Lunar Rocks

The Moon, like the Earth, is made of rocks. To learn about the Moon and its history, one has to learn something about lunar rocks—what they are made of, how they are studied and classified, and how our experiences in interpreting rocks on the Earth have been applied to rocks on the Moon. The lunar samples have provided our only direct information about the Moon’s minerals, chemical composition, and age. Lunar rocks have also provided information about the Moon’s origin, the evolution of its crust, and the timing of critical events such as volcanism and meteorite bombardment. The Moon is not a uniform, homogeneous planet. It consists of different rocks, formed in different ways at different times. Some of the Moon’s rocks are familiar. The basalt lava flows that cover the dark maria formed in the same way as many of the lava flows on Earth—by melting deep in the planet, followed by ascent of molten rock to the surface and eruption through fractures and vents. Other lunar rocks are less familiar; the numerous lunar breccias are made up of fragments of older rocks that have been shattered, crushed, and melted by meteoroid impacts, either during an ancient episode of intense meteoroid bombardment that ended about 3.8 b.y. ago or by the rarer impacts of more recent times. Within any group of rocks (e.g., basaltic lavas), the rocks are not identical. There are differences in mineral composition, in mineral shapes and sizes, and in each rock’s chemical composition. These differences have been studied in detail because they are the clues to a rock’s origin, history, and relations to other rocks. The immense database for lunar rocks, collected from the Apollo and Luna missions and from lunar meteorites, has made it possible to raise our understanding of some aspects of the origin, evolution, and history of the Moon to a level unmatched for any other planet—maybe even the Earth. The information obtained from lunar samples can also be applied to larger areas of the Moon, including regions never visited by astronauts or robot sample return vehicles. Natural X-ray and γ-ray signals mapped from orbit by the Apollo command modules provide chemical data from areas on the Moon that were never sampled (Chapter 10). In addition, Earth-based telescopes have been used—and are still being used—to measure the spectral characteristics of the light reflected from the Moon’s surface. By making similar spectral measurements in the laboratory on well-studied lunar samples, it is possible to better interpret the spectral data obtained for other lunar regions. The lunar samples provide the “ground truth” for extending our remote mapping to little-known areas of the Moon. Unfortunately, the many differences among lunar rocks also promote complex classifications, each with many subdivisions that have been developed (sometimes too enthusiastically) by lunar scientists. In some cases, terrestrial rock names have been successfully applied to their lunar equivalents. In others, new terms have had to be coined. These subdivisions, and their technical definitions, are 6