Most students of the moon favor the latter possibility. The debate centers on what the mechanism might have been. Several mechanisms have been proposed: the filling of a low-density, fragmented lunar crust with lava; a flow of lava into an impact crater; the upwelling of denser material from the lunar depths into giant impact basins; even the deposition of sediment in the maria by flowing water that later dried up. The last hypothesis carries the intriguing implication that water not only existed on the moon at one time but also played an important role in lunar history. In any case, the analysis of samples from the moon takes on added significance as a result of the mascon phenomenon. An exciting result from the preliminary measurements of Apollo 11 samples is that their density of 3.2 to 3.4 grams per cubic centimeter, which would be high for terrestrial rocks, may be related to the existence of the mascons.
Sites for Exploration
The present plan of the National Aeronautics and Space Administration is to make nine more manned explorations of the moon over the next three or four years. The sites for the first few will probably be determined on the basis of constraints similar to those that were in effect for the Apollo 11 mission, namely that a landing place must be on the side of the moon facing the earth, so that constant radio communication can be maintained between the earth and the landing party; that the site be in a region free of obstacles, and that it be accessible from a free-return orbit, meaning an orbit that will enable the astronauts to return to the earth with a minimum of power if the main engine in the command module should fail. These constraints restrict the next few landings to mare sites near the lunar equator.
Later it should be possible to venture farther afield and to land at or near other sites of particular scientific interest. A number of places are under discussion as possibilities for these landings, Instead of describing them all, we shall focus on four candidate sites and a long-traverse area that we believe offer significant clues for deciphering lunar history. No particular significance should be attached to the order in which we discuss the sites.
The first site is the small, extremely fresh crater Censorinus, A landing here could be expected to achieve three objectives: to establish the age of what is clearly a very young feature on the lunar surface, to investigate and characterize an unquestioned impact feature and to obtain samples of material from a region in the highlands, An alternative site, which would offer similar possibilities, is the crater Mosting C.
The second site represents the much more ambitious goal of exploring one of the major craters. Such a crater is Copernicus, which is about 70 kilometers in diameter and has prominent central peaks within it. The ejecta from this relatively young crater cover more than a tenth of the front face of the moon. The relief within the crater is more than 15,000 feet [4,572 meters], making it comparable to the most mountainous areas on the earth. An alternative site, with quite similar characteristics, is the crater Tycho. These large craters are of interest not only because they represent major events in the history of the moon but also because, by analogy with much smaller terrestrial craters, they should expose material from a range of depths up to 10 kilometers, and perhaps even more. It has been suggested that the central peaks in these craters may consist of material now at the surface that has come from depths of 10 to 15 kilometers or more. Thus, even though the material in a crater may be jumbled, broken and deformed by shock processes, it should provide a diverse sample of the outer few kilometers of the moon and a basis for interpreting its history.
Third, we point to the extremely interesting Marius Hills region. It is one of several areas where constructional features such as domes and built-up cones are more numerous than craters of a comparable size. The region is also associated with one of the longest lunar ridge systems, which crosses a large expanse of Oceanus Procellarum on the western half of the moon. The tectonic setting of the region is similar to that of terrestrial volcanic fields such as Iceland and the Azores. The setting and structure of the Marius Hills region suggest that it is an area of volcanic activity where igneous material has been added to the surface through vents.