Chapter 9 Cratered Worlds
9.4 The Origin of the Moon
Learning Objectives
By the end of this section, you will be able to:
- Describe the top three early hypotheses of the formation of the Moon
- Summarize the current “giant impact” concept of how the Moon formed
It is characteristic of modern science to ask how things originated. Understanding the origin of the Moon has proven to be challenging for planetary scientists, however. Part of the difficulty is simply that we know so much about the Moon (quite the opposite of our usual problem in astronomy). As we will see, one key problem is that the Moon is both tantalizingly similar to Earth and frustratingly different.
Early Ideas for the Origin of the Moon
Most of the earlier hypotheses for the Moon’s origin followed one of three general ideas:
- The fission hypothesis—the Moon was once part of Earth, but somehow separated from it early in their history.
- The sister hypothesis—the Moon formed together with (but independent of) Earth, as we believe many moons of the outer planets formed.
- The capture hypothesis—the Moon formed elsewhere in the solar system and was captured by Earth.
Unfortunately, there seem to be fundamental problems with each of these ideas. Perhaps the easiest hypothesis to reject is the capture hypothesis. Its primary drawback is that no one knows of any way that early Earth could have captured such a large moon from elsewhere. One body approaching another cannot go into orbit around it without a substantial loss of energy; this is the reason that spacecraft destined to orbit other planets are equipped with retro-rockets. Furthermore, if such a capture did take place, the captured object would go into a very eccentric orbit rather than the nearly circular orbit our Moon occupies today. Finally, there are too many compositional similarities between Earth and the Moon, particularly an identical fraction of the major isotopes1 of oxygen, to justify seeking a completely independent origin.
The fission hypothesis, which states that the Moon separated from Earth, was suggested in the late nineteenth century. Modern calculations have shown that this sort of spontaneous fission or splitting is impossible. Furthermore, it is difficult to understand how a Moon made out of terrestrial material in this way could have developed the many distinctive chemical differences now known to characterize our neighbor.
Scientists were therefore left with the sister hypothesis—that the Moon formed alongside Earth—or with some modification of the fission hypothesis that can find a more acceptable way for the lunar material to have separated from Earth. But the more we learned about our Moon, the less these old ideas seem to fit the bill.
The Giant Impact Hypothesis – Currently accepted
In an effort to resolve these apparent contradictions, scientists developed a fourth hypothesis for the origin of the Moon, one that involves a giant impact early in Earth’s history. There is increasing evidence that lar