Moon system, and the chemical similarities and differences between Earth and the Moon. None of the other terrestrial planets have a moon, except for the tiny moons of Mars, which are captured asteroids.

The general features of the giant-impact hypothesis were proposed in the 1980s, but new computer models have provided a clearer picture of the requirements and results (Canup, 2004a). A “Mars-sized” object has a mass about one-tenth of Earth’s, whereas the Moon has a mass about one-sixtieth of Earth’s. For the hypothesis to work, the impactor must hit Earth at a low angle and at a relatively low velocity (about 10 km/s). Models indicate that most of the impactor would become mixed with and incorporated into Earth during the collision (Figure 1.8) and the cores of the two planets would coalesce at the center of gravity of the combined system. The collision would eject a disk of molten rock and vapor into orbit around the newly enlarged Earth, and a portion of that disk would coalesce into the Moon. The energy of the impact would have melted virtually the entire Earth and may have resulted in the loss of most of Earth’s volatile elements (Question 2).

The impactor event, coming late in Earth’s formation, would have had an enormous effect. Many of Earth’s features may have been determined by the catastrophic collision, which marked the conclusion of the main phase of Earth’s formation. Any internal structure that formed within Earth’s mantle up to that time would probably have been destroyed, and the intense heating could have homogenized large parts of the interior. If the impact hypothesis is indeed correct, it dispels any doubt that the earliest Earth was extremely hot. The next section resumes the story of the early Earth with the aftermath of the Moon’s formation.


Many lines of recent evidence have provided critical information about how and when the Solar System began and how the planets formed. Astronomical observations from increasingly powerful telescopes have added a new dimension to models of star and planet formation, as have studies of asteroids, comets, and other planets via spacecraft. There is increasing crossover between geochemical studies and astronomical observations. With improved mass spectrometric methods, new details of meteorite isotopic compositions are forcing reevaluations of the standard models for the composition of Earth and meteorites, and studies of presolar grains are sharpening our understanding of stellar evolution and nucleosynthesis. Advanced computing capabilities are enabling more realistic simulations of nebular disk evolution, the consequences of collisions between planetesimals and planetary embryos, and the internal processes of proto-planetary bodies.

But we still do not understand the composition of Earth in enough detail to make sense of its subsequent evolution. Among the most important remaining questions are when and how Earth received its volatile components, how much of these components it still contains, whether Earth is exactly the same as chondritic meteorites with respect to refractory elements, and what the absolute concentrations of heat-producing elements are inside Earth. In a broader sense we need a better idea of the processes that formed planets during the first few million years of the Solar System, how much the planets were influenced by late events (tens to hundreds of millions of years after the beginning), how the chemical composition and size of planets were determined by early Solar System processes, and the origins of the various forms of isotopic heterogeneity.

Although theory and computation are essential tools, the starting point for posing and solving outstanding questions of Solar System evolution and planet formation remains observations and measurements of planets and other extraterrestrial objects. The materials and processes of planet formation are so varied and complex, and the scales so immense, that new breakthroughs in understanding will likely continue to follow real observations made by telescopes, spacecraft, and sensitive Earth-bound analytical equipment.


Assuming that the Moon formed as the result of a giant impact, the impact would have erased the existing rock record, adding enough heat to turn Earth into a mostly molten ball, probably to the very surface of the planet. The oldest rocks yet found on Earth are about 4,000 million years old, and there are precious few of them; only about 0.0001 percent of Earth’s crust is composed of rocks older than 3,600 million years (Nutman, 2006).

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