The Moon 249
In the closing stages of the accretion of the terrestrial
planets 50–100 million years afterT 0 (4567 million years
ago), the Earth suffered a grazing impact with an object
(named Theia) of about 0.10 Earth mass. This body is as-
sumed to have differentiated into a silicate mantle and a
metallic core. It came from the same general region of the
nebula as the Earth (the oxygen and chromium signatures
of Earth and Moon are identical and the impact velocities
are required to be low in the models).
Theia was disrupted by the collision and mostly went
into orbit about the Earth. Gravitational torques, due to
the asymmetrical shape of the Earth following the im-
pact, assisted in accelerating material into orbit. Expand-
ing gases from the vaporized part of the impactor also pro-
moted material into orbit. Following the impact, the mantle
material from Theia was accelerated, but its metallic core
remained as a coherent mass and was decelerated relative to
the Earth, so that it fell into the Earth within about 4 hours.
A metal-poor mass of silicate, mostly from the mantle of
Theia, remained in orbit.
In some variants of the hypothesis, this material immedi-
ately coalesced to form a totally molten Moon. In others, it
broke up into several moonlets that subsequently accreted
to form a partly molten Moon. This highly energetic event
accounts for the geochemical evidence that indicates that
at least half the Moon was molten shortly after accretion.
Figure 22 illustrates several stages of a computer simulation
of the formation of the Moon according to one version of
the single giant impact hypothesis.
Although the giant impact event vaporized much of the
material, the material now in the Moon does not seem to
have condensed from vapor. The extreme depletion of very
volatile elements and the bone-dry nature of the Moon may
be inherited from Theia and so have been a general feature
of the early inner solar nebula (all primary meteorite min-
erals are anhydrous) with volatiles and water added later to
the Earth from near Jupiter.
Unique events are notoriously difficult to accommodate
in most scientific disciplines. An obvious requirement in this
model is that a suitable population of impactors existed in
the early solar system. Evidence in support of the previous
existence of large objects in the early solar system comes
from the ubiquitous presence of heavily cratered ancient
planetary surfaces, from the large number of impact basins
with diameters up to 2000 km or so, and from the obliquities
or tilts of the planets, all of which demand collisions with
large objects in the final stages of accretion. The extreme
example is that an encounter between Uranus and an Earth-
sized body is required to tip that planet on its side. Thus, the
possibility of many large collisions in the early solar system
is well established, one of which had the right parameters to
form the Moon. The single impact scenario is thus consis-
tent with the planetesimal hypothesis for the formation of
the planets from a hierarchical sequence of smaller bodies.
FIGURE 22 A computer simulation of the origin of the Moon
by a glancing impact of a body larger than Mars with the early
Earth. This event occurred about 4500 million years ago during
the final stages of accretion of the terrestrial planets. Both the
impactor and the Earth have differentiated into a metallic core
and rocky silicate mantle. Following the collision, the mantle of
the impactor is ejected into orbit. The metallic core of the
impactor clumps together and falls into the Earth within about
4 hours in this simulation. Most terrestrial mantle material
ejected by the impact follows a ballistic trajectory and is
reaccreted by the Earth. The metal-poor, low-density Moon is
thus derived mainly from the silicate mantle of the impactor.
(Courtesy A. G. W. Cameron.)This research was conducted in part at the Lunar and
Planetary Institute, which is operated by the USRA under
contract CAN-NCC5-679 with NASA. This is LPI Contri-
bution 1260.Bibliography
Basaltic Volcanism Study Project (1981). “Basaltic Volcanism
on the Terrestrial Planets.” Pergamon, New York.