Answers to such questions call for the recovery and analysis of samples of lunar materials from a variety of regions on the moon. Most of the analysis will have to be done on the earth. Determining the age of a sample of lunar material or making a chemical and mineralogical analysis of it requires instruments that cannot be deployed on the lunar surface within the next few years, particularly with little or no prior knowledge of the character of the materials to be analyzed.
The study of returned lunar materials will in fact provide one of the most intriguing challenges ever faced by natural scientists. How much of the moon's history and how many of the lunar surface processes can be understood from a few isolated samples of lunar material, aided by the fairly detailed knowledge of the surface morphology obtained from photographs? The possibilities are considerable, because the lunar surface is not subject to many of the chemical processes that occur on the earth's surface, such as the changes accompanying erosion and sedimentation. Furthermore, the distribution of material over the surface of the moon by the impact of meteorites suggests that a substantial amount of material in any given place may have come from great distances without significant changes in chemical composition.
Efforts to trace the evolution of the moon, to understand its gross internal structure and to explain the characteristics of its major morphological features will require knowledge of the kind and amount of internal energy released by moonquakes, heat flow at the surface and volcanism. The occurrence of moonquakes would reveal something of the distribution of stress with depth. The seismic waves arising from moon quakes would provide a powerful tool for deducing the distribution of basic physical properties with depth. Measurement of heat flow at the surface, combined with estimates of the distribution of radioactive elements in the lunar rocks, would make possible a determination of whether or not internal energy is in fact the cause of volcanism on the moon. Data for attacking these problems will be needed from a number of widely distributed points on the moon.
The Problem of the Mascons
The space vehicles employed in the Lunar Orbiter missions not only made excellent photographs of the lunar surface but also yielded a startling discovery having to do with the gravitational field of the moon. If the moon were a symmetrical spheroid, internally as well as externally, a satellite would move around it in a well-defined elliptical orbit at a smoothly varying speed. In actuality the moon, like the earth, is not quite a symmetrical spheroid, which introduces perturbations in satellite orbits. Over and above these perturbations, however, there are others introduced by lateral variations in the moon's density. As the Lunar Orbiter vehicles were tracked in their orbits it was noted that they gained speed whenever they passed over one of the moon's ringed maria, or dark circular "seas". Analysis of these motions by Paul M. Muller and William L. Sjogren of the Jet Propulsion Laboratory led to the finding that over the major circular maria (Imbrium, Serenitatis, Crisium, Humorum and Nectaris) there is a substantial excess of gravity.
What is the cause of these gravitational variations? The large positive anomalies associated with the maria imply concentrations of mass, now abbreviated as "mascons". An example of the concentration involved is provided by the estimate that the gravitational anomaly over Mare Imbrium is equivalent to one produced by a sphere of nickel–iron 70 kilometers in diameter centered at a depth of 50 kilometers.
The discovery of lunar mascons has given rise to much speculation and debate about their origin. It has also revived interest in exploring the lunar maria, which many investigators had dismissed as unlikely to be as rewarding scientifically as other areas of the moon. Do the mascons represent remnants of giant iron asteroids that struck the moon and subsequently were buried and fragmented, or were they formed by some other mechanism?