446 Encyclopedia of the Solar System
limited, studies suggest that most of the small (<1kmdi-
ameter) craters on Europa are secondaries.
Comparisons of images fromVoyagerandGalileo, ac-
quired 20 years apart, show no definitive evidence for
current activity on Europa’s surface, although such compar-
isons are hampered by a lack of high-resolution global image
data taken at similar lighting geometries. Similarly, searches
for plumes such as those observed on Io and Enceladus
have proved unsuccessful. Nevertheless, if the surface is
only∼60 Ma old, it seems likely that Europa may still be
active today.
7.2 Surface History and Geological Evolution
Mapping of Europa’s landforms and their interactions with
each other yields a time history, orstratigraphy, of sur-
face evolution and shows whether theresurfacingstyle
has changed over time. Several areas across Europa’s sur-
face have been mapped at a variety of scales,and there
does appear to have been a change in geological activity
and style through the decipherable time-history of the sur-
face. The oldest type of terrain is the “ridged plains,” a
m ́elange of ridges and linear structures that are uniformly
bright overall, and in which it is difficult to pick out distinct
feature types. Bands are intermediate in the stratigraphic
column, while chaos and lenticulae are among the youngest
surface features, commonly disrupting bands and ridged
plains. Troughs and double ridges have formed through-
out Europa’s surface history and crosscut bands and some
lenticulae and chaos. There appears to have been more ac-
tivity in the earlier surface record, with a waning in the
number and width of features in the later stratigraphy.
Stratigraphic mapping therefore suggests that Europa’s
geological style has generally changed over time, from
ridged plains formation, to band formation, to chaos and
lenticulae formation, with the activity level simultaneously
waning. The mechanism for this change is uncertain, but
one plausible model that fits the observations is one in which
Europa’s ocean is slowly cooling, such that the ice above it
is thickening as the ocean freezes out. After the ice shell
reaches a critical thickness,solid-state convectionmay
be initiated, allowing ice diapirs to be convected toward
the surface. A thickening ice shell could be related to a
waning intensity of geological activity since the surface is
expected to be more mobile if the ice shell is thinner.
Because Europa’s surface is probably relatively young,
such a fundamental change in style might seem unlikely
over the last∼1% of the satellite’s history, and we must
speculate on its activity over the rest of its∼4.5 billion year
existence. Four possible scenarios have been proposed (Fig.
21): (1) Europa resurfaces itself in a steady-state and rela-
tively constant, but patchy style; (2) Europa is at a unique
time in its history, having undergone a recent major resur-
facing event; (3) global resurfacing is episodic or sporadic;
or (4) the satellite’s surface is actually much older than our
FIGURE 21 Possible schematic evolutionary models for
Europa’s surface. White represents epochs dominated by ridged
plains formation, and black represents mottled terrain formation.
Current analyses suggest that Europa is either at a special time
in its history or, more likely, that it undergoes episodic
resurfacing. (After Pappalardo et al., 1999.)cratering models suggest. From the standpoint of the dy-
namical evolution of the Galilean satellite system, there
is good reason to believe that Europa’s surface evolution
could be cyclical (i.e., scenario above). As participants in
the Laplace resonance, the orbital characteristics of Io, Eu-
ropa, and Ganymede are inherently linked to each other,
and also to their interior thermal characteristics. Io experi-
ences the greatest amount of tidal heating and largely drives
the predicted cycling. The eccentric orbit of Io can cause a
great amount of tidal heating, which tends to drive its orbit
toward circularity, and in turn decreases its tidal heating.
The decreased tidal heating causes Io to cool, but it also
allows its eccentricity to increase again, thereby increasing
the tidal heating and Io’s temperature, thus completing the
cycle. This cyclical evolution of Io’s tidal heating and orbital
characteristics pulls Europa (and Ganymede) along for the
ride through the Laplace resonance. In this way, Europa
can experience cyclical variations in its orbital characteris-
tics and tidal heating on time scales of perhaps 100 Ma,
and therefore may resurface itself on approximately these
timescales.
The coupled thermal and orbital evolution of the
Galilean satellites can cause significant variations in the
thickness of Europa’s ice shell and level of geological activity
through time. In this scenario, Io and Europa are currently
in a diminishing phase of activity. The observed surface
characteristics of Europa may represent the latest, waning
stage of a long cyclical thermal and geological history.8. Astrobiological Potential
Based on our terrestrial view, the primary ingredients for
life are water, organic compounds, and chemical energy.