Sky and Telescope - August 2018 - 29

ISOTOPES: G REGG DINDER M A N / S&T, SOURCE K E VIN RIG H TER (N ASA JSC) VIA
ERIK ASPH AUG / ANNUAL R E VIE W OF E ARTH AND PL ANE TARY SCIENCES 2014

Changing Tides
At roughly the same time that the giant-impact hypothesis
appeared, astronomers realized they could use the varying
ratios of certain isotopes - atoms of the same element with
a varying number of neutrons - to probe the origins of solar
system bodies. These variations reﬂect the composition of the
unique regions of the solar nebula each object sampled during its formation. This composition affects the proportions
of heavy to light isotopes in an object as a whole, acting as a
ﬁngerprint for the object's formation history.
Scientists ﬁrst used the varying ratios of oxygen's three
isotopes. Initial analysis of the Apollo samples showed that,
by this measure, the Moon and Earth are virtually identical,
whereas other bodies in the solar system have distinctly different oxygen-isotopic ratios. The differences between Earth

Change in oxygen-17 fraction from Earth's
(parts per thousand)

after a Mars-size body crashed into the nearly fully-formed
Earth. In the scenario that has grown out of this initial work,
the impactor delivered a glancing blow, powerful enough to
launch huge quantities of silicate rock from both bodies into
orbit. By that time, most of the iron and other heavy elements
had already settled into the cores of each protoplanet, which
merged inside Earth after the impact. Meanwhile, the ejected
rocks formed a disk around Earth and quickly coalesced to
form the Moon. Due to the high temperatures created during
the collision, volatile elements, including water, would have
escaped from the disk, leaving behind a dry, gas-free, and
metal-poor Moon.
This giant-impact hypothesis successfully explains most
of the peculiarities of the Moon: its geochemistry, lack of a
metallic core, late formation relative to other objects in the
solar system, and large mass ratio compared to its host planet
- more than 50 times those of the giant planets. The scenario also accounts for the high angular momentum of the
Earth-Moon system as a whole. If all the angular momentum
could somehow be transferred to the Earth alone, our planet
would spin around in only 5 hours.
Despite its virtues, the giant-impact hypothesis didn't gain
much traction among planetary scientists until the mid1980s, when modern planet-formation models showed that
titanic collisions might have been fairly common during the
birth of terrestrial planets. According to these models, giant
impacts mark the beginning of the end of a long process in
which bits of rocky material gradually clump together, growing in size to form boulders, then planetesimals, and then
planets. Such accretion is competitive, with the larger bodies
absorbing the smaller ones in their feeding zones like big ﬁsh
devouring all the small fry in a pond. In the ﬁnal stages, the
larger bodies can collide as well, crashing into each other in a
series of calamitous encounters.
As this new paradigm became dominant, so did the giantimpact hypothesis for the origin of the Moon. By the 1990s,
most scientists considered the question largely settled. Eventually the hypothetical impactor earned the name Theia, after
the mother of the Moon in Greek mythology.

0.4
0.3

Mars

0.2
0.1
0
-0.1
-0.2
-0.3

Moon
Earth
Angrites

Eucrites (Vesta)
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Ratio of oxygen-18 to oxygen-16 (parts per thousand)

p OXYGEN ISOTOPES Mars, the asteroid Vesta, and angrite meteorites
all have notably different ratios of oxygen's three isotopes than terrestrial
samples do. Samples of the Moon, however, plot right on top of Earth's
values. The x-axis spread in each population is due to chemical processing within each body.

and Mars, for instance, are 100 times greater than those
between Earth and its Moon. This similarity was initially
taken as a point in favor for the giant-impact hypothesis: If
a body as large as Theia crossed paths with Earth, scientists
argued, then it must have formed in the same region of the
solar system and had a similar isotopic composition.
This evidence would soon become problematic, though.
Computer simulations developed in 2001 by Robin Canup
(Southwest Research Institute) and Erik Asphaug (now at
University of Arizona) indicated that in the "standard" giantimpact hypothesis, which involved a glancing blow, Theia's
mantle contributes most of the material that ends up forming
the Moon - somewhere between 70% and 90%. Concurrently, as laboratory techniques became more sophisticated,
reﬁned measurements showed with more precision that
terrestrial and lunar isotopic ratios are virtually identical for
most elements, including some that in principle should be
different, like tungsten.
Unlike oxygen, tungsten's isotopes do not merely reﬂect a
particular mix of the materials that formed a planet. Instead,
tungsten-182 is generated when hafnium-182, a radioactive
element with a relatively short half-life of 9 million years,
eventually decays into it. While tungsten tends to bond with
iron, hafnium has an afﬁnity for silicates. As a result, hafnium and tungsten end up in different parts of a young planet
as its interior separates into layers: Iron drags the tungsten
with it as it sinks into the growing core, while hafnium
remains in the mantle. If the core forms faster than all the
hafnium-182 can decay, whatever hafnium-182 is left ultimately seeds the mantle with tungsten-182.
Since each planet's core likely assembles with its own pace
and process, it's very unlikely that Theia and the proto-Earth
could have had matching ratios of tungsten-182 to the element's other stable isotopes.
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Sky and Telescope - August 2018

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