Jupiter's moon Europa was revealed by the Galileo spacecraft, and previous missions, to be a fascinating ice-covered world with some of the best prospects for the presence of liquid water beyond Earth. Results from various instruments on the Galileo spacecraft revealed the presence of a surface layer approximately 100 km thick with the density of water or water ice. These results also suggest that while the top 10 km or so are likely frozen solid, the majority of this water could exist in a liquid form beneath an icy crust. Coupled with the likely presence of the chemical building blocks of life, and a variety of possible energy sources ranging from tidal heating from Jupiter's gravity to radiative processing of the surface, Europa has emerged as one of the top Solar System locations in terms of potential habitability.
Habitability is the focus of the recently released 2006 Solar System Exploration Roadmap for NASA's Science Mission Directorate. As stated in the Roadmap, "A unifying theme for the exploration of our Solar System is habitability -- the ability of worlds to support life. As living, self-aware, sentient entities, we seek to know whether life is or was present elsewhere in our planetary backyard, how we and our planet came to be, and what are the future prospects for terrestrial life on and off the Earth." Highlighting the importance of this exploration focus, the roadmap states "The Solar System Exploration program described here directly addresses the key science questions regarding habitability in the universe."
Europa as a potentially habitable environment thus gains a prominent place in the list of missions described in the 2006 Roadmap. In particular, a mission to Europa is given the highest priority as the next large "Flagship" class mission. Regarding the contributions of a mission to Europa, the Roadmap states:
While there are many uncertainties regarding the geology and chemistry of this environment and potential life-supporting energy sources within it, confirmation of the existence and determination of the characteristics of Europa's ocean would allow us to conclude whether it is or ever has been a habitable environment. A positive finding would provide tremendous impetus for future surface and subsurface chemical and geophysical Europa exploration. Both Ganymede and Callisto also show evidence for subsurface oceans similar to that of Europa. If the formation of oceans is found to be a common phenomenon, the implications for life in the cosmos could be stunning. Comparative intensive studies of Callisto, Ganymede, and Europa could therefore prove to be one of the most important contributions we can make to the understanding of habitability in the Solar System.
The Roadmap describes a Europa Explorer mission concept. This spacecraft would use gravity-assist flybys of both Venus and Earth, and thus would be able to deliver about three times the payload mass to the Jupiter system of previous concepts like Europa Orbiter. Technology currently exists to allow this mission to survive in the high-radiation environment around Europa, and the mission would be able to carry out a minimum 90-day nominal mission that could be extended for up to a year depending on spacecraft conditions. Before the spacecraft entered Europa orbit, scientists would also be able to take great advantage of the two-year period in Jupiter orbit while the spacecraft used multiple flybys of the icy Galilean satellites to position itself to enter orbit around Europa.
The distance and complexity of a mission to orbit Europa means that it truly requires the large Flagship class of missions as described in the Roadmap. Flyby missions, such as the upcoming New Horizons mission to Pluto, are possible in the cost-capped New Frontiers mission class, but the complex and versatile multi-function missions that would be necessary to perform a detailed analysis of Europa require the commitment of people and resources of a Flagship-class mission. NASA's previous Flagship-style missions, such as Voyager, Viking, Magellan, Galileo, and Cassini, have returned or are returning huge amounts of data and have made fundamental discoveries about the nature of the solar system. The roadmap states: "Large missions capable of reaching distant locations in the outer Solar System with powerful complements of instruments, enabling serendipitous discovery, and conducting adaptive observational programs responsive to these new discoveries is the province of Flagship missions. Flagship missions are an essential component of the Roadmap, if NASA is to make fundamental new discoveries in the Solar System, address the key scientific questions, and maintain public pride and excitement in America's leadership in deep space exploration."
The time is right to begin work on the next Flagship mission, to Europa. A new start is needed now, providing sufficient development time to ensure that the mission can be launched in the middle of the next decade. Even without an official new start, relatively modest investments in refined mission design studies and Europa technology refinement during the next two years can allow NASA to keep the door open toward a Europa Explorer launch as soon as future funding permits.
The time is right scientifically for a Europa mission, as well. Europa science has matured over the last decade, and the current Galileo dataset for Europa has become well characterized. To enable the next new breakthroughs in understanding the nature of Europa's probable watery environment beneath its icy surface, achieving the high priority science goals of understanding Europa's potential for life, requires new, more sophisticated measurements that can only come from a dedicated orbiter. The science questions for the future exploration of Europa are well defined, and the Europa mission concept is mature and requires no new technology development.
Starting work on a Europa mission now, as suggested by the Solar System Exploration Roadmap, is the right thing to do. Europa's interior ocean may be the best environment for life in the solar system beyond planet Earth. There is a substantial scientific basis to believe that Europa has the fundamental ingredients necessary for life: water, organic molecules, a chemical energy source, and a stable environment. Understanding Europa's potential for life brings us closer to addressing one of the most fundamental scientific questions that humans can ask: Are we alone in the cosmos? It is only by committing the time and resources to a capable Europa mission that we will be able to begin to answer this essential question.