Page 229 - Fundamentals of Physical Volcanology

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206 CHAPTER 13

the large metal core of Mercury froze at some point
in its geological history.
We know from the mean density of Mercury that
it has an iron core that is unusually large for the size
of the planet. This has led to the suggestion that
Mercury, like the Earth, was involved in a giant
collision with another planet-sized body in its very
early history. The event presumably occurred after
Mercury had at least partly differentiated into a
core, mantle and crust, and the impact stripped off
much of the crust and mantle but left the core un-
disturbed. Mercury did not retain part of the debris,
as the Earth did to form its Moon, but lost it all.
If this suggestion is correct, it has some impor-
tant implications for what can be seen on the
surface. If much of the evolution of Mercury into a
Fig. 13.15 Possible smooth volcanic plains on the surface mantle and crust had finished when the impact
of Mercury. The undulating ridges are probably thrust faults took place, then at least a large part of what can
formed after the flows were erupted. The largest impact
be seen on the surface should in fact be olivine-rich
crater in the image is ∼100 km in diameter. (Mariner 9 frame
rocks that normally would be associated with the
FDS 167, courtesy NASA/JPL/Northwestern University.)
mantle. Alternately, if significant further evolution
of the mantle occurred after the giant impact, per-
haps what we should see is a relatively thin basaltic
crust. Our only evidence comes from simple
spectroscopic data from the Mariner 10 space-
craft, augmented by spectroscopic observations
from Earth-based telescopes that are hindered by
Mercury’s small size, great distance from Earth, and
closeness to the Sun. The main conclusions from
these data are that Mercury’s surface rocks contain
smaller amounts of iron and titanium than those
of the Moon, and are more akin to the lunar high-
land rocks than to the basalts of the maria. Clearly
our understanding of the possible volcanic history
of Mercury is very primitive. Two spacecraft, Mes-
senger and BepiColombo, are planned to arrive at
Mercury in the period 2010 to 2020, and we shall
have to wait for results from these probes before
Fig. 13.16 The arcuate scarp to the far right in this 400 km much progress can be made.
wide image of plains on Mercury may be a lobate lava flow
front. (Mariner 9 frame FDS 166738, courtesy
NASA/JPL/Northwestern University.)
13.8 Io
edges and fronts of lava flow units (Fig. 13.16), but The intense tidal flexing of Io by Jupiter produces
elsewhere similar features seen at lower resolution internal heating at a prodigious rate. As a result Io is
have been traced for many hundreds of kilometers very vigorously volcanically active. The activity was
and are interpreted to be the traces of thrust faults detected by the two Voyager spacecraft that flew
produced by a general shrinking of the crust when by Io in 1979 and monitored in depth between