The heavily cratered lunar highlands (see cover) preserve remnants of the moon's origin at 4.5–4.6 billion years ago and subsequent evolution to 3.8–3.9 billion years ago. Rocks produced during the same time appear to be totally missing from the record on Earth. Thus the lunar highlands afford us the opportunity to study processes that were operative during planetary formation and early evolution; such studies can be expected to yield new insights on the history of the earth and other terrestrial planets during the first few hundred million years of their existence.
Studies of the lunar highlands have made great strides since the collection of highland rocks by the Apollo 14,15,16, and 1 7 missions. Many types of information have been interpreted and synthesized, including petrologic and geochemical data collected on highlands samples and photographic and geochemical data, covering large portions of the moon's surface, collected by orbiting spacecraft. Research on igneous processes, such as fractional crystallization and magma mixing, has increased our understanding of lunar petrogenetic processes. Research on impact cratering processes (involving improved mathematical modeling, the study of small craters that are produced experimentally, and investigations of large terrestrial impact craters) has added greatly to our ability to interpret lunar history. In addition, the formulation of models of planetary accretion, heating, and internal mass transfer has increased our knowledge of the earliest processes of planetary formation.