Sky and Telescope - April 2016 - 20

Lunar Mystery
is generated partially melts the planets' interiors. The
result is the generation of magma, which can cool slowly
deep inside a planetary crust (a process called plutonism)
or be rapidly pushed out onto its surface (volcanism).
Many different radioactive elements produce heat
inside both Earth and the Moon. These elements are
too large to fit into the crystal structures of the major
rock-forming minerals. So as minerals crystallize from
the magma, such elements tend to be left behind in the
magmatic liquid that remains. On the Moon, this material has been given the name KREEP, which stands for
potassium (K), rare-earth elements (REE), and phosphorus (P). We first discovered KREEP in Apollo 12 samples
collected in Oceanus Procellarum, the largest expanse of
maria. Because KREEP includes the radioactive, heatproducing elements uranium and thorium, a map of
high radioactivity on the lunar surface is also a map of
the KREEP content of different regions.
As shown by the 1998 Lunar Prospector mission,
KREEP is not distributed evenly around the Moon.
Instead, it's concentrated largely on the western nearside, within and around Oceanus Procellarum. A second, much lower concentration is found in the southern
central farside, near the small maria on the floor of the
South Pole-Aitken Basin, the largest (2,600 km) and oldest (perhaps 4.3 billion years) basin on the Moon. Because
volcanism is driven by internal heat, scientists thought
that high KREEP levels near the largest maria might
mean that radioactivity had generated more heat in these
places, resulting in the eruption of more lava. Higher
KREEP abundances beneath the nearside than under the

farside could explain why the Moon has two faces.
The difficulties in this case lie in two areas. First, the
distribution of maria only partly correlates with high
levels of radioactive elements. Although Procellarum is
both very large and has lots of KREEP, other significant
mare deposits occur in zones strongly depleted in such
material. Such areas include both eastern nearside maria
(such as Crisium, Smythii, and Fecunditatis) and some
farside maria (Moscoviense and Orientale).
Second, the radioactive elements detected from orbit
all occur within the topmost meter of the Moon's crust.
Yet mare magmas are generated deep within the Moon,
hundreds of kilometers lower. Any relationship between
the surface and mantle compositions is likely to be both
indirect and complex.
But even if the ultimate cause of the Moon's two-faced
nature is a local enrichment of radioactive elements, this
merely begs the question: why, then, are the heat-producing elements distributed unevenly on the Moon? There
isn't a straightforward answer to this question, though
some ideas have been proposed.
One recent paper suggested that since the surface
of early Earth would have been molten after the impact
that created the Moon, the radiant heat from this glowing sphere would have kept the Moon's nearside from
cooling as quickly as the farside did. Such a temperature
gradient between near- and farsides would supposedly
lead to the creation of a chemical gradient, with a higher
concentration of refractory elements (that is, those with
high melting points, such as aluminum) on the farside.
In this model, the thicker farside crust arises because

R AR E E AR TH
E LE ME N T S
Mare
Imbrium
Oceanus
Procellarum

Mare
Serenitatis

Mare
Crisium

Mare
Smythii

Mare Moscoviense

Mare
Fecunditatis

Mare
Orientale

Mare
Humorum
South Pole-
Aiken Basin
Thorium, ppm
NASA

1

2

4 6 8 10 12

Rare earth elements are a
group of metals that includes scandium, yttrium,
and the 15 lanthanide elements (atomic numbers 57
through 71). They're used
in many modern technologies, such as cell phones
and hybrid-car batteries. In
terms of abundance they're
actually far more common
in Earth's crust than gold,
but they rarely exist in concentrations high enough to
make mining economical.

RADIOACTIVE MOON Spectra from the Lunar Prospector mission reveal levels of the radioactive element thorium on the lunar surface. Scientists
think thorium, along with uranium and KREEP (potassium, rare earth elements, and phosphorous), helped melt the lunar interior, producing the
magma that migrated to the surface to create the mare lava plains. But although the highest thorium concentrations appear in Oceanus Procellarum
(orange and green area), maria locations don't necessarily match up with high radioactive levels.

20

April 2016 sky & telescope



Sky and Telescope - April 2016

Table of Contents for the Digital Edition of Sky and Telescope - April 2016

Contents
Sky and Telescope - April 2016 - Cover1
Sky and Telescope - April 2016 - Cover2
Sky and Telescope - April 2016 - 1
Sky and Telescope - April 2016 - Contents
Sky and Telescope - April 2016 - 3
Sky and Telescope - April 2016 - A
Sky and Telescope - April 2016 - B
Sky and Telescope - April 2016 - 4
Sky and Telescope - April 2016 - 5
Sky and Telescope - April 2016 - 6
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