Apollo Expeditions to the Moon|
A NEW VIEW OF THE MOONWhat 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:
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.