The origin of the Moon is uncertain, although several theories have been advanced to account for it. However, both the similarities and the differences between the Moon and Earth conspire to confound many promising attempts to explain the Moon's existence.

One theory (the sister, or coformation, theory) suggests that the Moon formed as a separate object near Earth and in much the same way as our own planet formed—the "blob" of material that eventually coalesced into Earth gave rise to the Moon at about the same time. The two objects thus formed as a double-planet system, each revolving about the common center of mass. Although once favored by many astronomers, this idea suffers from a major flaw: the Moon differs in both density and composition from Earth, making it hard to understand how both could have originated from the same preplanetary material.

A second theory (the capture theory) maintains that the Moon formed far from Earth and was later captured by it. In this way, the density and composition of the two objects need not be similar, for the Moon presumably materialized in a quite different region of the early solar system. The objection to this theory is that the Moon's capture would be an extraordinarily difficult event; it might even be an impossible one. Why? Because the mass of our Moon is so large relative to that of Earth. It's not that our Moon is the largest natural satellite in the solar system, but it is unusually large compared with its parent planet. Mathematical modeling suggests that it is quite implausible that Earth and the Moon could have interacted in just the right way for the Moon to have been captured during a close encounter sometime in the past. Furthermore, although there are indeed significant composition differences between our world and its companion, there are also many similarities—particularly between the mantles of the two bodies—that make it unlikely that they formed entirely independently of one another.

A third, older, theory (the daughter, or fission, theory) speculates that the Moon originated out of Earth itself. The Pacific Ocean basin has often been mentioned as the place from which protolunar matter may have been torn—the result, perhaps, of the rapid spin of a young, molten Earth. Indeed, there are some chemical similarities between the matter in the Moon's outer mantle and that in Earth's Pacific basin. However, this theory offers no solution to the fundamental mystery of how Earth could possibly have been spinning so fast that it ejected an object as large as our Moon. Also, computer simulations indicate that the ejection of the Moon into a stable orbit simply would not have occurred. As a result, the daughter theory, in this form at least, is no longer taken seriously.

Today, many astronomers favor a hybrid of the capture and daughter themes. This idea—often called the impact theory—postulates a collision by a large, Mars-sized object with a youthful and molten Earth. Such collisions may have been quite frequent in the early solar system (see Chapter 15). The collision presumed by the impact theory would have been more a glancing blow than a direct impact. The matter dislodged from our planet then reassembled to form the Moon.

Computer simulations of such a catastrophic event show that most of the bits and pieces of splattered Earth could have coalesced into a stable orbit. Figure 8.26 shows some of the stages of one such calculation. If Earth had already formed an iron core by the time the collision occurred, then the Moon would indeed have ended up with a composition similar to that of Earth's mantle. During the collision, any iron core in the impacting object itself would have been left behind in Earth, eventually to become part of Earth's core. Thus both the Moon's overall similarity to that of Earth's mantle and its lack of a dense central core are naturally explained. Over the past decade, planetary scientists have come to realize that collisions such as this probably played very important roles in the formation of all the terrestrial planets.

Figure 8.26 This sequence shows a simulated collision between Earth and an object the size of Mars. The sequence proceeds top to bottom and zooms out dramatically. The arrow in the final frame shows the newly formed Moon.