Sky and Telescope - April 2016 - 19

Earth

The Crust of the Moon
The lunar crust consists of aluminum- and calcium-rich
rocks, similar to the terrestrial rock anorthosite. Anorthosite is made up almost entirely of one mineral: plagioclase, which has a relatively low density. Small rocks rich
in plagioclase were ﬁrst found in samples returned by
Apollo 11. From this evidence and from looking at the
highlands' topography and density, scientists concluded
that the early Moon must have been nearly totally molten, covered by an ocean of magma in which low-density
minerals ﬂoated to the top (forming an anorthositic
crust) while denser, iron-rich minerals such as olivine
sank to the bottom, ultimately becoming the mantle. It
was this mantle that later partly remelted, through the
slow release of heat by radioactive elements, to create the
magmas that erupted as mare basalts.
The astronauts deployed long-lived instruments on
the lunar surface, including seismometers that measured moonquakes. Study of these quakes showed
that the Moon has a crust, a mantle, and possibly even
a small metallic core. The lunar crust at the Apollo
landing sites is between 35 and 40 km thick, similar to
parts of Earth's continental crust. Interestingly, gravity
data from orbiting spacecraft show that the crust on the
farside is thicker than on the nearside, for reasons that
remain unclear. In addition, the Moon's center of mass
is oﬀset from its geometric center by a couple of kilometers in the direction of Earth. This oﬀset is probably
what keeps the nearside visible and the farside facing
away, because it would have forced the Moon's rotation
and revolution periods to synchronize.
Armed with these ﬁndings, lunar scientists sought
to explain the two faces' geologic diﬀerences. They
ﬁrst postulated that the diﬀerence in crustal thickness
between the two hemispheres might explain why there
are far more maria on the nearside. How would such a
scenario work?
As mentioned, basalts are produced from the partial
melting of the deep lunar mantle, forming bodies of
liquid rock that are less dense than their surroundings
and, therefore, buoyant. These liquids migrate upward
along grain boundaries and cracks until they reach a
point where they either escape to the surface and erupt
or stop moving because the pressure from the overlying
rock is no longer high enough to make them buoyant.
Assuming all mare basalts came from the same "zone"
of melting, scientists suggested that, because the crust
is thinner on the nearside, the magmas could reach the
surface there and erupt, but rising the same distance on
the farside would still leave them below ground level.

Partially
molten
zone
Core

Mantle

CM CF

Equipotential
surface
Flooded
basin

S&T: GREGG DINDERMAN

Crust
* Not to scale

S&T: GREGG DINDERMAN, SOURCE: SUSAN PULLAN / GEOLOGICAL SURVEY CANADA

farside. In fact, basins are (more or less) equally distributed over both near- and farsides. Some other factor
must have caused the volcanic ﬂooding of almost all the
nearside basins and only a very few of the farside ones.

LUNAR INTERIOR The Moon's center of mass (CM) is oﬀset
from its geometric center (called the center of ﬁgure, CF) by
about 2 km toward Earth. This oﬀset led to the gravitational
lockup that keeps the lunar nearside facing our planet.

This explanation was attractive for a lot of reasons,
especially as it uniﬁed several disparate observations
into a generalized model that nicely explained a lunar
mystery. But experience in science shows us that grand
unifying theories are usually wrong - or, at best,
incomplete. In this case, continued studies of the lunar
samples returned by the Apollo missions demolished
this density equilibrium idea. The composition of the liquid rock that ﬁ lled maria changes from region to region,
which means that the magmas' densities were diﬀerent.
That implies that, even if material all came from the
same depth (unlikely), it wouldn't necessarily have risen
the same distance. Thus, the contrast in the number
of near- and farside maria can't merely be the result of
magmas of similar densities rising to similar levels.

Lunar Heat
All the rocky planets contain radioactive elements that
spontaneously decay into other elements, releasing
radiation and generating heat. A classic example is the
element uranium, half of which decays into lead over
4.5 billion years. Radioactive decay has been occurring
inside the planets since they formed, and the heat that

A THIN VE N E E R
Although they can span hundreds of kilometers, maria are
typically only a few hundred meters thick or less. They're
usually thickest near basins' centers - sometimes reaching 2 to 4 km deep - and thinnest near the edges.

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Sky and Telescope - April 2016

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