70 Scientific American, July 2019
O
n August 1 , 1971 , while exploring the eAstern edge of the lAvA plAin
known as Mare Imbrium on the silent, serene lunar surface, Apollo 15
astronauts David Scott and James Irwin found something remarkable:
a profoundly old piece of lunar crust, a relic more than four billion
years old that carried clues to the moon’s formation. Seeing the glint
of ancient crystals embedded in what would later be called the Genesis
rock, Scott immediately knew its potential importance for solving the
mystery of how the moon was made. “I think we found what we came for,” he radioed to mis-
sion control as he and Irwin retrieved the rock and placed it in a bag. It would become a key
part of what is the Apollo program’s greatest scientific legacy.
Studies of the Genesis Rock and the nearly 400 kilograms of
other samples hauled back to Earth by the Apollo astronauts
overturned our understanding of lunar history. In what amount-
ed to a scientific reboot, these precious samples nullified the
then prevailing theories—that the moon had been gravitational-
ly captured by Earth or had formed alongside it—while reveal-
ing important new details, such as the fact that the newborn sat-
ellite had been covered by a magma ocean.
The immense energy required to form the moon’s magma
ocean pointed to a radical new idea for lunar origin: the notion
that Earth’s closest companion had formed from a giant impact,
a collision between the proto-Earth and another planetary body.
The concept built on calculations showing that growing planets
would collide with one another, as well as the curious fact that
the moon’s composition is uncannily similar to that of Earth’s
rocky mantle. Some researchers even proposed that such an
impact had set the young Earth’s spin, establishing what would
become our planet’s 24-hour cycle of day and night. The canoni-
cal giant impact hypothesis that emerged from these early stud-
ies proposes that a glancing collision with a Mars-size body cre-
ated a hot disk of rocky debris around Earth. The moon then
coalesced from the disk—a scenario that can explain the moon’s
large mass and dearth of water and other volatiles.
Yet the giant impact hypothesis is not without flaws. Chief
among them is the astounding chemical relationship between
Earth and the moon. These two bodies are made from the same
source material, as if they are planetary twins, whereas the canon-
ical hypothesis predicts the moon should mostly be made of its
Mars-size progenitor. That progenitor should differ in composi-
tion from the proto-Earth because planets growing from the disk
of gas and dust around the young sun would each incorporate dis-
tinctive mixes of building blocks based on their orbital location.
Scientists can discern these differences by making very precise
measurements of the relative abundances of isotopes in rocks,
yielding unique “isotopic fingerprints” for every planetary body in
the solar system—except for Earth and the moon, which, bizarre-
ly, appear to be almost the same.
This isotopic crisis has haunted the giant impact hypothesis
for decades, but no better explanation has emerged for the lunar
origin. Now, however, in another scientific reboot we have discov-
ered that most giant impacts do not make a planet surrounded by
a debris disk. In fact, most giant impacts do not make a planet at
all. Instead they make an entirely new class of astronomical object,
a transient hybrid between planet and disk called a synestia that
could explain many of the moon’s most mysterious features.
HIDING IN PLAIN SIGHT
the discovery of synestiAs traces back to a few years ago, when
we (Lock and Stewart) were puzzling over whether or not a giant
moon-forming impact had set the length of Earth’s day. That
IN BRIEF
Earth’s moon formed nearly 4.5 billion years ago, in
the aftermath of a cataclysmic collision between the
proto-Earth and another protoplanet.
The giant impact hypothesis has dominated scien-
tific discussions of lunar origins for decades, in part
because it neatly explains the moon’s large size and
lack of water. But the current theory cannot easily
account for other lunar properties, such as its uncan-
ny resemblance to Earth in terms of composition.
A synestia —an impact-generated hybrid between a
planet and a disk—is an entirely new class of astro-
nomical object proposed to explain the moon’s birth
and curious compositional similarity to Earth. Synes-
tias may be regular outcomes of the planet-formation
process throughout the cosmos.
Simon J. Lock is a planetary scientist and postdoctoral
researcher at the California Institute of Technology.
Sarah T. Stewart is a professor of planetary science and
geophysics at the University of California, Davis. In 2018
the MacArthur Foundation awarded her a “genius” grant
for her work on synestias.
