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OCR for page 6
he outer solar system is dominat-
ed by the giant ice and gas plan-
ets, whose enormous gravitational
fields sweep up much of the debris
that passes through their orbits. As
the solar system evolved, this debris
formed a host of satellites around
Jupiter, Saturn, Uranus, and Neptune,
and very large satellites formed
around all but Uranus. These major
moons, all larger than Pluto and two
larger than Mercury, are interesting
worlds in their own right.
Why are these worlds worthy of
national and international exploration
and research? The most compelling
Icy Europa as seen by Voyager 2 (top] and Galileo (above]. The view from Galileo of
Europa's Conamara region shows objects as small as 60 m across. The discontinu-
ities in the linear features create the strong impression that pieces of the surface
have fractured and drifted before refreezing into new positions.
motivation relates to understanding
the origin and survival of life and the
limits on where life can be found in a
planetary system. Water is essential to
life as we know it, and the large icy
satellites may contain the largest reser-
voirs of liquid water in the solar sys-
tem. fupiter's moon Europa may be
the best place in the solar system
beyond Earth to search for signs of
life. Saturn's moon Titan provides a
natural laboratory for the study of
organic chemistry over spans of time
and distance unattainable in terrestrial
laboratories. The origin and evolution
of the satellite systems of the giant
planets also provide analogs for under-
standing planetary systems around
other stars, some of which may be
abodes for life. Perhaps teeming with
~ Fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
life or perhaps sterile today, these
moons seem certain to contain the
basic ingredients for life. Knowing
whether or not life exists on them
today is equally compelling.
Europa holds the most promise for
understanding the biological potential
of icy satellites. There is convincing
evidence that the incessant pushing
and pulling on Europa by fupiter's grav-
itational field generates sufficient tidal
heating to sustain a global ocean of liq-
uid water just a few tens of kilometers
beneath the satellite's icy surface.
Moreover, there is geological evidence
of recent transfer of material between
the surface and the water layer. If a
cold europan ocean is in direct contact
with Europa's warmer rocky mantle,
the result could be an environment in
which complex chemical processes can
occur. Such an environment could lead
to the beginnings of life.
A Europa mission with the goal of
confirming the presence of an interior
ocean is the first step in understanding
icy satellites' potential as abodes for
life. Characterizing Europa's ice shell
and understanding its geological histo-
ry are also vital components of such a
mission. By exploring the extent to
which organic chemistry progresses
toward life in extreme planetary envi-
ronments, a Europa mission would
help us to understand how tidal heat-
ing can affect the evolution of worlds.
Europa's internal structure may include a water layer some 100 km thick.
OCR for page 7
Europa Geophysical Explorer
C. ~ ~ ~ ~ r r ~ ~ r,~,~,~,~,~,r,r,r,r,r,.;
i ~ ~ ~ ~ a a a a ~
Profile
Europa Geophysical Explorer
Mission Type: Orbiter
Cost Class: Large
Priority Measurements:
· Obtain high-resolution images of
Europa's surface.
· Characterize its internal heat
sources.
· Determine its surface composition.
· Sound the ice shell to determine its
thickness and structure.
· Search for temporal variations in its
magnetic properties.
These concepts are key to understand-
ing the origin and evolution of water-
rich environments in icy satellites.
The Europa Geophysical Explorer
mission proposed by the SSE Survey is
an extension of orbiter concepts stud-
ied by NASA in the l990s, and would
pave the way for more ambitious
Europa missions in the future, such as
a lander or even a submarine to
explore the ocean (if it exists). The
Europa Geophysical Explorer would be
tasked with confirming the presence
of an ocean, identifying areas of
recent surface activity, finding areas
with possible biotic or prebiotic com-
pounds, and characterizing the
europan environment in preparation
for further study.
The large cost (in excess of $1 bil-
lion) of this mission would place it in
the same class as Galileo and Cassini/
Huygens. Such large missions have
been a traditional focus for interna-
tional cooperation in which NASA
and other national space agencies can
leverage their resources to accomplish
what might otherwise be too costly to
achieve. Galileo and Cassini/Huygens
provide perfect examples of partner-
ships that have proved highly success-
ful. NASA should engage prospective
international partners in the planning
and implementation of the Europa
Geophysical Explorer.
Guiding Themes Addressed Important Planetary Science Questions Addressed
In addition to mapping the satellite's topography, the Europa Geophysical Explorer
will use radar to probe the structure of Europa's icy surface layer.
Representative terms from entire chapter:
geophysical explorer
