442 Encyclopedia of the Solar System
FIGURE 15 Model showing how rising
diapir may impinge upon brine-rich ice
(reddish material) lowering the melting
temperature and thermally
disaggregating the surface (right). (See
Collins et al., 2000.)icebergs floating buoyantly on top of the watery matrix. This
model requires that Europa has a very thin shell, less than
∼6 km; otherwise, the warm base of the ice shell would
flow to maintain its thickness faster than the ice shell could
melt from below. Moreover, this model requires that the
ocean is only weaklystratifiedin temperature and salinity
because if stratification were strong, then heat could not be
transferred from the ocean floor to the base of the ice shell.
In addition, a large, concentrated mantle heat source would
need to be stable for hundreds of years. If Europa has a tidal
energy budget that scales to Io’s (i.e., an icy shell overlying
the Io-like tidally heated mantle), then it could potentially
have sufficient heat sources for surface melt-through, but
the actual level of mantle activity is unknown.
A proposed alternative model for chaos formation is anal-
ogous to that for lenticulae, where ice diapirs have risen
buoyantly through the ice crust, breaking or otherwise in-
teracting with the surface (Fig. 15). This mechanism would
explain why some chaos areas stand several hundred me-
ters above the surrounding plains, something that is hard
to explain if they formed atop liquid water, but feasible if
buoyant diapirs rose to their level of neutral buoyancy. It
has been suggested that partial melting of a salty ice shell
could allow surface material to flow. If matrix material is a
mixture of disaggregated ice and low-melting-temperature
brines, then partial melting could explain the apparent flu-
idity of materials associated with many chaos regions and
the mobility of blocks. Given the morphological similarities
between lenticulae and chaos, it seems entirely plausible
that they have similar origins through diapiric upwelling.
It is possible that chaos terrains form from a number of
separate lenticulae that link together by fractures, form-
ing distinct plates that can separate and mobilize. The di-
apiric model for chaos formation is not the whole story,
however; it does have difficulty explaining partial melting
of the matrix because initially warm ice diapirs would be
expected to cool significantly as they approach the surface,
before they would be able to rotate and translate surface
crustal blocks. It is possible that tidal heating would con-
centrate in the warm ice of a rising diapir, countering its
cooling. If so, chaos would represent yet another manifes-
tation of the tidal effects imposed by Jupiter and the Laplace
resonance.
4.2.3 IMPACT STRUCTURES
Although formed in ice, rather than silicate rock, Europa’s
craters have the same range of morphological features as
craters on other bodies, including bowl shapes, central
peaks, bright ray systems, andsecondary craterfields.
Europa’s craters are shallower than those formed on silicate
bodies, however, probably because of viscous relaxation of
the ice in which they form. Another difference is the size
at which the transition from simple, bowl-shaped craters
to more complex craters with central peaks occurs. On the
Moon, a rocky body with similar gravity, this transition oc-
curs at∼15–20 km in diameter, while on Europa it occurs at
only∼5–6 km, presumably because the ice crust is relatively
weak compared to rock. Simple or bowl-shaped craters are
too small to undergo rim collapse or other significant mod-
ifications during formation.
The 24 km diameter crater Pwyll (Fig. 16) is thought to
be the youngest large impact crater on Europa because it
exhibits a bright ray system that extends for over∼1000 km
and can be seen in global views of the satellite. These raysFIGURE 16 Several views of the crater Pwyll. (Left) Global
color-enhanced view showing bright ejecta and rays from
material thrown over 1000 km by the impact. Their white color
indicates exposure of fresh, icy material. (Top right) Higher
resolution image of Pwyll, showing distinct rim and central peak,
along with ejecta around the impact. (Bottom right) Topographic
model of Pwyll, created from stereo imaging. This shows the
fresh but shallow topography of the crater. (NASA/JPL/DLR.)