Sky and Telescope - August 2018 - 26

PLANET PUMMELING

by Javier Barbuzano

Moon
Mess
The

A planet-scale smash
created Earth's satellite.
But how did it manage to
make a moon so like Earth?

here is orange soil! It's all over! Orange!" Harrison
Schmitt's excitement crackled over Apollo 17's radio.
His companion, surrounded by the gray lunar
landscape, wondered if Schmitt had been on the Moon too
long. He came to take a look. "Hey, he's not going out of
his wits," Eugene Cernan chimed in, seeing the material
Schmitt's footsteps had uncovered and catching his excitement. "It really is."
Schmitt, a geologist and the ﬁrst scientist-astronaut
to set foot on our planet's natural satellite, knew he was
seeing something important. The two astronauts hurried
to dig a trench, snap images, and collect samples. Further
analysis on Earth later revealed that the orange soil was 3.7
billion years old and had formed during a volcanic eruption.
The orange soil wasn't the only thing worth bringing
back home. When the Apollo 17 command module ﬁnally
splashed down in the Paciﬁc Ocean on December 19, 1972,
it carried some 110 kilograms (243 pounds) of lunar rocks
and dust in its guts. In total, the Apollo teams brought back
to Earth 382 kg of samples. Among them were green, yellow, and orange volcanic beads; black basalts from the dark
maria regions; twisted breccias cemented together by meteorite impacts; and a light-toned rock called anorthosite.
Although furthering lunar science was just one of the
many objectives of the Apollo program, conceived during
the Cold War for political and technological reasons, it
produced an unprecedented amount of knowledge about
the Moon. Virtually everything we know about our natural
satellite was discovered or conﬁrmed thanks to Apollo's
scientiﬁc windfall.
But one of the most basic questions remains open: How
did the Moon form? For a long time scientists thought
they had found the answer in the Apollo lunar samples,
but more recent analyses of the very same rocks, combined
AUGUST 2 018 * SK Y & TELESCOPE

with powerful computer models, have shown there is still
much we don't know about the birth of Earth's companion.

Probable but Unpredictable
Before Apollo, scientists had imagined three possible
scenarios for the formation of the Moon: co-accretion, ﬁssion, and capture. Co-accretion means that Earth and the
Moon coalesced together as a pair from a primeval cloud
of gas and dust. In the capture scenario, the Moon formed
elsewhere and was yanked into an Earth-circling orbit when
the two bodies came too close. In the ﬁssion model, the
still-molten Earth started rotating faster and faster until it
split itself in two.
However, once scientists could study actual lunar
samples, those three ideas became obsolete. Chemically,
the Moon looks like what you'd expect after vaporizing
Earth material and letting it condense in the vacuum of
space. It lacks most of the easily vaporized elements regularly found in meteorites and terrestrial rocks, including
water and hydrogen, and has little iron. The oldest rocks
on the Moon, the low-density anorthosites, are almost
completely devoid of heavy metals and must have formed
after a global event melted the entire
Moon, allowing these buoyant rocks to
ﬂoat to the top of a magma ocean and
solidify on its surface as the Moon's
original, frothy crust. None of the
three scenarios would produce these
characteristics.
Instead, by 1974, two pairs of scientists working independently (William
Hartmann and Donald Davis, Alastair
Cameron and William Ward) realized
that the Moon could have been created

Sky and Telescope - August 2018

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