July 2019, ScientificAmerican.com 71Proto-Earth Proto-EarthProto-Moon Proto-MoonProto-Earth coreIllustration by Jen Christiansen
Making the Moon
Evidence strongly suggests the moon formed some 4.5 billion years ago from a collision between the proto-Earth and another
protoplanet. But certain aspects—such as the moon’s relative depletion in volatile elements compared with Earth—could be better
explained by lunar formation from a synestia, a transient object produced in the aftermath of a giant impact.A GRAZING COLLISION
The canonical giant impact model accounts for many features of
the moon, such as its ancient magma ocean and its small iron core.13A SYNESTIA-BAKED MOON
A modification of the giant impact model in which the moon forms and
“bakes” in a high-temperature, high-pressure environment can
better explain other details, such as the moon’s nearly identical
isotopic composition to Earth and relative dearth of volatile elements.
In the canonical
giant impact
model, a Mars-
size protoplanet
(orange) strikes
the proto-Earth
(blue) a glanc -
ing blow.This impact
would have
entirely disrupted
the Mars-size
protoplanet as
well as the
proto-Ear th’s
crust and much
of its mantle.The impactor’s iron core would be
incorporated into Earth, leaving behind
a moon-forming debris disk mostly
made from the impactor’s mantle.Rapidly coalescing from this disk in perhaps a century,
the newborn moon would have a small iron core and
a magma ocean.21342More violent
collisions (such
as the direct hit
of a larger impac -
tor on a rapidly
rotating, slightly
smaller proto-
Earth) result in ...A more extensive
va porization
and blending of
Earth’s crust
and mantle with
the disrupted
impactor to
create ...A doughnut-shaped mass of rock vapor—a synestia —
in which the moon forms in perhaps a few decades.
Most volatile elements remain in the vapor, falling back
to Earth as the synestia cools.Thorough mixing of material from both progenitor
bodies may account for the twinlike compositions
of Earth and the moon.4