GEOCHEMICAL AND GEOPHYSICAL FOSSILS

In tracing the evolution of the Earth, instead of using physical and chemical methods to reproduce phenomena in the laboratory, research is carried out using the “fossils” of these phenomena. We have already explained how the isotopic ratios recorded in rocks, the oceans, and the atmosphere serve effectively as the “fossils” of ancient chemical differentiation events. Meteorites are still the most important source of information on the origin of the Earth, being “fossils” in which the early state of the Solar System is frozen. We have also seen that the remanent magnetism of rocks can be used as a “fossil” of the Earth’s magnetic field in the past. Studying the evolution of the Earth, therefore, often calls for an ingenious approach, requiring more than simple applications of mathematical, physical, and chemical methods. In this chapter we will discuss a novel approach in this spirit, which may be called “lunar paleopedology” (pedology is the scientific discipline of studying soil). After the monumental achievement of the landing on the Moon in 1969 under the Apollo project, lunar soils sampled by astronauts have been offering a new type of “fossil record” not only for the Moon, but also for the Sun and Earth. We start with a short discussion of the origin of the Earth–Moon system.

ORIGIN OF THE EARTH–MOON SYSTEM

Although the origin of the Moon is still a big mystery, a currently predominant view is that the Moon formed as a result of the impact of a Mars-sized proto-planet with a proto-Earth.[1] An almost identical isotopic composition of oxygen (the most abundant element in planetary objects) between the Moon and Earth suggests that both objects had undergone elemental and isotopic homogenization, most likely through high-temperature vaporization caused by the impact. Numerical calculations of such a giant impact have commonly indicated that the temperature in the impacted material (a proto-Earth) and impactor could have reached more than ten thousand degrees centigrade, enough to vaporize all of the materials involved.[2] The formation age of the Earth–Moon system (or the timing of the giant impact) has been estimated to be about 4.5 billion years ago from an extinct short-lived radioactive isotope 182Hf, whose half-life is 9 million years (see Chapter 2).