Earth’s Moon - Introduction to Planetary Geomorphology - page 59

CHAPTER 4

Earth s Moon

'

Moon, today we recognize that the history of our closest
planetary neighbor is inexorably linked to that of our
home planet, Earth.
For planetary geology, it is fortunate that the Moon was
the first extraterrestrial object to be studied. Because the
Moon lacks an atmosphere and appears never to have had
liquid water flowing on its surface, the Moon is a rela-
tively simple geologic object and serves as a training
ground for studying the formation and evolution of plan-
etary surfaces.

4.1 Introduction

Throughout the history of humankind, other than the Sun,
no other planetary object has held our attention as much as
the Moon. The Moon figures prominently in mythology
and literature, with notions of vampires and werewolves
that were driven by the phases of the Moon. The very term
“ lunatic ” derives from the idea that mentally unstable
individuals are in uenced by the Moon. Aside from
these aspects, scienti cally, the Moon holds much for
study, especially in terms of planetary geomorphology.
Even with the naked eye, we can see that its surface is
not uniform. Some areas are dark and circular (the “ eyes ”
of the Man in the Moon) and other areas are very bright.
These characteristics led to the terms maria (Latin for
seas) for the dark areas for their fanciful resemblance
to water areas and terrae (Latin for land), or highlands,
for the notion that there were continents surrounding
the seas.
At 3,476 km, the diameter of the Moon is nearly
the width of the United States; its surface area of
3.79 × 10 7 km 2 is about the same as the land area of
Africa and Australia combined. In many ways, the Earth -
Moon system is unique in the Solar System, and, because
the Moon is comparatively so large, some planetary scien-
tists view Earth - Moon as a “ binary ” planet. As is true with
many natural satellites, our Moon is locked in synchronous
rotation in its orbit around Earth, meaning that it always
shows the same “ face, ” termed the near side,towardEarth
and hides the far side from direct viewing ( Fig. 4.1 ).
Librations, or “ wobbles, ” in the Moon ' s movement enable
slightly more than a hemisphere to be seen in both polar
areas and on the eastern and western sides, or limbs,of
the Moon.
After more than four decades of analyses of lunar data
returned from dozens of successful spacecraft ( Table 4.1 )
and the return of nearly a half ton of samples from the

4.2 Lunar exploration

4.2.1 Pre-Apollo studies

In the early 1600s, Galileo Galilei turned the newly
invented telescope toward the heavens. Although his tele-
scope was a simple tube with a couple of lenses, as we
shall see later, he made remarkable discoveries that revo-
lutionized ideas regarding the Solar System. It is, how-
ever, surprising that his sketches of the Moon are rather
crude ( Fig. 4.2 ). Although Galileo showed the distinctive
dark and light terrains as well as craters, his drawings do
not seem to match well to speci c surface features.
Within a few decades of Galileo ' s observations,
improvements in telescopes resulted in maps of the
Moon that are rather accurate ( Fig. 4.3 ), portraying ter-
rains and individual features in correct positions. Users
of telescopic photographs of the Moon should be aware
that the images are often inverted in relation to views
with the naked eye and in reference to the geographic
convention used today, in which north is in an “ up ”

position.
The craters observed on the Moon through telescopes
stimulated a great deal of interest and today many of the
ideas of lunar crater formation seem rather bizarre. For
example, the famous English scientist Sir Robert Hooke

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