Mercury 133
FIGURE 18 (a) A thermal history model for inner core radius as
a function of time for three values of initial core sulfur content.
The colors show the ranges in sulfur content from 0.2 to>5%,
and the solid, dotted, and dashed lines are for sulfur contents of
0.2, 1, and 5%, respectively. (b) Decrease in Mercury’s radius due
to mantle cooling and inner core growth for three values of initial
core sulfur content as in (a). (Modified from Vilas et al., 1988.)
early in its history, perhaps during the first 700 to 800
million years. Today the planet may still be contracting as
the present fluid outer core continues to cool.
7. Origin
The origin of Mercury and how it acquired such a large
fraction of iron compared to the other terrestrial planets
is not well determined. Chemical condensation models for
Mercury’s present position in the innermost part of the so-
lar nebula, from which the solar system formed, cannot
account for the large fraction of iron that must be present
to explain its high density. Although these early models are
probably inaccurate, revised models that take into account
material supplied from feeding zones in more distant re-
gions of the inner solar system only result in a mean un-
compressed density of about 4200 kg/m^3 , rather than the
observed 5300 kg/m^3. Furthermore, at Mercury’s present
distance, the models predict the almost complete absence
of sulfur (100 parts per trillion FeS), which is apparently re-
quired to account for the presently molten outer core. Other
volatile elements and compounds, such as water, should also
be severely depleted (<1 part per billion of hydrogen).
Three hypotheses have been put forward to explain the
discrepancy between the predicted and observed iron abun-
dance. One (selective accretion) involves an enrichment of
iron due to mechanical and dynamical accretion processes
in the innermost part of the solar system; the other two
(postaccretion vaporization and giant impact) invoke re-
moval of a large fraction of the silicate mantle from a once
larger proto-Mercury. In the selective accretion model,
the differential response of iron and silicates to impact
fragmentation and aerodynamic sorting leads to iron enrich-
ment owing to the higher gas density and shorter dynami-
cal timescales in the innermost part of the solar nebula. In
this model, the removal process for silicates from Mercury’s
present position is more effective than for iron, leading to
iron enrichment. The postaccretion vaporization hypothesis
proposes that intense bombardment by solar electromag-
netic and corpuscular radiation in the earliest phases of the
Sun’s evolution vaporized and drove off much of the silicate
fraction of Mercury leaving the core intact. In the giant im-
pact hypothesis, a planet-sized object impacts Mercury and
essentially blasts away much of the planet’s silicate mantle
leaving the core largely intact.
Discriminating among these models is difficult, but may
be possible from the chemical composition of the sili-
cate mantle (Fig. 19). For the selective accretion model,
Mercury’s silicate portion should contain about 3.6–4.5%
alumina, about 1% alkali oxides (Na and K), and between 0.5
and 6% FeO. Postaccretion vaporization should lead to very
severe depletion of alkali oxides (∼0%) and FeO (<0.1%),
and extreme enrichment of refractory oxides (∼40%). If
a giant impact stripped away the crust and upper man-
tle late in accretion, then alkali oxides may be depleted
(0.01–0.1%), with refractory oxides between∼0.1 and 1%
and FeO between 0.5 and 6%. Unfortunately our current
knowledge of Mercury’s silicate composition is extremely
poor, but near and mid-infrared spectroscopic measure-
ments favor low FeO- and alkali-bearing feldspars. If the
tenuous atmosphere of sodium and potassium is being out-
gassed from the interior, as suggested by some, then the
postaccretion vaporization model may be unlikely. Decid-
ing between the other two models is not possible with our
current state of ignorance about the silicate composition.
Since the selective accretion hypothesis requires Mercury
to have formed near its present position, then sulfur should
be nearly absent, unless the solar nebula temperatures in
this region were considerably lower than predicted by the
chemical equilibrium condensation model.
Support for the giant impact hypothesis comes from
three-dimensional computer simulations of terrestrial
planet formation for several starting conditions. Since these