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.
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.