Apollo Expeditions to the Moon
CHAPTER 15.3
HOT DEBATES ABOUT VOLCANOES
What have been the relative roles of impact cratering and
volcanism? For the crater-ridden Moon the obvious first question
is how were the craters formed: by meteorites hitting the surface
at high velocity, or as the remains of once active volcanoes?
Debates on this subject generated about as much heat as the
volcanoes themselves. Many argued for, or gave the appearance of
arguing for, one extreme or the other. More sober debaters
recognized that both cratering and volcanism played a role and
sought to discern their relative importance. It is clear that the
Moon has changed considerably since it was first formed, but most
of that change occurred more than three billion years ago.
Apparently the first 1.5 billion years were a period of violent
evolution, involving the Mare Imbrium event and other cataclysmic
impacts, followed by the vast flooding of the mare plains with a
series of lava flows. In contrast, the last three billion years
have been relatively quiet, with occasional impacts like those
that generated the craters Copernicus and Tycho, but no great
lava events.
The implication of this for Earth is that during its first
billion years Earth also must have been subjected to severe
bombardment, generating huge craters like those still seen on the
Moon, and this may also have been true for other planets as well.
Certainly the Mariner 9 pictures of Mars and Mariner 10 pictures
of Mercury show that both planets experienced substantial
cratering, while terrestrial radars indicate the same for Venus.
The evidence mounts that violent meteorite bombardment was
widespread in the solar system in times past. On Earth, however,
erosion, crumpling of the crust, and subsequent volcanism have
erased most of the evidence of this early catastrophic period.
Is the Moon still active today? Infrequent observations of
sudden localized glows or hazes on the Moon have caused a stir
when they occurred, and gave rise to speculation as to whether
these were due to current volcanic activity, or were merely
trapped gases shaken loose by moonquakes or meteorite impacts.
There was speculation about whether our satellite was a dead
planet or still a live one. Five Apollo seismometers were set up
at different landing sites, and four of them are still working.
At times of lunar perigee, these detect moonquakes of very deep
foci centered at 500 to 620 miles below the lunar crust. The
energy released over a year by these moonquakes, however, is a
billion times less than that released by earthquakes over a
similar period. No evidence of current volcanic activity on the
Moon has been found from either the unmanned or manned space
missions In most places the soil just below the surface appears
to have been relatively undisturbed for millions of years, which
is consistent with the rarity of large-scale meteorite impacts
that Earth experiences today. At the very surface a slow erosion
takes place by micrometeorite impacts and solar-wind particles,
wearing away on the average a few molecular layers a year on
exposed surfaces. Material does move around. Some material falls
and slides down slopes, some may be moved around by electrostatic
forces, and much of what movement occurs is due to splashes from
meteorite impacts. But all in all, the Moon appears to be
extremely quiet now, in comparison with its earliest history or
with Earth today.
Is the interior of the Moon hot or cold? Discussion of this
subject once made the sparks fly. Assuming the Moon contains
radioactive elements, as does Earth, then the heat generated by
their decay should warm up the interior. But would this source
provide enough heat to melt the Moon? Whatever the cause, the
lava flows apparent on the surface of the Moon show that at least
part of the Moon's interior actually was molten at one time. But
on the other side of the picture, the distinctly out-of-round
shape implies a rigidity that a very hot and plastic Moon would
not have. The Apollo measurements have added fuel to this flaming
controversy. Anomalous concentrations of mass called mascons have
been discovered in the great circular mare basins, detected by
the way in which they distort the Moon's gravitational field. In
a hot, plastic Moon, these mascons would have sunk until the
gravity field was restored to equilibrium. The fact that they
persist today indicates a rigid, cool Moon. Yet melting and lava
flooding in the upper layers of the Moon are widespread.
Electrical conductivities inside the Moon, deduced from the
Moon's reaction to the electrical charges and magnetic fields in
the solar wind, indicate that the outer layers, down to 500 to
620 miles in depth, are now well below the melting point. Below
those levels, however, is a region where seismic shear waves are
markedly attenuated, indicating partial melting of the lunar
material there. Moreover while magnetic measurements do not
reveal any dipole magnetic field at present like that of the
Earth, remnant magnetization in rock samples, and substantial
local magnetic fields. suggest that the Moon may well have had a
dipole field in the past. Since it is believed that such a
planetary magnetic dipole field is generated by circulation in a
liquid core, this implies that at one time the core of the Moon
was molten. This would further imply that the Moon was at one
time quite hot throughout, which gets us right back to the
difficulty of explaining how the mascons and the aspherical shape
can continue to exist. There is a real puzzle here that needs
sorting out.
Does the Moon have any atmosphere at all? Before Apollo it
was already clear that the Moon could have very little
atmosphere. It was expected that heavier gases like argon and
krypton would be found clinging close to the surface, but in the
lower gravitational field lighter gases would long since have
escaped. In large measure this has been confirmed. Argon
generated by radioactivity in the lunar crust, and hydrogen,
helium, and neon from the solar wind, account for most of what
little atmosphere there is, and that is over a billion times less
than the Earth's atmosphere.
