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.
advertisement
