A Brief History of Astronomy
Earth and its Moon
The composition of the atmosphere of Earth is 21 percent molecular oxygen, 78 percent molecular nitrogen, and 1 percent argon. Trace amounts of carbon dioxide, water vapor, and other gases are also present.
Direct chemical analysis of surface rocks shows their composition to be primarily oxygen, silicon, aluminum, and iron, in that order of abundance. Such rocks have an average density of about 2.7 g/cm3, whereas the overall mean density of Earth is 5.5 g/cm3, an observation that has two important consequences. First, the interior of the planet must have much denser materials than at the surface. The overlying weight of rock will compress interior rocks to some extent, but the necessary density requires the presence of intrinsically dense elements that must also be those that are cosmologically relatively abundant; that is, iron and nickel. Second, the differentiation of Earth's chemistry into lighter outer materials and heavier inner materials suggests that early in its history, the planet must have been reasonably molten in order to allow the heavier materials to sink into the interior.
