Geology Reference
In-Depth Information
8.5.5 Europa summary
In many respects, Europa is a hybrid planet. It is a rocky
object similar in composition and size to Earth
'
s Moon but
contains an outer shell of water and ice, a characteristic
common among the outer planet satellites. As with Io and
Venus, Europa
'
is surface is very young, leading to a com-
pressed geologic time scale (
Fig. 4.50)
. There is a strong
possibility that Europa is currently being resurfaced.
Tidal stresses in the ice shell and to some extent within
the rocky interior generate tectonic and volcanic processes,
including the possibility of sea-
oor volcanism similar to
that in Earth
'
s oceans. These processes lead to complex
surface features, as portrayed in the diagrams of
Fig. 8.27
.
Coupled with the probable presence of organic compounds
and key inorganic elements, Europa is a primary target in
the search for habitable zones beyond the Earth.
Figure 8.26. Not all young ridges are dark; the prominent ridge-pair
extending east
west (right to left) cuts across the set of older dark
ridges on the left. Bright ice avalanches have been shed from the
west
-
east ridge and cover not only the dark ridge-set, but also the
underlying plains. The area shown is 20 km wide (NASA Galileo
PIA01179).
-
8.6 Ganymede
such, their size would imply that the thickness of the
ice shell was at least 10
-
20 km at the time of formation
(McKinnon,
1999
). Alternatively, Rick Greenberg et al.
(
1999
) suggest that the lenticulae could be small forms
of chaos formed by wholesale melting of the ice crust.
Ganymede is the largest satellite in the Solar System and is
the only moon known to generate its own magnetic
field. It
is about the same size as Mercury, but Ganymede
'
slow
density (1.9 g/cm
3
) suggests that it is composed of about
60% water and 40% rocky materials. Geophysical models
indicate that it is differentiated into an iron and sul
de
-
iron
core about 1,600 km across, a silicate mantle, and an outer
icezone800
-
1,000 km thick
(Fig. 8.4)
. A saltwater liquid
ocean is thought to be sandwiched within the ice at about
170 km below the frozen surface. Images of Ganymede
taken by Voyager and Galileo cover about 80% of the
surface and show two prominent terrains
(Fig. 8.28)
:dark,
heavily cratered regions considered to be ancient and bright
terrain characterized by sets of ridges and grooves.
8.5.4 Gradation features
Gradation on Europa occurs through mass wasting and
space weathering. Mass wasting is seen along the bases of
steep slopes
(Fig. 8.26)
where blocks of dirty ice have
accumulated. It is likely that similar processes have
degraded the walls of impact craters which have also
been deformed by viscous relaxation.
On a regional scale there is a marked difference
between the leading and trailing hemispheres of Europa
(Fig. 8.15)
. The trailing hemisphere is darker, tends to be
red in comparison with the leading hemisphere, and
probably results from the implantation of materials by
Jupiter
8.6.1 Physiography
Overall, Ganymede is very bright and even its dark terrains
are actually brighter than the lunar highlands. The surface
also appears to be spectrally
“
red,
”
with the dark terrain
being somewhat redder than the bright terrain. Moreover,
as in the case of Europa, the trailing hemisphere is darker
than the leading hemisphere and could be enriched in sulfur
dioxide from exogenic implantation. Dark non-ice materials
at lower latitudes are possibly hydrated brines similar to
those inferred for Europa while minor constituents include
CO
2
,SO
2
, and possibly organic materials.
s magnetosphere. Speeds of materials within the
magnetosphere are faster than Europa
'
s orbit, which
means that materials within the magnetosphere would
“
catch up
”
with Europa and impact the trailing hemi-
sphere. Some of this material includes sulfur and sulfur-
rich compounds from Io
'
s volcanic plumes. In addition,
ions impacting the trailing hemisphere would alter
the molecular structure of the ice, causing it to darken
with time.
'
