Sky and Telescope - August 2018 - 32

Theia

Proto-Earth

Vaporization

Surface cools
by radiating,
causing
condensation.

Rotating synestia

Condensed rock droplets
and/or particles create
a torrential rain
inside synestia.

Condensates
rapidly cool
and accrete
into lunar seeds.

Rotation axis

The synestia cools
and contracts.

The Moon grows by
accreting condensates.

The synestia continues to contract
and eventually forms Earth.
The Moon
separates
from the
synestia.

PLANETARY DANISH Recent simulations by Simon Lock and Sarah
Stewart suggest that a high-energy collision that vaporized both the
impactor and proto-Earth could explain why the Moon and Earth
ended up so chemically similar. After the massive collision (1), the debris would have quickly formed a vast connected structure, or synestia,
inside which turbulent convection would have mixed material into a
fairly uniform soup. As the synestia's surface cooled, rock vapor there
would condense into droplets and/or dust, falling as a torrential rain
into the interior and building up into lunar seeds (2). Over the course of
a year or so, the Moon grows from these moonlets and condensates
(3). After several to tens of years, the synestia has cooled and shrunk
enough that the Moon separates from it (4). The contracting synestia
will eventually create Earth. The gray region is the iron core. Depending
on the type of impact, most of the impactor's core merges with that of
the proto-Earth, with the rest either ejected from the system, dissolved
in the synestia's hot outer regions, or put into orbit as larger chunks.

AUGUST 2 018 * SK Y & TELESCOPE

anymore, many other volatiles like carbon have been found
in lunar rocks, and the list of elements with matching isotopic compositions keeps growing.
Rather than killing Theia, most researchers are trying to ﬁ x
the giant-impact scenario. The idea seems too good to discard.
To reconcile Theia with the new observations, scientists
are trying one of two approaches. The ﬁrst is to come up with
a mechanism that could have equilibrated the isotopic compositions of Earth and Theia after the impact.
In 2007, Kaveh Pahlevan (now at Arizona State University)
and David Stevenson (Caltech) developed a collision model
that could have rendered an Earth-like Moon. They proposed
that, right after the giant impact, the protolunar disk was so
hot that it allowed the transfer of material from Earth into
orbit through convection and turbulent mixing. However,
the mixing must have been very efﬁcient to completely
mask Theia's isotopic contribution before the Moon
formed and the disk disappeared. Also, extremely rapid
mixing would require lots of convection, which would
also cause rapid cooling and thus leave less time to mix
the material well.
In 2012, Sarah Stewart (now at University of California,
´uk (now at SETI Institute) tried to ﬁnd
Davis) and Matija C
a mechanism that would decrease Theia's contribution to the
Moon. They showed that a faster, nearly head-on collision
could launch more of our planet's mantle into orbit - if Earth
were also spinning very fast (with a period of about 2½ hours)
at the time of the impact. Although the extra energy produces
a Moon-Earth system with much higher angular momentum
´uk calculated that
than what we see today, Stewart and C
certain orbital resonances could have transferred that excess
angular momentum to Earth's orbital velocity, slightly enlarging its path around the Sun. This effect, combined with the
bodies' tidal pulls on each other, ended up de-spinning Earth
to its current 24-hour day.
This realization opened the door to more energetic impact
scenarios and other exotic ideas. Among those to cross
through that door was Simon Lock (Harvard University),
one of Stewart's graduate students. While running computer
simulations, Lock and Stewart realized that sometimes giant
impacts vaporize so much material that the planet ends up
looking like something completely different: a molten, bagelshaped structure with a metal-rich bulge where it should have
the hole. The bulge is the core of the planet, connected to
an outer torus of silicate vapor and molten silicate droplets.
Since there isn't a gap between the core and the torus orbiting around it, Lock and Stewart coined the name synestia, or
"connected structure."
In this model, the Moon forms within the torus of the
synestia. As the rock vapor cools down, it begins to condense
into droplets of liquid. As these grow, they might form several
moonlets that eventually merge together.
Lock's synestia could be the perfect superheated mixer in
which the isotopic ratios of the impactor and the proto-Earth
could equilibrate. There would be enough energy to mobilize

IMPACT CA RTOON: G REGG DINDER M A N / S&T,
SY NESTIA CU TAWAYS: SIM ON LOCK

Planet Pummeling

Sky and Telescope - August 2018

Table of Contents for the Digital Edition of Sky and Telescope - August 2018