78 Scientific American, July 2019
evidence collected during the Apollo program, is that
some 4.5 billion years ago, a body about the size of
Mars (referred to as Theia) hit Earth, fragmenting itself
and ejecting part of Earth’s crust and mantle into space.
Eventually the ejected terrestrial material mixed with
the remnants of Theia, accumulating into a satellite
that cooled and became the moon.
This model has been influenced by many observa-
tions from the Apollo samples and surface experiments,
which include:I R O N : The moon has surprisingly little iron. Surface
geophysics experiments deployed by Apollo missions
showed that compared with the terrestrial planets,
the moon’s core comprises a very small portion of its
volume compared with the terrestrial planets—just
25 percent of its total radius. The relative lack of iron
suggested by the moon’s small core is evidence that
Earth had already formed an iron-rich center when
the giant impact occurred, leaving little iron to form
the moon.D R Y N E S S : The lunar samples proved to be extremely
dry and almost entirely depleted of volatiles—ele-
ments or molecules with low boiling points that easily
evaporate, such as water, carbon dioxide, nitrogen
and hydrogen. To explain this depletion, scientists
suggest the massive amount of energy and heat gener-
ated from the giant impact may have driven volatiles
from the fragments of the proto-moon.MAGMA OCEAN: One of the most influential hypothe-
ses to come from the lunar samples is the idea that
there was an ocean of magma on the early moon. Apol-
lo 11 samples showed that the lunar highlands (bright,
high-standing regions as opposed to the dark lunar
maria in low-lying areas) contain high concentrations
of the mineral plagioclase. The texture of the rocks con-
taining this mineral suggested that it formed from a
large body of molten rock that cooled, and the light pla-
gioclase crystals floated to the top. Because similar
rocks had been found by previous robotic missions at
other locations, and the lunar highlands are wide-
spread, the layer of magma must have covered most, or
all, of the moon’s surface. Two independent groups pro-
posed the idea of this early magma ocean in 1970, just
six months after the return of the first Apollo samples.
Several additional lines of evidence from geochemistry
and geophysics support the magma ocean model, which
is still being developed today.One piece of evidence that complicates the giant
impact model is the concentration of various iso-
topes—atoms of an element that have a different mass
from the “regular” atoms—in Apollo samples. Using a
process called laser fluorination, in 2001 and 2012
researchers found that the compositions of both oxy-
gen and titanium isotopes are almost identical
between the moon and Earth. If the moon formed
from a mixture of Theia and Earth materials, why
does it have an Earth-like isotopic composition? ThisCURATION
processors
transfer an
Apollo 15
sample out of
an airlock in its
stainless-steel
storage cabinet.