of water and other volatile elements and compounds, and the chemical complementarity of the dark lunar basaltic lowlands and the bright highland rocks—led to enormous advances in theories of planet formation. Moon rocks provide one of the most persuasive pieces of evidence that Earth and the Moon have a common origin. The isotopic composition of oxygen varies dramatically within the Solar System (Figure 1.6) but is identical in Earth and the Moon. An important difference is the size of their metallic cores—one-third of the mass of Earth but only about 2 percent of the mass of the Moon. Another difference is that Earth has water, as well as other volatile species and oxidized (ferric) iron; the Moon has virtually no water and all of its iron is in the reduced (ferrous) state.
Studies of lunar rocks have helped persuade many geologists that the Moon was formed when a Mars-sized object collided with the still-forming Earth about 40 million years after the formation of the Solar System. This “giant-impact” hypothesis would explain the relatively large mass of the Moon relative to Earth, the large amount of angular momentum in the Earth-