• 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.

COMPLEX concluded that Europa is an exciting object for further study, with the potential for major new discoveries in planetary geology and geophysics, and the potential for studies of extraterrestrial life. In addition, COMPLEX concluded that the results obtained by Galileo (revealing geologically recent or ongoing geologic activity, regarding the possible presence of liquid water, and indicating the potential for present or past biological activity) make Europa a high-priority target for further exploration.

The two highest-priority overall science goals identified in the 1999 report by COMPLEX for Europa exploration reflect the emphasis on the potential for life as a major driver in Europa’s exploration. They are the following:

  1. 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

  2. Understanding the chemical evolution that has occurred in the liquid-water environment and the potential for an origin and the possible continued existence of life on Europa.

Even if there turns out to be no life or no sophisticated prebiotic chemistry, these goals remain legitimate drivers for a better understanding of Europa’s geologic history.

The particular scientific goals of the first mission are expected to be determination of 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 one or more near-polar-orbiting spacecraft.

EUROPA’S RADIATION ENVIRONMENT

The intense radiation near the surface of Europa is a key factor governing the viability of organisms that might be carried to Europa on spacecraft. Ionizing radiation causes biological effects, such as genetic damage, that result in significant morbidity or death once sufficient damage accumulates. The intensity of the radiation environment in the Jupiter system has been measured by several spacecraft, and its variation with depth below the surface of the ice can be predicted. Accordingly, the rate at which microorganisms with a specified radiation tolerance would succumb in the vicinity of Europa can be determined.

Europa lies deep within the magnetosphere of Jupiter, which is the volume of space above Jupiter’s atmosphere that is affected by Jupiter ’s magnetic field. This magnetosphere extends up to 10 million km from Jupiter (i.e., it encompasses a volume 1,000 times that of the Sun) and is filled with ionizing, magnetically trapped particle radiation. The mechanism of magnetic trapping of radiation at Jupiter is the same as that which operates in Earth’s van Allen belts. Jupiter’s magnetosphere is, aside from the Sun, the dominant source of energetic charged particles and radio emissions in the solar system.

First discovered as a radio source, the magnetosphere of Jupiter interacts strongly with the innermost Galilean satellite Io, as evidenced by the modulation of decametric radio emissions at the orbital period of Io. The study of Jupiter’s decimetric radio emissions led to the first determinations of the approximate strength and direction of the jovian magnetic moment. The first spacecraft visits to Jupiter, in 1973 and 1974 by Pioneer 10 and Pioneer 11, respectively, confirmed the existence of the magnetosphere and revealed its disklike configuration, which rotates approximately with the planet itself. The later visits by Voyager 1 and Voyager 2 in 1979 revealed the importance of Io ’s plasma torus, a region of sulfur- and oxygen-dominated plasmas maintained by the escape of SO2 and other S- and O-bearing molecules from Io. The plasma torus mediates the interaction of Io with the jovian



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