Editors note: This article was written by Apollo 17 lunar module pilot and geologist Harrison Schmitt. It appears here in an abbreviated form. The entire text and additional photos can be found at www.hq.nasa.gov/office/pao/History/SP-350/ch-14-1.html Also see: www.hq.nasa.gov/office/pao/History/SP-350/toc.html for more Apollo articles.
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 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 (Figure 1 )? „ 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. How did the visits of Apollo 15, 16, and 17 to Hadley-Apennines, Descartes, and Taurus-Littrow relate to it (Figure 2 )? 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:
- The existence of a melted shell from about 4.6 to 4.4 billion years ago.
- Bombardment to form the cratered highlands from about 4.4 to 4.1 billion years ago.
- The creation of the large basins from about 4.1 to 3.9 billion years ago.
- A brief period of formation of light-colored plains about 3.9 billion years ago.
- The eruption of the basaltic maria from about 3.8 to about 3.1 billion years ago.
- The gradual transition to a quiet crust from about 3.0 billion years ago until the present.
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 erup