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CHAPTER 12

The Moon

Stuart Ross Taylor

Australian National University

Canberra, Australia

Lunar and Planetary Institute

Houston, Texas

1. Introduction 5. Lunar Structure 9. Lunar Composition


2. Physical Properties 6. Impact Processes 10. The Origin of the Moon


3. Geophysics 7. The Maria Bibliography


4. Lunar Surface 8. Lunar Highland Crust

T

he Moon is a unique satellite in the solar system, the
largest relative to its planet. It has a radius of 1738 km,
a density of 3.344 g/cm^3 (Earth density=5.52 g/cm^3 ), and
a mass that is 1/81 that of the Earth. Its orbit is inclined at
5.09oto the plane of the ecliptic. It rotates on its axis once
every 27 days. Themoment of inertiavalue is 0.3931, con-
sistent with a small increase of density toward the center.
The current consensus is that the Moon formed as a conse-
quence of the collision with the Earth of a Mars-sized body
about 4.5 billion years ago. The rocky mantle of the impactor
spun out to form the Moon, while the core of the impactor
fell into the growing Earth. This model explains the high
spin of the Earth–Moon system, the strange lunar orbit,
the low density of the Moon relative to the Earth, and the
bone-dry and refractory composition of our satellite. The
model also provides a source of energy to melt the early
Moon. The geochemical and petrological evidence is clear
that the Moon was molten and floated a crust of feldspar
about 4.45 billion years ago. This forms the present white
highland crust. The interior crystallized into a sequence
of mineral zones by about 4.4 billion years ago. Possibly
a small metallic core formed, although the composition of
the core is still under debate. Major impacts produced many
craters and multiring basins, probably during a spike or “cat-
aclysm” around 3.9–4.0 billion years ago. The oldest basin
observed is the South Pole–Aitken Basin and the youngest
is the Orientale Basin, which formed 3.85 billion years ago.

Beginning about 4.3 billion years ago, and peaking between

3.8 and 3.2 billion years ago, partial melting occurred in the

lunar interior, and basaltic lavas flooded the low-lying basins

on the surface. This occurred mostly on the nearside, where

the crust is thinner. Major activity ceased around 3.0 billion

years ago, although minor activity may have continued until

1.0–1.3 billion years ago and the Moon has suffered only a

few major impacts (forming, for example, the young rayed

craters such as Copernicus and Tycho) since that time.

1. Introduction

The Earth’s Moon (Fig. 1) is a unique satellite in the so-

lar system. None of the other terrestrial planets possesses

comparable moons: Phobos and Deimos, the tiny satellites

of Mars, are probably captured asteroids. Most of the 130

or so satellites of the outer planets are composed of low-

density rock-ice mixtures and either formed in accretion

disks around their parent planets or were captured. None of

them resembles the Moon so that the origin of our unique

satellite has been an outstanding problem. It is in plain

sight, accessible even to naked-eye observation, yet it has

remained until recently one of the most enigmatic objects

in the solar system.

The Moon has played a pivotal role in human develop-

ment. Both the axial tilt and the 24-hour rotation period

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