Europa 433
of rock and ice. If the jovian subnebula were cold enough,
some lower temperature condensates such as CO 2 could
have been incorporated as Europa and the other Galilean
satellites formed.
Europa’s early heat of accretion combined with heat
from radioactive decay would have warmed the satellite’s
interior and formed a primordial ocean, which was likely
reduced and sulfidic. Thermal and geochemical evolution
would have caused some oxidation of the ocean through
time, forming sulfates. Refined models of Europa’s accre-
tion and chemical evolution are bringing improved under-
standing of the satellite’s initial conditions.
3. Stress Mechanisms and Global
Tectonic Patterns
3.1 Europa’s Internal Structure
Although numerous models exist for the thickness and ther-
mal state of Europa’s shell, there is a lack of information
regarding key input parameters such as the thermal and
mechanical properties of its ice. These models can put
bounds on the shell thickness and make predictions about
how it has varied over time. Such predictions can be com-
pared to geological and geophysical measurements. Mea-
surements of Europa’s gravity field from theGalileospace-
craft constrain itsmoment of inertia. Further assumptions
of likely composition and density of its internal layers sug-
gest that Europa has a∼100 km thick layer of H 2 O overlying
a rockymantle, which surrounds an iron-richcore(Fig. 2).
The most definitive evidence that there is an ocean
within Europa at the present time comes from theGalileo
spacecraft’s magnetometer, combined with theoretical stud-
ies. Because Jupiter’s powerful magnetic field is tilted by
10 ◦relative to the planet’s equatorial plane in which the
satellites rotate, the satellites experience Jupiter’s magnetic
field as time-varying. For Europa, each 5.5 hours the satel-
lite finds itself alternately above then below the magnetic
equator of Jupiter. Surprisingly, the Galileo magnetome-
ter measured a magnetic field in the vicinity of Europa,
which alternately flips to oppose the external jovian mag-
netic field. This implies that Europa is behaving as a con-
ductor, generating an induced magnetic field in response
to the jovian field. Modeling of theGalileoobservations
suggests that there is a conductive layer—probably a briny
ocean—possibly many tens of kilometers deep, within the
outer portion of Europa.
The thickness of the ice shell overlying the ocean is sig-
nificant for models of Europa’s thermal evolution, geolog-
ical processes, and astrobiology. Future missions will want
to sample material from the ocean to understand Europa’s
potential for life, but the means to accomplish this task are
dependent upon the ease by which material from the ocean
can be accessed, and the ways in which this material may
have been processed. Because the thickness of Europa’s
FIGURE 2 Interior structure of Europa. Rocky mantle (brown)
and iron-rich core (gray) are synchronously locked in position
with respect to Jupiter, but the ice shell (white) may rotate
nonsynchronously—slightly faster than the interior—if
decoupled by the water layer (blue). Layer thicknesses are not to
scale. (NASA/JPL/Brown University.)ice shell cannot be determined from gravity and magnetic
data, we must search for clues in the geophysical history
and geological record preserved in the icy surface.3.2 Tidal Evolution
The principal energy source that heats Europa’s interior
and drives its tectonics today comes from its orbital inter-
action with Jupiter. Anorbital resonanceoccurs when
two satellites have orbital periods that are related by in-
teger relationships, allowing them to exert a gravitational
influence over each other and affecting the eccentricity of
their orbits. The three Galilean satellites are involved in
theLaplace resonance, in which the orbital periods of
Ganymede:Europa:Io are in a near 1:2:4 ratio, but more
important, the mutualconjunctionsof the Io–Europa pair
and of the Europa–Ganymede pairprecessaround Jupiter
at precisely the same rate. Like a child on a swing pushed
at the optimal moment, the recurring mutual conjunctions
force and maintain eccentricities in their orbits (Fig. 3).
Although it is not known exactly when or how the moons
came to form the precise clock that is the Laplace reso-
nance, one model suggests this resonance was progressively
achieved after Io moved outward into a near 2:1 resonance
with Europa, and then the Io–Europa pair moved outward
until Ganymede was captured into its own near 2:1 reso-
nance with Europa. Europa’s forced eccentricity is key to
its youthful and complex surface, as will be described in the
following sections.