THE SKY'S NO LONGER THE LIMIT This lunar picture was taken from the Apollo 11 spacecraft July 20, 1969, during the lunar landing mission.
© NASA

Editor's note: This article originally appeared in the October 1969 issue of Scientific American magazine. We are posting it to commemorate the 40th anniversary of Apollo 11's moon landing.

The success of the Apollo 11 mission in putting men on the moon and bringing them back safely with samples of lunar material marks the beginning of what promises to be a period of fruitful exploration of the moon by men and machines. The objective will be to answer a large number of questions about the origin and evolution of the moon, its geology, its chemical and physical structure and what light its history can shed on the history of other bodies in the solar system. Our purpose in this article is to discuss the major questions the coming manned expeditions to the moon will be taking up and to describe the techniques likely to be employed on such missions.

In an astronomical sense the moon is usually considered to be a satellite of the earth. From the viewpoint of planetary processes, however, the moon can be regarded as the smallest of the "terrestrial" planets (the others being the earth, Mars, Venus and Mercury). Because its distance from the sun is about equal to that of the earth, the moon is subject to external influences similar to those affecting the earth. The moon's smaller size, however, implies a history quite different from the earth's.

Most planet-wide processes result from internal sources of energy and the means of its dissipation. The amount of internal energy and the means of dissipation are dependent on the size of the planet. On the earth the dissipation of energy has been accompanied by the transport of volcanic fluids from the interior of the earth to the surface, by the long-term development of a light crust and a dense core and by large-scale movements of the earth's crust, mantle and core. These processes, together with erosion and the chemical interaction of materials on the surface with the atmosphere and the hydrosphere, continually destroy the earth's surface features. For example, even the largest volcanoes are leveled by erosion within a few million years after volcanic activity ceases. It is extremely unlikely that any of the earth's original surface features still exist unchanged. No examples are known, and almost certainly none will be found.

It was once thought that many of the surface features of the moon date back to the moon's formation. The detailed views of the lunar surface provided by the photographs from the Lunar Orbiter space vehicles have somewhat diminished this possibility, because they indicate that erosion and other processes of change do take place on the moon. Preliminary analysis of the samples returned by Apollo 11 nonetheless indicates that the material is very old, perhaps three billion years old. The highlands may be even older. The possibility that some of the material lying on the lunar surface is chemically unchanged since the formation of the planet remains high.

The Major Questions
Fundamental scientific questions about the moon are often stated in terms of terrestrial characteristics, which are of course more familiar. In inquiring about the gross chemical and physical structure of the moon, for instance, one wonders if the moon is chemically and mineralogically differentiated as the earth is. The processes such as volcanism have occurred on the moon, what has been their history over long periods of time? Did the moon ever have an atmosphere? Have protobiological materials ever existed or evolved on the moon?