Geology Reference
In-Depth Information
(a)
(b)
Figure 8.21. This view of Europa
is trailing hemisphere is 335 km
across. Bright plains mark the right side, while domes occur in a
cluster in the upper left. The dark irregular patch in the upper right is
a zone of chaos; cutting east
'
west across the upper part of the image
is a gray band, or spreading center, where the crust was separated
and
filled in with ice or water (NASA Galileo PIA01125).
-
Figure 8.22. (a) This part of Europa
s trailing hemisphere near the
equator shows two large ridge systems, older plains, and dark spots.
Below the
'
Fractures are as small as a few meters wide and can
exceed 1,000 km in length. Some fractures cut across
nearly all surface features, indicating that the ice shell is
subject to deformation on the most recent time scales. The
youngest ridges and fractures could be active today in
response to tidal
flexing. Young ridges could also be
places where the ocean has recently exchanged material
with the surface and would be prime targets as potential
habitable niches.
Chaos terrain is characterized by fractured plates of ice,
many of which have shifted into new positions within a
matrix
(Fig. 8.22)
. Much like a jigsaw puzzle, many
plates can be
fit back together. Some ice blocks appear
to have disaggregated and foundered into the surrounding
finer-textured matrix, while other chaos areas stand
higher than the surrounding terrain. Models of chaos
formation, reviewed by Collins and Nimmo (
2009
), sug-
gest whole or partial melting of the ice shell, perhaps
enhanced by local pockets of brine that would have
lower melting temperatures. Chaos terrain commonly
has associated dark reddish material thought to be derived
from the subsurface, possibly from the ocean. However,
these and related models are poorly constrained because
formed by cross-cutting ridges is a dark area of the icy
crust severely disrupted into chaos terrain, named Conamara Chaos.
Galileo data suggest that the dark ridges, spots, domes, and disrupted
terrains are all of essentially the same composition, suggesting that
material was brought to the surface by cryovolcanism; the area is
250 kmby 300 km (NASAGalileo PIA03002). (b) A high-resolution view
of the disrupted ice crust in the Conamara region; the area shown is
70 km by 30 km (NASA Galileo PIA01127).
“
X
”
the processes within and below Europa
'
s ice shell are not
known.
8.5.3 Volcanic features
Volcanism of two forms is suggested for Europa. The
rst
involves silicate volcanism at the base of Europa
s water
shell at the interface with the rocky interior. Tidal stresses
of the sort that generates Io
'
s volcanism could
ex the
rocky mantle to produce enough heat to melt the rock,
leading to volcanism analogous to that seen on Earth
'
s sea
floor. If such activity occurs within Europa, it could main-
tain liquid water beneath the ice shell and lead to convec-
tion and zones of up-welling.
'
