Unanswered Questions
While the giant impact hypothesis is the current leading model on lunar formation, it leaves some issues unresolved. For example, if Theia came from elsewhere in the solar system, it would likely have contained different ratios of oxygen isotopes than Earth, resulting in a Moon with different isotope ratios. Surprisingly, however, the isotope ratios of oxygen and many other elements from Earth and the Moon look essentially identical.
Many variations of the giant impact model have been proposed to address this and other challenges with the giant impact scenario. These variations fall generally into three main categories:
- By coincidence, Theia had a composition very similar to Earth.
- Theia and Earth had different compositions, but the vaporization and mixing of materials from the impact evened out the differences.
- Aspects of the impact occurred differently than originally thought.
A few of these ideas are summarized in the diagrams below.
Twins by Coincidence
Theia and Earth accreted from the same pool of materials in the solar nebula and therefore had the same isotopic composition.
Synestia
The impact between Theia and Earth vaporized both bodies and formed a structure named a synestia—a rotating, donut-shaped cloud of hot debris. The materials in the synestia mixed thoroughly, resulting in an Earth and Moon with identical isotopic composition.
Click to view larger image.
Click to view larger image.
Multiple Impacts and Moonlets
Several smaller objects collided successively with Earth. Each impact result in a disk of debris around Earth that coalesced to form a moonlet. Over time, the moonlets eventually combined into the single Moon we have today.
Half-Earths
A collision occurred between two similar-sized bodies, each about half of Earth’s current mass. The low velocity impact thoroughly mixed the materials from both bodies, resulting in a Moon with a composition derived approximately half from Earth and half from the impactor.
Click to view larger image.
Click to view larger image.
Apollo samples have been invaluable in learning about the Moon and continue to yield data today. As scientists develop better tools, analysis techniques, and computer models, new evidence—such as new samples from future lunar missions—may support or weaken various aspects of the hypotheses of the Moon’s origin and therefore our understanding of Earth’s history.
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