explainable only by the presence of a salty ocean.64,65,66 The problem with accepting the latter as a definitive detection of a europan ocean is that a similar electromagnetic response is seen at Callisto, yet this moon displays no surface morphological evidence for the existence of a subsurface ocean. Could such an ocean exist on Callisto without being reflected in the geologic processes that have created its surface?
A summary, then, of the major outstanding scientific issues and questions for Europa includes the following:
Is there liquid water on Europa and, if so, what is its spatial distribution? If there is a globally distributed "ocean" of water, how thick is the layer of ice that covers it, and what are the properties of the liquid? If there is not liquid water today, has there been any in the relatively recent past, and what is the time dependence of its occurrence?
Are the kilometer-scale ice rafts seen on Europa's surface a product of the movement of ice on an underlying liquid-water sea or through a warm, soft (but not necessarily melted) ice? What is the overall relationship between the surficial geologic units and the history of liquid water?
What is the composition of the deep interior of Europa, including both the presumed silicate mantle and the iron (or FeS) deep core? Is the core solid or liquid, and is it actively convecting to get rid of heat? Is there a global magnetic field produced by motions of the core? What are the dynamics of the interior, especially regarding the possible physical decoupling of the rotation of the surface ice from the deep interior and the possible non-synchronous rotation of the surface or of the entire satellite? What is the magnitude of tidal heating of the interior, and how is the heating distributed within the interior?
What is the composition of the non-ice component (such as salts) of the surface materials that are seen in imaging and spectroscopic investigations? How do they vary over the surface? What are the source and history of these materials, and how do they relate to the geologic history of the surface and the potential for the at-least-intermittent presence of liquid water?
What is the nature of the ice-tectonic processes that have affected the surface, and how are they reflected in the features that are seen (such as triple bands and spots)? Is there active or ongoing cryovolcanism? What are the absolute ages of the various surface geological units?
What is the composition of the neutral atmosphere and of the ionosphere? What are the sources and sinks of these species? What are the spatial and temporal variations in the atmosphere, and how do they relate to the physical processes that might control them? What is the composition of the magnetospheric ions that can sputter the surface, and of the sputtering products?
What are the characteristics of the radiation environment at the surface of Europa (currently and in the past), and what are the implications for organic/biotic chemistry and the survival of life on the surface?
What is the abundance of geochemical sources of energy that could support an origin of life on Europa or its continued existence? Is there extant life, or has there been life in the past? If there has been liquid water, access to biogenic elements, and a source of energy but there is no life present, what factors might explain the lack of occurrence of life, and does the potential exist for an independent origin of life in the future?
These fundamental science questions about the nature and evolution of Europa can be addressed through an ongoing program of telescopic and spacecraft exploration. Some of these issues may be tackled in part or in full by NASA's proposed Europa Orbiter mission. It is almost certainly the case that they cannot all be addressed by a single, short-lived spacecraft mission. Rather, answering these questions will require an ongoing program that progresses from flybys to orbiters to landers to subsurface penetrators. In the following chapters, COMPLEX discusses how these scientific questions can be addressed.
1. B.K. Lucchitta and L.A. Soderblom, "Geology of Europa," in Satellites of Jupiter, D. Morrison, ed., University of Arizona Press, Tucson, Arizona, 1982.
2. M.C. Malin and D.C. Pieri, "Europa," in Satellites, J.A. Burns and M.S. Matthews, eds., University of Arizona Press, Tucson, Arizona, 1986.