The Moon is not like Earth. It does not have oceans, lakes, rivers, or streams.
It does not have wind-blown ice fields at its poles. Roses and morning glories
do not sprout from its charcoal gray, dusty surface. Redwoods do not tower
above its cratered ground. Dinosaur foot prints cannot be found. Paramecium
never conjugated, amoebae never split, and dogs never barked. The wind never
blew. People never lived there but they have wondered about it for centuries,
and a few lucky ones have even visited it.
THE LUNAR LANDSCAPE
Highlands and lowlands
The major features of the Moon's surface can be seen by just looking up
at it. It has lighter and darker areas. These distinctive terrains are the
bright lunar highlands (also known as the lunar terrae, which is Latin for
land) and the darker plains called the lunar maria, Latin for seas,
which they resembled to Thomas Hariot and Galileo Galilei, the first scientists
to examine the Moon with telescopes. The names terrae and maria were given
to lunar terrains by Hariot and Galileo's contemporary, Johannes Kepler.
In fact, the idea that the highlands and maria correspond to lands and seas
appears to have been popular among ancient Greeks long before telescopes
were invented. Although we now know they are not seas, we still use the
term maria, and its singular form, mare.
The highlands and craters
Closer inspection shows that the highlands comprise countless overlapping
craters, ranging in size from the smallest visible in photographs (1 meter
on the best Apollo photographs) to more than 1000 km. Essentially all of
these craters formed when meteorites crashed into the Moon. Before either
robotic or piloted spacecraft went to the Moon, many scientists thought
that most lunar craters were volcanic in origin. But as we found out more
about the nature of lunar craters and studied impact craters on Earth, it
became clear that the Moon has been bombarded by cosmic projectiles. The
samples returned by the Apollo missions confirmed the pervasive role impact
processes play in shaping the lunar landscape.
The term 'meteorite impact' is used to describe the process of surface
bombardment by cosmic object. The objects themselves are variously referred
to as impactors or 'projectiles'.
The impact process is explosive. A large impactor does not simply bore
its way into a planet's surface. When it hits, it is moving extremely
fast, more than 20 km/sec (70,000 km/hour). This meeting is not tender.
High-pressure waves are sent back into the impactor and into the target
planet. The impactor is so overwhelmed by the passage of the shock wave
that almost all of it vaporizes, never to be seen again. The target material
is compressed strongly, then decompressed. A little is vaporized, some
melted, but most (a mass of about 10,000 times the mass of the impactor)
is tossed out of the target area, piling up around the hole so produced.
The bottom of the crater is lower than the original ground surface, the
piled up material on the rim is higher. This is the characteristic shape
of an impact crater and is different from volcanic calderas (no piled
up materials) or cinder cones (the central pit is above the original ground
surface). A small amount of the target is also tossed great distances
along arcuate paths called rays.
Real impacts cannot be readily simulated in a classroom. In fact, there
are very few facilities where we can simulate high-velocity impacts. Nevertheless,
classroom experiments using marbles, ball bearings, or other objects can
still illustrate many important points about the impact process. For example,
objects impacting at a variety of velocities (hence kinetic energies)
produce craters with a variety of sizes; the more energy, the larger the
crater. [See the 'Impact Craters' activity on Pages 61, 70.]
The maria cover 16% of the lunar surface and are composed of lava
flows that filled relatively low places, mostly inside immense impact
basins. So, although the Moon does not have many volcanic craters,
it did experience volcanic activity. Close examination of the relationships
between the highlands and the maria shows that this activity took
place after the highlands formed and after most of the cratering
took place. Thus, the maria are younger than the highlands. [See
the 'Clay Lava Flows' activity on Pages 71-76 and the Lava Layering
activity on Pages 77-82]
How do we know that the dark plains are covered with lava flows?
Why not some other kind of rock? Even before the Apollo missions
brought back samples from the maria, there were strong suspicions
that the plains were volcanic. They contain some features that look
very much like lava flows. Other features resemble lava channels,
which form in some types of lava flows on Earth. Still other features
resemble collapses along underground volcanic features called lava
tubes. These and other features convinced most lunar scientists
before the Apollo missions that the maria were lava plains. This
insight was confirmed by samples collected from the maria: they
are a type of volcanic rock called basalt.
The maria fill many of the gigantic impact basins that decorate
the Moon's nearside. (The Moon keeps the same hemisphere towards
Earth because Earth's gravity has locked in the Moon's rotation.)
Some scientists contended during the 1960s that this demonstrated
a cause and effect: impact caused not only the formation of a large
crater but led to melting of the lunar interior as well. Thus, it
was argued, the impacts triggered the volcanism. However, careful
geologic mapping using high-quality telescopic images, showed that
the mare must be considerably younger than the basins in which they
reside. For example, the impact that formed the large Imbrue basin
(the Man-in-the-Moon's right eye) hurled material outwards and sculpted
the mountains surrounding the Serenitatis basin (the left eye);
thus, Serenitatis must be older. The Serenitatis basin is also home
to Mare Serenitatis. If the lava in Mare Serentatis formed when
the basin did, they ought to show the effects of the giant impact
that formed Imbrium. They show no signs of it. Furthermore, the
maria contain far fewer craters than do basin deposits, hence have
been around a shorter time (the older the surface, the greater the
number of craters). The Apollo samples, of course, confirmed these
astute geological observations and showed that the maria filling
some basins formed a billion years after the basin formed.
One other type of deposit associated with the maria, though it
blankets highlands areas as well, is known as dark mantle deposits.
They cannot be seen except with telescopes or from spacecraft near
the Moon, but are important nonetheless. Before Apollo, most scientists
believed that the dark mantle deposits were formed by explosive
volcanic eruptions known as pyroclastic eruptions (literally, 'pieces
of fire'). Some deposits seemed to be associated with low, broad,
dark cinder cones, consistent with the idea that they were formed
by pyroclastic eruptions—this is how cinder cones form on Earth.
This bit of geologic deduction was proven by the Apollo 17 mission
and its sampling of the 'orange soil', a collection of tiny glass
droplets like those found in terrestrial pyroclastic eruptions.
mysteries persist about the maria. For one, why are volcanoes missing
except for the cinder cones associated with dark mantle deposits?
Second, if no obvious volcanoes exist, where did the lavas erupt
from? In some cases, we can see that lava emerged from the margins
of enormous impact basins, perhaps along cracks concentric to the
basin. But in most cases, we cannot see the places where the lava
erupted. Another curious feature is that almost all the maria occur
on the Earth-facing side of the Moon. Most scientists guess that
this asymmetry is caused by the highlands crust being thicker on
the lunar farside, making it diffcult for basalts to make it all
the way through to the surface. [See the 'Moon Anomalies' activity
on Pages 91-98.]