FIGURE 2.2 Europa’s cycloidal ridges, such as these seen at 60 degrees north and 80 degrees west, may be the surface expression of diurnal tidal stresses propagating through a thin shell of ice overlying a subsurface ocean. Courtesy of the Jet Propulsion Laboratory.

Because of the likely existence of liquid water, at least as a transient or intermittent species, life could exist within or below Europa ’s icy shell. The other requirements for life—access to biogenic elements and access to a source of energy—may be available at the water-rock boundary at the bottom of the water layer.6 Some researchers have argued against this idea on the grounds that Europa’s ocean is a closed system and, therefore, its water would rapidly become chemically reduced as the result of interactions with hot rocks in hydrothermal systems,7 which would mean that a europan ocean would not be an energetically favorable environment for life. This contrary view has, however, been challenged on the grounds that even if the ocean is reducing, abundant redox chemistry could still take place, providing an energy source for metabolism.8

Another possibility is that life exists not in the deep oceanic interior of Europa but near the surface. If this is the case, then the ultimate power source for a europan biosphere may be chemical species created by interactions between the surface ice and energetic particles in the jovian radiation environment.9 While no evidence for life exists, it is the potential for life that makes Europa an exciting target for further exploration.

The most important questions about Europa include whether liquid water exists and whether it has lubricated the motion of surface blocks seen in the Galileo images. Researchers also seek to learn the composition of the deep interior and the non-icy parts of the surface. The nature of the tectonic processes and the abundance of geochemical energy sources are important as well for learning about the potential for—or the history of—life on Europa.

EUROPA EXPLORATION STRATEGY

The important questions about Europa that are outlined above can be addressed through a series of spacecraft missions that carry out the following key measurements, extracted from the 1999 NRC report by the Committee on Planetary and Lunar Exploration (COMPLEX):10



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