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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? This

CURATION
processors
transfer an
Apollo 15
sample out of
an airlock in its
stainless-steel
storage cabinet.