Multimedia Mania/abacus-multimedia-mania.iso/dp/0097/00978.txt

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$Unique_ID{bob00978}
$Pretitle{}
$Title{Apollo Expeditions To The Moon
Chapter 14: The Great Voyages Of Exploration}
$Subtitle{}
$Author{Schmitt, Harrison H.}
$Affiliation{NASA}
$Subject{moon
apollo
lunar
earth
years
crust
billion
miles
now
ago
see
pictures
see
figures
}
$Date{1975}
$Log{See Lunar Field Camps*0097801.scf
See The Rover Folded*0097802.scf
See Scoop Sampling*0097803.scf
See Point of View*0097804.scf
}
Title:       Apollo Expeditions To The Moon
Author:      Schmitt, Harrison H.
Affiliation: NASA
Date:        1975

Chapter 14: The Great Voyages Of Exploration

     First I want to share a new view of Earth, using the corrected vision of
space.  Like our childhood home, we really see the Earth only as we prepare to
leave it.  There are the basically familar views from the now well-traveled
orbits: banded sunrises and sunsets changing in seconds from black to purple
to red to yellow to searing daylight and then back; tinted oceans and
continents with structural patterns wrought by aging during four and a half
billion years; shadowed clouds and snows ever-varying in their mysteries and
beauty; and the warm fields of lights and homes, now seen without the
boundaries in our minds.

     Again like the childhood home that we now only visit - hanging in time
but unchanged in the mind - we see the full Earth revolve beneath us.  All the
tracks of man's earlier greatness and folly are displayed in the window: the
Roman world, the explorers' paths around the continents, the trails across
older frontiers, the great migrations of peoples.  The strange perspective is
that of the entire Earth filling only one window, and gradually not even doing
that.  No longer is it the Earth of our past, but only a delicate blue globe
in space.  With something of the sadness felt as loved ones age, we see the
full Earth change to half and then to a crescent and then to a faint moonlit
hole in space.  The line of night crosses water, land, and cloud, sending its
armies of shadows ahead.  We see that night, like time itself, masks but does
not destroy beauty.

     In sunlight, the sparkling sea shows its ever-changing character in the
Sun's reflection, in varying hues of blue and green around the turquoise
island beads, and in its icy competition with polar lands.  The arcing,
changing sails of clouds, following whirling, streaking pathways of wind, mark
the passage of the airy lifeblood of the planet.

     The revolving equatorial view concentrates our attention.  There is the
vast unbroken expanse of the Indian Ocean, south of the even more vast green
and tan continent of Asia.  In another complete view there are all of the
blending masses of greens, reds, and yellows of Africa from the Mediterranean
to the Cape of Good Hope, from Cap Vert to the Red Sea.  Then we see across
the great Atlantic from matching coast to matching coast.  Scanning all of
South America with one glance, we seemingly cease to move as the planet turns
beneath us.  And then there is the South Pacific.  At one point only the
brilliant ranges and plains of Antarctica remind a viewer that land still
exists.  The red continent of Australia finally conquers the illusion that the
Earth is ocean alone, becoming the Earth's natural desert beacon.

     When at last we are held to our own cyclic wandering about the Moon, we
see Earthrise, that first and lasting symbol of a generation's spirit,
imagination, and daring.  That lonesome, marbled bit of blue with ancient seas
and continental rafts is our planet, our home as men travel the solar system.
The challenge for all of us is to guard and protect that home, together, as
people of Earth.

A New View of the Moon

     What will historians write many years from now about the Apollo
expeditions to the Moon?  Perhaps they will note that it was a technological
leap not undertaken under the threat of war; competition, yes, but not war.
Surely they will say that Apollo marked man's evolution into the solar system,
an evolution no longer marked by the slow rates of biological change but from
then paced only by his intellect and collective will.  Finally, I believe that
they will record that it was then that men first acquired an understanding of
a second planet.

     What then is the nature of this understanding, and how did the visits of
Apollo 15, 16, and 17 to Hadley-Apennines, Descartes, and Taurus-Littrow
relate to it?

     The origins of the Moon and the Earth remain obscure, although the
boundaries of possibility are now much more limited.  The details of the
silicate chemistry of the rocks of the Moon and Earth now make us reasonably
confident that these familiar bodies were formed about 4.6 billion years ago
in about the same part of the youthful solar system.  However, the two bodies
evolved separately.

     As many scientists now view the results of our Apollo studies, the Moon,
once formed, evolved through six major phases.  Of great future importance is
the strong possibility that the first five of these phases also occurred on
Earth, although other processes have obscured their effects.  Thus, the Moon
appears to be an ever more open window into our past.

     The known phases of lunar evolution are as follows:

     1.  The existence of a melted shell from about 4.6 to 4.4 billion years
ago.

     2.  Bombardment to form the cratered highlands from about 4.4 to 4.1
billion years ago.

     3.  The creation of the large basins from about 4.1 to 3.9 billion years
ago.

     4.  A brief period of formation of light-colored plains about 3.9 billion
years ago.

     5.  The eruption of the basaltic maria from about 3.8 to about 3.1
billion years ago.

     6.  The gradual transition to a quiet crust from about 3.0 billion years
ago until the present.

     The detail by which we understand these six phases of lunar evolution is
quite great.  It derives from analysis of returned samples and observations of
their geologic setting on the Moon, from the interpretation of geophysical and
geochemical data from stations that still operate on the Moon or that
previously operated in lunar orbit, and from our experience on Earth.

     During the melted shell phase from about 4.6 to 4.4 billion years ago, at
least the outer 200 miles of the Moon was molten or partially molten.  As this
shell cooled, the formation and settling of crystals of differing composition
resulted in the creation of major chemical differences between various layers
tens to hundreds of miles thick.  A crust, mantle, and core apparently were
formed at this time.  The crust consisted of light-colored minerals rich in
calcium and aluminum (largely the mineral plagioclase); the mantle contained
dark minerals rich in magnesium and iron (largely the minerals pyroxene and
olivine); and the core probably was composed of dense, molten material rich in
iron and sulfur.

Inconceivable Violence

     The cratered highland phase that followed was extremely, almost
inconceivably violent.  The debris left over from the creation of the planets
bombarded the light-colored crust.  These highland surfaces have survived as
the bright portions of the full Moon we see today.  They were pulverized,
remelted, reaggregated, and, finally, saturated with craters at least 30 to 60
miles in diameter.  The sheer violence of those times is difficult to
comprehend.

     The large basin phase was the time when very large basins were formed.
This appears to have been the result of a distinctly more massive scale of
bombardment than that which preceded their formation.  These large basins
dominate the surface character of the front side of the Moon and are
responsible for the major chemical differences we have measured between
various large surface regions.

     The light-colored plains phase that followed was a brief, still
controversial period in which most old basins appear to have been partially
filled with debris largely derived from the surrounding light-colored crust.
The events that created these plains are poorly understood partly because
several different processes related to both meteor impact and internal
vulcanism may have produced similar plains.

     The basaltic maria phase was the main period during which the
accumulation of heat from radioactive elements within the Moon produced
melting and volcanic eruptions.  Those eruptions filled all of the large
basins with thick masses of dark-colored basalt called the maria. (These sea-
like regions are the dark portions of the full Moon.) The lunar basalts are
very different from basalts on Earth; they contain much less sodium, carbon,
and water and commonly have much more titanium, iron, and heavy elements.  At
least the upper parts of the maria are ancient lava flows up to 300 feet
thick.  Many flows differ significantly from each other in chemical and
mineral characteristics, differences that vary with both the age and the
region.

     The quiet crust phase from about 3.0 billion years ago to the present was
largely just that - quiet.  Compared to the past, very little happened except
for the formation of scattered, very bright craters like Tycho and Copernicus,
the creation of regional fault systems like the Hyginus Rille, and the
appearance of mysterious light-colored swirls like Reiner Gamma. Eruptions of
basaltic maria also seem to have continued along a ridge and volcanic system
that stretches for 1200 miles along the north-south axis of Mare Procellarum.
Some of the events may be indications of continuing internal activity and
stress beneath a now strong crust, such as the slow, solid convection of the
lunar mantle.

     For the most part, the surface of the Moon appears to have completed
recording its history about three billion years ago.  It has been largely
unchanged except for the continued eroding rain of small meteors and now by
the first primitive probings of men.

[See Lunar Field Camps: Field camps on the Moon were provisioned with oxygen,
water, food and power for about 70 hours plus some reserves.]

     The Moon is as chemically and structurally differentiated as the Earth,
lacking only the continued refinements of internal melting, solid convection,
surfacial weathering, and recycling of the crust.  It moves through space as
an ancient text, related to the history of the Earth only through the
interpretations of our minds.  It also exists as an archive of our Sun,
possibly preserving in its soils much information of importance to man's
future.

     If we are to continue to read the text, we must continue to go there and
beyond.

The Missions of Understanding

     The last three Apollo journeys were great missions of understanding
during which our interpretation of the evolution of the Moon evolved.  In July
1971 the first of these missions, Apollo 15, visited Hadley Rille at the foot
of the Apennine Mountains.  Apollo 15 gave lunar exploration a new scale in
duration and complexity.  Col. David R. Scott, Col. James B. Irwin, and Lt.
Col. Alfred M. Worden looked at the whole planet for 13 days through the eyes
of precision cameras and electronics as well as the eyes of men.  Scott and
Irwin spent nearly 67 hours on the Moon's surface, and were the first to use a
wheeled surface vehicle, the Rover, to inspect a wide variety of geological
features.  Finally, before returning to Earth, they placed a small satellite
in lunar orbit that greatly expanded our knowledge of the distribution and
geological correlation of gravitational and magnetic variations within the
Moon's crust.

[See The Rover Folded: Folded up to fit within its storage bay in the LM
descent stage, the little car was designed so that is almost assembled itself.]

     The varied samples and observations from the vicinity of Hadley Rille and
the mountain ring of Imbrium called the Apennines pushed knowledge of lunar
processes back past the four-billion-year barrier we had seemed to see on
previous missions.  We also discovered that lunar history behind this barrier
was partially masked by multiple cycles of impact melting and fragmentation.
Nevertheless, the rock fragments we sampled gave vague glimpses into the first
half-billion years of lunar evolution and into some details of the nature of
the melted shell.  Part of this view into the past was provided by the
well-known "Genesis Rock" of anorthosite (a plagioclase-rich rock).  In
addition, we expanded our understanding of the complex volcanic processes that
created the present surfaces of the maria.  These processes were now seen to
have included not only the internal separation of minerals within lava flows
but possible processes of volcanic erosion and fracturing that could have
created the rilles.

     The Apollo 15 astronauts placed instruments on the Moon which, in
conjunction with earlier missions, finally established a geophysical net of
stations.  Of particular importance was a net of seismometers by which we
began to decipher the inner structure of the Moon.  Correlations of
information from these stations with other facts enabled us to interpret
several major portions of the interior.  The Moon's crustal rocks, rich in the
calcium and aluminum silicate plagioclase, are broken extensively near the
surface but more coherent at depths from 15 to 40 miles.  The crust rests on
an upper mantle 125 to 200 miles thick that contains the magnesium and iron
silicates, pyroxene and olivine.  From about 200 or 250 to about 400 miles
deep, the lower mantle is possibly similar to some types of stony meteorites
called chondrites.  From about 400 miles to about 700 miles deep, the
chondrite material appears to be locally melted and seismically active. There
are also many reasons now to believe that the Moon has an iron-rich core from
about 700 miles deep to its center at 1080 miles that produced a global
magnetic field until only recent times.

     The geophysical station at Hadley-Apennines also told us that the flow of
heat from the Moon was possibly two times that expected for a body having
approximately the same radioisotopic composition as the Earth's mantle.  If
true, this tended to confirm earlier suggestions that much of the
radioisotopic material in the Moon was concentrated in its crust.  Otherwise,
the interior of the Moon would be more fluid and show greater activity than we
sense with the seismometers.

     We began with Apollo 15 to be able to correlate our landing areas around
the whole Moon by virtue of very-high-quality photographs and geochemical x-
ray and gamma-ray mapping from orbit.  The x-ray remote sensing investigations
disclosed the provincial nature of lunar chemistry, particularly by
highlighting differences in aluminum-to-silicon and magnesium-to-silicon
ratios within the maria and the highlands.  By outlining variations in the
distribution of uranium, thorium, and potassium, the gamma-ray information
suggested that large basin-forming events were capable of creating geochemical
provinces by the ejection of material from depths of six or more miles.

[See Scoop Sampling: Sampling by scoop was the main way we obtained the large
numbers of small samples that provide good statistical information about the
composition of the surface.]

     Possibly of equal importance with all these findings by Apollo 15 was the
discovery - shared through television by millions of people - that there
existed beauty and majesty in views of nature that had previously been outside
human experience.

     The mission of Apollo 16 to Descartes in April 1972 revealed that we were
not yet ready to understand the earliest chapters of lunar history exposed in
the southern highlands.  In the samples that Capt.  John W. Young, Comdr.
Thomas K. Mattingly, and Col. Charles M. Duke, Jr., obtained in the Descartes
area, the major central events of that history seemed to be compressed in time
far more than we had guessed.  There are indications that the formation of the
youngest major lunar basins, the eruption of light-colored plains materials,
and the earliest extrusions of mare basalts required only about 100 million
years of time around 3.9 billion years ago.

     The extreme complexity of the problem of interpreting the lunar highland
rocks and processes became evident even as the Apollo 16 mission progressed.
Rather than discovering materials of clearly volcanic origin as many expected,
the men found samples that suggested an interlocking sequence of igneous and
impact processes.  A new chemical rock group known as "very high aluminum
basalts" could be defined, although its ancestry relative to other lunar
materials was obscured by later events that gave the cratered highlands their
present form.  The results of Apollo 16 have within them an integrated look at
almost all previously and subsequently identified highland rock types.  With
this complexity comes a unique, as yet unexploited, opportunity to understand
the formation and modification of the Moon's early crust and potentially that
of the Earth.

     The materials found in the Descartes region were similar to those sampled
slightly earlier by Luna 20 in the Apollonius region.  But there were
significant differences in the aluminum content of debris representative of
the two regions.  Also there were differences in the abundance of fragments of
distinctive crystalline rocks known as the anorthosite-norite-troctolite
suite.  After Apollo 15, this suite of rocks had been recognized as possibly
being a much reworked leftover of at least portions of the ancient lunar
crust.  Luna 20 and Apollo 16 confirmed its great importance to the
understanding of the ancient melted shell.

A Major Thermal Event?

     The last crystallization age of some of the Apollo 16 rocks appeared to
be about 3.9 billion years, and continued to indicate that this age is a major
turning point in lunar history.  This general age for the cooling of
highland-like materials also was found to hold for the ejecta blanket of the
Imbrium Basin at Fra Mauro, for the rocks of the Apennines, and later for some
of the highland rocks at Taurus-Littrow.  This limit suggested (1) a major
thermal event associated with the formation of several large basins over a
relatively short time, or (2) a major thermal event associated with the
formation of the light-colored plains, or (3) the rapid cessation of the
period of major cratering that continually reworked the highlands until most
vestiges of original ages had disappeared and only the last local impact event
was recorded.  As we attempt to explain the absence of very old rocks on
Earth, we should not forget these possibilities for resetting our own geologic
clocks.

     Apollo 16 continued the broad-scale geological, geochemical, and
geophysical mapping of the Moon's crust from orbit begun by Apollo 15.  This
mapping greatly expanded our knowledge of geochemical provinces and
geophysical variations, and has helped to lead to many of the generalizations
it is now possible to make about the evolution of the lunar crust.

     Apollo 17 carried Capt. Eugene A. Cernan, Capt. Ronald Evans, and me in
December 1972 to the valley of Taurus-Littrow near the coast of the great
frozen basaltic "sea" of Serenitatis.  The unique visual character and beauty
of this valley was, I hope, seen by most people on television as we saw it in
person.  The unique scientific character of this valley has helped to lessen
our sadness that Apollo explorations ended with our visit.  It would have been
hard to find a better locality in which to synthesize and expand our ideas
about the evolution of the Moon.

[See Point of View: Our conceptions are altered when the point of view is
shifted. The Apollo 15 astronauts took this picture from lunar orbit. At the
same time, we on Earth were seeing a nearly full moon.]