Page 233 - Fundamentals of Physical Volcanology

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The mass and diameter of Europa are known very have developed on Europa. Unfortunately the ocean
accurately, thus giving us its mean density, and we is so deep that there is little chance that surface
also have some idea of the distribution of density features on the ice would directly reflect what is
within it from the tracking of the Galileo spacecraft happening 100 km below.
during close approaches. The most likely internal
structure is a differentiated silicate body with a
water ocean about 100 km deep, the outer 5–10 km 13.10 Differentiated asteroids
of which are frozen to form the observed ice sur-
face. The young age of that surface implies that Of the few asteroids yet visited by spacecraft, none
water frequently escapes onto the surface from has been a body that we think evolved to the
beneath the ice to cover up impact craters, and point of having volcanic eruptions. Most of our
indeed there are enormous numbers of long thin information about asteroids is obtained from the
fractures from which water appears to have flowed meteorites broken off them, and usually it is not
a short distance before freezing. The rheological known which asteroid has produced a given
changes in these water flows as ice crystals form in meteorite. In a few cases, however, there are good
them should theoretically have some similarities enough spectroscopic observations from Earth
to the development of a yield strength in a cooling telescopes (and the Hubble space telescope) to be
silicate lava flow, but this has not yet been studied sure that certain groups of meteorites come from
in detail. There are also patches of crust on Europa a particular body. By far the strongest case can
where raised blocks of ice with old fracture pat- be made for the Howardite, Eucrite, and Diogenite
terns lie near one another and can be fitted together groups of meteorites coming from one of the
like a jigsaw. These seem to be places where the largest asteroids, the 520 km diameter 4 Vesta
crust melted nearly all the way to the surface, and (Fig. 13.21). These meteorites include all of the
the unmelted top layer fractured into icebergs kinds of rocks one would expect to get from a dif-
that drifted apart in water that welled-up between
them before freezing again.
All this evidence points to vigorous thermal con-
vection in the ocean under the ice, and possibly
even slow convection within the ice layer itself.
The same calculations of tidal flexing that confirm

the massive heat source inside Io also imply a
smaller but still significant source inside Europa,
and it is not out of the question that volcanic activ-
ity takes place in the silicate body beneath the
ocean even today. As seen earlier, the high pressure
would limit the activity to the eruption of lava with
no exsolved gas bubbles. This would mean that
there was little opportunity for a complex density
profile to evolve in the silicate crust, and so no neu-
tral buoyancy level at which magma would pref-
erentially accumulate to form magma reservoirs.
Thus any magma would have to travel directly to
the surface from its source zone in the mantle, or Fig. 13.21 The best available image of the asteroid 4 Vesta,
taken by the Hubble Space Telescope. Note the asymmetric
would stall as an intrusion at some depth dictated
shape and the large depression, evidence of a large impact
by the amount of cooling that it had experienced as
cratering event. The image has been digitally restored to a
it rose; this is very similar to what happened on our
resolution of ~10 km per picture element. (Image credit:
Moon. We might speculate that if water is the key to Ben Zellner (Georgia Southern University), Peter Thomas
the Earth having plate tectonics, the process might (Cornell University) and NASA.)