Chapter 4. The Moon

4.0 Introduction
In many ways the Moon is a geologic Rosetta stone: an airless, waterless body untouched by erosion, containing clues to events that occurred in the early years of the solar system, which have revealed some of the details regarding its origin and providing new insight about the evolution of Earth. Although they also posed new questions, the thousands of satellite photographs brought back from the Moon have permitted us to map its surface with greater accuracy than Earth could be mapped a few decades ago. We now have over 380 kg of rocks from nine places on the Moon, rocks that have been analyzed by hundreds of scientists from many different countries. Data from a variety of experiments have revealed much about the Moon's deep interior. As it turns out, the Moon is truly a whole new world, with rocks and surface features that provide a record of events that occurred during the first billion years of the solar system. This record is not preserved on Earth because all rocks formed during the first 800 million years of Earth's history were recycled back into the interior. The importance of the Moon in studying the principles of geology is that it provides an insight into the basic mechanics of planetary evolution and events that occurred early in the solar system. Much of the knowledge we have of how planets are born and of the events that transpired during the early part of their histories has been gained from studies of the Moon.
At the outset, it is important to note that we assume that the physical and chemical laws that govern nature are constant. For example, we use observations about how chemical reactions occur today, such as the combination of oxygen and hydrogen at specific temperatures and pressures to produce water, and infer that similar conditions produced the same results in the past. This is the basic assumption of all sciences. Moreover, much of what we "know" about the planets, as in all science, is a mixture of observation and theory---a mixture that is always subject to change. Scientific knowledge is pieced together slowly by observation, experiment, and inference. The account of the origin and differentiation of planets we present is such a theory or model; it explains our current understanding of facts and observations. It will certainly be revised as we continue to explore the solar system and beyond, but the basic elements of the theory are firmly established.
4.1 Major Concepts
The surface of the Moon can be divided into two major regions: (a) the relatively low, smooth, dark areas called maria (seas) and (b) the densely cratered, rugged highlands, originally called terrae (land).
Most of the craters of the Moon resulted from the impact of meteorites, a process fundamental in planetary development.
The geologic time scale for the Moon has been established using the principles of superposition and cross-cutting relations. Radiometric dating of rocks returned from the Moon has provided an absolute time scale.
The lunar maria are vast plains of basaltic lava, extruded about 4.0 to 2.5 billion years ago. Other volcanic features on the Moon include sinuous rilles and low shield volcanoes.
The major tectonic features on the Moon, mare ridges and linear rilles, are products of minor vertical movements.
The Moon is a differentiated planetary body with a crust about 70 km thick. The lithosphere is approximately 1000 km thick. The deeper interior may consist of a partially molten asthenosphere and a small metallic core.
The tectonic and thermal evolution of the Moon was very rapid and terminated more than two billion years ago. The Moon has no surface fluids, so that little surface modification has occurred since the termination of its tectonic activity.
The major events in the Moon's history were: (a) accretion of material ejected from Earth after a massive collision with a Mars-sized object, (b) and differentiation with the formation of the lunar crust by crystallization of a magma ocean, (b) intense meteoritic bombardment, (c) extrusion of the mare lavas, and (d) light bombardment.
4.2 The Moon as a Planet
In July 1969, a human stood for the first time on the surface of another planet, seeing landscape features that were truly alien and returning with a priceless burden of Moon rocks and other information obtainable in no other way. Nonetheless, many of the facts listed in Table 1 were known long before we began to explore space; they represent years of diligent study. For example, it was discovered centuries ago that the Moon revolves about Earth and not the Sun and is thus a natural satellite (the largest in the inner solar system). Long ago the distance from Earth to the Moon was measured and the diameter of the Moon determined. Early astronomers realized that the Moon's rotation period and its period of revolution are the same; thus it keeps one hemisphere facing Earth at all times. Moreover, many of the Moon's surface features have become well known, especially since the days of Galileo, the first to study the Moon through a telescope. Even the density and gravitational field of the Moon had been determined long before our generation. But not until the 1960s---and the inception of space travel with its sophisticated satellites and probes and the eventual Moon landing---did man begin to appreciate the significance of the Moon as a planet. In spite of its small size and forbidding surface, the Moon has revealed secrets that pertain to the ultimate creation of our planet, Earth, and our neighbors beyond.