132 Encyclopedia of the Solar System
FIGURE 17 The 130-km-long
Vostok scarp transects two craters 80
and 65 km in diameter. The
northwest rim of the lower crater
(Guido d’Arezzo) has been offset
about 10 km by thrusting of the
eastern part of the crater over the
western part. The diagram to
the right shows the geologic
relationship of the thrust fault and
the offset crater rim. (Modified from
Strom and Sprague, 2003.)the period of cataclysmic bombardment (Fig. 18). This
would imply that the observed tectonic framework began
at about the same time, and that smooth and intercrater
plains were emplaced before inner core formation. In-
deed, the geologic evidence indicates that at least the
observed tectonic framework began to form relatively
late in Mercury’s history; certainly after intercrater plains
formation and possibly after smooth plains formation.
However, under initially molten conditions, the thermal
history models indicate that the lithosphere has always
been in contraction. The surface record of the period of
intense contraction caused by mantle cooling has probably
been erased by the period of cataclysmic bombardment
and intercrater plains formation that occurred from about
3.9 to 3.8 billion years ago. That would explain why there
is no evidence for old compressive structures.
If the upper value of a 2-km radius decrease, inferred
from the thrust faults, was due solely to cooling and solid-
ification of the inner core, then the core sulfur abundance
is probably 2–3%, and the present fluid outer core is about
500 or 600 km thick. If the lower value of a 0.5-km radius
decrease is correct, then there must be more than 5% sulfur
in the core, and the present fluid outer core would be over
1000 km thick.
If the smooth and intercrater plains are volcanic flows,
then they must have had some way to easily reach the
surface to form such extensive deposits. Early lithospheric
compressive stresses would make it difficult for lavas to
reach the surface, but the lithosphere may have been rela-
tively thin at this time (<50 km). Large impacts would be
expected to strongly fracture it, possibly providing egress for
lavas to reach the surface and bury compressive structures.6.5 Geologic History
Mercury’s earliest history is very uncertain. If a portion of
the mantle was stripped away, as invoked by most scenarios
to explain its high mean density, then Mercury’s earliest
recorded surface history began after core formation,
and a possible mantle-stripping event (see Section 7).
The earliest events are the formation of intercrater plains
(≥3.9 billion years ago) during the period of late heavy bom-
bardment. These plains may have been erupted through
fractures caused by large impacts in a thin lithosphere.
Near the end of late heavy bombardment, the Caloris Basin
was formed by a large impact that caused the hilly and
lineated terrain from seismic waves focused at the antipodal
region. Further eruption of lava within and surrounding
the Caloris and other large basins formed the smooth plains
about 3.8 billion years ago. The system of thrust faults
formed after the intercrater plains, but how soon after is
not known. If the observed thrust faults resulted only from
core cooling, then they may have begun after smooth plains
formation and resulted in a decrease in Mercury’s radius. As
the core continued to cool and the lithosphere thickened,
compressive stresses closed off the magma sources, and
volcanism ceased near the end of late heavy bombardment.
All of Mercury’s volcanic events probably took place very