The Great Voyages Of Exploration

By HARRISON H. SCHMITT

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 familiar 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- changing 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 sec 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 volcanism 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.

The Moon is as chemically and structurally differentiated as the Earth, lacking only the continued refinements of internal melting, solid convection, surficial 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.

The varied samples and observations from the vicinity of Hadley Rille and the mountain ring of Imbrium called the Apennine’s 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.

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.

At Taurus-Littrow we looked at and sampled the ancient lunar record ranging back from the extrusion of the oldest known mare basalts, through the formation of the fragmental rocks of the Serenitatis mountain ring, and thence back into fragments in these rocks that may reflect the very origins of the lunar crust. We also found and are now studying volcanic materials and debris-forming processes that range forward from the formation of the earliest mare basalt surface through 3.8 billion years of modification of that surface.

The pre-mare events in the Taurus-Littrow region that culminated in the formation of the Serenitatis Basin produced at least three major and distinctive units of complex fragmental rocks. The oldest of these rock units contains distinctive fragments of crystalline magnesium and iron-rich rocks that appear to be the remains of the crystallization of the melted shell. This conclusion is supported by a crushed rock of magnesium olivine with an apparent crystallization age of 4.6 billion years. The old fragmental rock unit containing these ancient fragments was intruded and locally altered by another unit which was partially molten at the time of intrusion about 3.9 billion years ago. Such intrusive fragmental rocks are probably the direct result of the massive impact event that formed the nearby Serenitatis Basin; however, an internal volcanic origin cannot yet be ruled out. The third fragmental rock unit seems to cap the tops of the mountains and it may be the ejecta from one of the several large old basins within range of the valley. This unit contains a wide variety of fragments of the lunar crust, including barium-rich granitic rock.

The valley of Taurus-Littrow and other low areas nearby appear to be a fortuitous window that exposes some of the oldest, if not the oldest, mare basalt extrusives on the Moon. They are about 3.8 billion years old, and are 50 to 100 million years older than the basalts at Tranquility Base. They also contain titanium oxide in amounts up to 13 percent by weight.

BEADS OF ORANGE GLASS

The modification of the surface of the valley basalt included the addition of mantles of beads of chemically distinctive orange glass (the “orange soil") and black devitrified glass. The beads appear to have been formed by volcanic processes having their origins in the deep interior of the Moon. The titanium-, magnesium-, and iron-rich nature of these silicate glasses surprised us in many ways. Their approximately 10- to 30-million-year age of exposure to the Sun is young and was expected for the dark mantling deposits seen in photographs; but their 3.5- to 3.7-billion-year age of cooling from a liquid was not expected. The explanation for this difference is not yet obvious.

All the Apollo missions left the Moon before the lunar sunrise had progressed into the vast regions of the lunar west: Mare Procellarum, where the mysterious features of that region’s central ridge system still await the crew of a mission diverted after Apollo 13, and Mare Orientale, whose stark alpine rings have been viewed only in the subdued blue light of Earth. The promise of these regions and of the far side of the Moon has not diminished. They seemingly watch the progression of sunrises, awaiting the landing craft of another generation of explorers.

A NEW WAY OF EXPLORATION

The great explorations of history were unique, but in some ways all voyages of exploration are alike. There is a means of transportation; food, tools, and a way of living; methods of investigation, mapping, and sampling; and the collaboration of fellow explorers. What distinguishes one voyage from another is the place gone to and the discoveries made there. In our time, the places were on the Moon, and the discoveries have revealed another planet.

In Apollo, our ships were Saturns and CSMs, the latter more than just transport and resupply ships, for they were also orbiting laboratories, bearing spectrometers, lasers, and precision cameras. Our sketchbooks and notebooks were cameras and voice recordings. The alert experts of Mission Control were our guardian angels, in addition to those issued by Providence. The landed lunar module was our base camp, and the lunar rover was our steed. On the Apollo 17 mission, the LM Challenger could support us in its base-camp role for 75 hours, with a contingency margin of 12 hours more. In one-sixth gravity, its hammocks provided comfortable sleeping positions, with none of the lumps of camp cots or pine boughs. The total absence of black flies or mosquitoes was wonderful; and the rehydrated food was better than the grub dished up by some camp cooks.

Out on the surface, the backpack could support us for seven to eight hours, depending on physical exertion. We carried enough extra oxygen, water, and batteries in Challenger to recharge the backpack twice, for three major excursions outside. We also had an emergency supply of oxygen in the backpack that could provide at least 30 minutes of suit pressure and oxygen at large rates of flow, in case the suit was holed. Only one aspect of work in the pressure suit was very difficult and that was the effort of gripping tools against pressure within the glove. Like squeezing a tennis ball repetitively for nine hours, this was very fatiguing to forearm muscles; we also separated our fingernails from the quick as the nails scraped against the glove fingers. The forearm fatigue disappeared after each night’s sleep; sore fingernails did not disappear until days after we left the Moon.

TOURING THE MOON

Encumbered by a spacesuit, an astronaut on foot could not venture very far from the LM; carrying tools and samples made his forays more difficult. On the last three lunar missions a lightweight electric car greatly increased the productivity of the scientific traverses. Mission rules restricted us from going more than 6 miles from the LM - the distance we could walk back in a pinch - but even so the area that could be investigated was ten times greater than before. The Rover’s mobility was quite high; it could climb and descend slopes above 25°. Crossing a steep slope was uneasy for the man on the downhill side, but there were no rollovers. On the level we averaged close to our top speed of 7 mph. Once, going down the Lee-Lincoln scarp, we set an informal lunar speed record for four-wheeled vehicles of 11 mph.

Carried to the Moon in a nose-down, floorpan-out position, the Rover could be deployed by an Astronaut paying out two nylon tapes. In the first stage the car swings out from its storage bay. Then the rear part of the chassis unfolds and locks, and the rear wheels unfold. In the third stage the front chassis and wheels snap out. Finally, the astronaut lowers it to the surface, and unfolds the seats and footrests. Torsion-bar springs and latches mode assembly semiautomatic. Power for the Rover came from two 36-volt silver-zinc batteries driving an independent 1/4-hp motor in each wheel. A navigation system kept track of the bearing and range to the LM.