europa 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. LABORATORY, THEORETICAL, AND TELESCOPIC STUDIES Much additional laboratory and theoretical work is required in order to interpret and understand existing and likely future spacecraft observations. This will be the case whether or not liquid water has been present during geologically recent periods. As this is an area of active pursuit, specific measurement requirements are not enumerated here. However, the measurements generally center on reflectance spectroscopy of hydrated salts, saltice mixtures, and other potential components of the europan surface; measurements are needed of the physical and chemical effects of radiation from the near-Jupiter environment on surface materials, including salts, ices, and possible organic compounds, and the sputtering properties of these same materials. 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. Theoretical analysis is involved not just in interpreting laboratory measurements but also in understanding the structure and composition of the interior and the interactions of Europa with its environment. Such theoretical analysis, for example, led to the first predictions of tidal heating of Europa and the potential for liquid water to exist. COMPLEX recommends that 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. Earth-based (both ground-based and orbital) telescopic observations and analyses will play an important role in further exploring the nature of the atmosphere and surface, providing input into understanding the geological and geochemical context of spacecraft measurements of Europa and the composition and evolution of its atmospheric and surface materials. In order to be able to take advantage of the next generation of telescopes and instruments that are currently being developed, COMPLEX recommends that new large telescopes and instrumentation that are being developed incorporate, from the beginning of the design stage, 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. TECHNOLOGY DEVELOPMENT New technology and methods will be required for the operation of spacecraft in the challenging europan environment, as well as for exploring the properties of a possible ocean and its potential for life. It is necessary, for example, to measure the thickness and structure of an ice layer that has unknown dielectric and mechanical properties, to physically penetrate through substantial thicknesses of ice in order to reach liquid water, to move in a controlled manner in making measurements in a sub-ice layer of liquid water, and to relay information derived during subsurface activities to the surface and then to Earth. COMPLEX recommends that low-mass, radiation-hardened instruments be developed for use on orbiting and surface spacecraft. COMPLEX further recommends that 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. COMPLEX recommends the development of 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.
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A Science Strategy for the Exploration of Europa TERRESTRIAL ANALOGS Research on terrestrial analogs of the Europa environment will allow the development of and provide proof-of-concept testing for new technologies required for the exploration of Europa. The use of radar to explore thick glacial ice sheets on Earth is an obvious example of the relevance of this approach. The exploration of the ice-water and ice-rock boundaries on terrestrial ice as an abode for life also may offer valuable lessons in exploration strategy. COMPLEX recommends that 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. COMPLEX recommends that 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. INTERDISCIPLINARY AND INTERAGENCY ISSUES The exploration of Europa is an interdisciplinary venture, and various aspects of the necessary science and technology background are being investigated at present by federal agencies other than NASA, including the National Science Foundation, the Department of Defense, the Department of Energy, and the National Oceanic and Atmospheric Administration and the Office of Air and Space Commercialization within the Department of Commerce. Some of the Earth-analog studies, for example those in Greenland and Antarctica, will require sensitivity to international agreements and treaties. Additional international issues could arise in connection with the possible biological contamination of Europa by spacecraft from Earth, the potential for back-contamination of Earth if samples from Europa that may contain biologically active materials should ever be returned to Earth, or in launching spacecraft that carry radioisotope thermoelectric generators. COMPLEX recommends that NASA, to avoid ''reinventing the wheel," look to other government agencies to deal with some of the scientific and technological issues and to cooperate with governments of other countries in exploring Earth analogs. COMPLEX endorses the planetary protection procedures and policies articulated in previous NASA and NRC documents and recommends that appropriate planetary protection measures be determined and implemented on all relevant spacecraft missions.2 REFERENCES 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. 2. See, for example, Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making, National Academy Press, Washington, D.C., 1998.