Apollo Expeditions to the Moon
CHAPTER 15.5
THE NAGGING QUESTION OF ORIGIN
Where did the Moon come from? This is the big puzzler of
them all. There were three major theories. The Moon came from
Earth, possibly wrenched from what is now the Pacific Ocean. Or
the Moon formed in the vicinity of Earth at the same time that
Earth was forming. Or the Moon formed somewhere else in the solar
system, and was later captured by Earth. Most students of the
subject reject the first possibility, because it proves very
difficult to explain all the steps that must have occurred to
bring this about. That leaves various versions of the last two as
principal contenders. Apollo has done little to favor one over
the other. Indeed, some feel that we will never be able to say
for sure. But the question is a nagging one, and scientists will
continue to argue over it.
The deep significance of the Apollo investigations lies in
the fact that these measurements and observations give us a
detailed insight into a planetary body other than Earth, thereby
helping us to understand better our own planet. Before space
probes, lunar and planetary science had for a long time been
inactive, due in part to lack of new data to spark serious
thought. With the vast quantities of lunar data returned by
Apollo and other lunar missions, together with rich new space-
probe data from Mars, Venus, Mercury. and Jupiter, and new
discoveries on Earth such as the slow spreading of ocean floors
and the drifting of continents, a new field of comparative study
of planets has virtually exploded into world science. In this
comparative investigation of the planets, the Moon is an
important link.
Already it is clear that bodies of planetary size will
undergo considerable evolution after their formation. Most of
this evolution takes place early, and it is probably less in the
case of a Moon-sized planet, leaving the planet relatively quiet
for most of its history. A planet the size of Mars, though
substantially smaller than Earth, remains active for much longer
than a Moon-sized body, as the Mariner 9 pictures clearly show.
With its huge volcanoes, its giant canyon several times deeper
than the Grand Canyon and long enough to span the United States,
its variable polar caps, its suggestion of colliding crustal
plates, Mars is clearly still an active planet. Venus, the size
of the Earth, with its very hot surface and extremely dense and
dynamic atmosphere, may well prove to be more active volcanically
and tectonically than Earth. Mercury, intermediate between Mars
and the Moon in size, is heavily cratered and seems to be much
like the lunar highlands.
It will be fascinating when we can complete this perspective
by studying the giant outer planets on the one hand, and the very
small bodies like the comets and asteroids on the other. As
Pioneer 10 and 11 have shown, Jupiter is extremely dynamic. We
may expect the same to prove true of the other giant planets when
we get a chance to see them close at hand. Moreover, the large
planets, consisting, as they do, mainly of the lighter elements
like hydrogen and helium, should provide a revealing insight into
conditions in the solar nebula at the time the planets of the
solar system were condensing out of the nebula. The same should
be true of the comets and asteroids. The asteroids in particular
are probably too small to have undergone any gravitational or
radioactive melting, and therefore should in their interiors be
as they were at the time of their formation, unless they, too,
turn out to be fragments of what were once larger bodies.
