July 2019, ScientificAmerican.com 73have condensed to droplets of magma that rained down into its
interior. The rate of the magma rain would have been 10 times
that of the most intense rainfall ever measured on Earth. In this
scenario, the moon would begin as a small orb of molten rock
and metal—some of the material that was not vaporized in the
initial impact. Dwarfed by the synestia’s immensity, the nascent
moon would, in fact, have orbited within the synestia’s glowing,
vaporous depths, surrounded by vast quantities of high-pressure
gaseous rock, growing with each absorbed droplet of falling
magma rain. The synestia would shrink as it cooled, so that after
tens of years, it would have sufficiently contracted for its outer
edge to recede within the orbit of the moon. In that moment, the
moon would emerge, born from the dying synestia.
This story may explain why Earth and the moon are isotopic
twins because the synestia formed from the vaporized and well-
mixed material derived from the two colliding bodies. Further-
more, the synestia’s torrential magma rains and turbulent
vapors would have driven even more mixing throughout a large
fraction of the body. If the synestia was sufficiently well mixed,
the moon would have acquired the same isotopic ratios as Earth.
A synestia can also explain several other lunar mysteries that
the canonical giant impact hypothesis does not. For example,
although the moon has the same isotopic fingerprint as Earth, it
does not have exactly the same chemical composition. The moon
has lower abundances of extremely volatile elements, such as
hydrogen and nitrogen, and moderately volatile elements, such
as sodium and potassium, as compared with Earth. These pecu-
liar features are not definitively explained by the canonical
hypothesis. Yet they arise naturally from “baking” a growing
moon at a few thousand degrees in the “oven” of a synestia.
More volatile elements would have preferentially stayed in the
vapor of the synestia, so the moon would never have acquired
Earth-like abundances of these elements. The volatile elements
that stayed in the vapor would be carried inward with the shrink-
ing synestia to become part of Earth. With help from our col-
leagues Misha Petaev and Stein Jacobsen, both at Harvard Univer-
sity, we demonstrated that the pattern and abundance of moder-
ately volatile lunar elements can be explained by the moon coming
into chemical equilibrium with the vaporized elements inside the
synestia. Simply put, being born in a synestia naturally explains
why the moon has a similar composition to Earth but has a lower
abundance of volatile elements. Our simple recipe for making the
moon’s chemistry is as follows: vaporize two colliding planetary
bodies, mix well and bake at 4,000 degrees Celsius (more than
7,000 degrees F) in a convection oven for 10 to 100 years.
Finally, synestias may explain otherwise mysterious quirks in
the moon’s orbit. Strangely, the moon does not orbit Earth in the
same plane in which Earth orbits the sun, which is called the
ecliptic plane. Instead the moon’s orbit is inclined to the ecliptic
plane by about five degrees. The tilt of the orbit is why we do not
have total lunar eclipses every month but only on the rare occa-
sions when Earth, the moon and the sun align. Yet after a giant
impact, if the moon formed from a circumplanetary disk or a
synestia, the naive expectation would be that it should be orbit-
ing in the ecliptic plane. So why is the lunar orbit inclined?
A new model for how the orbit of the moon changes with
time by SETI Institute theorist Matija C ́uk and his colleagues
can explain both the inclination of the lunar orbit and the length
of Earth’s day. The giant impact may have knocked the proto-
Earth on its side and produced a synestia with its rotation axis
tilted close to the ecliptic plane. The moon would have formed
in the plane of Earth’s equator, with its orbit also tilted far from
the ecliptic. Over time, resonant interactions with the sun would
have pulled the rotation axis of Earth more upright to its pres-
ent-day 23-degree tilt. Earth’s spin would have been slowed in
the process, with our planet being pushed slightly farther away
from the sun to conserve angular momentum. As the moon dis-
sipated its orbital energy by raising tides on Earth, it would
slowly move away from the planet, decreasing the lunar inclina-
tion to the ecliptic to its present orientation. Thus, a single giant
impact that created a tilted synestia could explain many of the
key dynamical characteristics of Earth and its satellite.
In sum, the synestia’s natural elegance and explanatory pow-
er have rescued the giant impact hypothesis—and permanently
changed the playing field for studies of the origin of the moon.FULFILLING APOLLO’S LEGACY
without the dAtA from the rocks collected by the Apollo astro-
nauts, we could have been satisfied with an incomplete, or even
erroneous, idea for how the moon was created. The challenge of
explaining the data led to the discovery of synestias. Now our
new challenge is to further develop our understanding of synes-
tias and their role in planet formation. We are only at the begin-
ning of this quest.
Our model of a moon-forming synestia can be tested by
improving its chemical and isotopic predictions for lunar com-
position. We are still learning from the samples collected by the
Apollo missions—half a century of progress in instrumentation
is allowing the extraction of more accurate and detailed data.
But the Apollo samples are a limited resource with enormous
gaps in coverage and completeness. More than ever, we need
rocks from the lunar mantle to build better chemical models for
the moon’s bulk composition. Returning to the moon to obtain
samples from the mantle, parts of which should be exposed in
and around massive impact craters, will let us make fresh pre-
dictions for that vital measurement. Meanwhile rocks right
here on Earth may provide additional important clues for lunar
origins. It has recently been realized that the deepest regions of
Earth’s mantle contain traces of material that survived the
moon-forming giant impact. Whatever process formed the
moon could not have erased these chemical records. By combin-
ing data from Earth and the moon, we hope to piece together
our view of the synestia that made both bodies.
Help in understanding synestias may also come from beyond
our solar system. So far we have seen them only as mathemati-
cal objects on our computer screens, but synestias may not
remain a purely theoretical notion much longer. Many tele-
scopes, in space and on the ground, are staring at the heavens
in search of exoplanets silhouetted against the bright faces of
their stars. Because their shapes are very different from a spher-
ical planet, synestias would cast unusual shadows on our tele-
scopes. Other new and emerging facilities are snapping baby
pictures of planets around very young stars that may still be in
the giant impact stage of formation. Perhaps some of those
snapshots will reveal a puffy, glowing doughnut of rock vapor
where a planet used to be. Soon we may glimpse our first natu-
ral synestia and witness a near replay of the creative destruc-
tion that led to the formation of our own Earth and moon.