134 Encyclopedia of the Solar System
FIGURE 19 Possible bulk composition of the silicate mantle for
the three models of Mercury’s origin; selective accretion (SA),
postaccretion vaporization (V), and giant impact (GI). The
composition is parameterized for the FeO content, the alkali
content (soda plus potash) in (a), and the refractory oxide
content (calcium plus aluminum plus titanium oxides) in (b). The
modifying effects of late infall of 0–5% of average chondritic
meteorite material on several regolith compositions are indicated
by arrows labeled 0–5. (Modified from Vilas et al., 1988.)
simulations are by nature stochastic, a range of outcomes
is possible. They suggest, however, that significant frac-
tions of the terrestrial planets may have accreted from
material formed in widely separated parts of the inner so-
lar system. The simulations indicate that during its accre-
tion Mercury may have experienced large excursions in its
semimajor axis. These semimajor axis excursions may have
ranged from as much as 0.4–1.4 AU due to energetic im-
pacts during accretion (Fig. 20). Consequently, Mercury
could have accumulated material originally formed over
the entire terrestrial planet range of heliocentric distances.
About half of Mercury’s mass could have accumulated at
distances between about 0.8 and 1.2 AU (Fig. 21). If so,
then Mercury may have acquired its sulfur from material
that formed in regions of the solar nebula where sulfur was
stable. Plausible models estimate FeS contents of 0.1–3%.
However, the most extreme models of accretional mix-
FIGURE 20 Results of a computer simulation of terrestrial
planet evolution showing the change of “Mercury’s” semimajor
axis during its accretion. In this case “Mercury’s” semimajor axis
spans the entire terrestrial planet region (0.5–1.4 AU) during the
planet’s growth. (Modified from Vilas et al., 1988.)ing result in homogenizing the entire terrestrial planet
region, contrary to the observed large systematic density
differences.
The simulations also indicate that byproducts of terres-
trial planet formation are planet-sized objects up to three
times the mass of Mars that become perturbed into ec-
centric orbits (meane∼0.15 or larger) and eventually col-FIGURE 21 Results of a computer simulation of terrestrial
planet evolution showing the region (semimajor axis) from which
the terrestrial planets acquired their mass. In this simulation,
“Mercury” acquires about half its mass from regions between 0.8
(green) and 1.2 AU (blue). (Modified from Vilas et al., 1988.)