Q: If the Giant Impact Hypothesis is correct and the Moon is made from Earth and Theia (the size of Mars that hit Earth), what happened to the rest of Theia after the crash?
Steven Peckham
Portsmouth, Rhode Island
A: In the 1970s, Donald R. Davis and I suggested that the Moon was formed when a planet the size of Mars, later called Theia, hit Earth created about 4.5 billion years ago. At the time, the Giant Impact Hypothesis had little to say about what happened to the influencer himself.
In the years since, many researchers have modeled what the effect would look like. After entering Earth, the outer rocky shells of both Earth and Theia were detonated in a disc of debris around our planet. From this disc came the Moon together; thus, models indicate that most of Theia’s material ended up as part of the Moon. Any iron heart that could have been cast went with the heart of the Earth itself.
The Giant Influence Hypothesis, as your question refers to, has yet to be resolved. One issue with the theory is that samples of lunar rocks show that the Moon and Earth have very similar ratios of isotopes – equivalent to elemental fingerprints for celestial materials. For example, the ratio of oxygen-16 to oxygen-18 is about the same as that of the Moon and that of the Earth. These results are surprising, since other major groups in the solar system are less similar, especially in oxygen density. A few years ago, symbols seemed to indicate that Theia had come from the distant solar system, making the hypothesis more problematic.
It is true that meteorites from other parts of the solar system are not as isotopic to Earth or the Moon. But a possible explanation is in the form of a rare group of meteorites called enstatite chondrites, which are also almost identical to Earth. These meteorites are probably the building blocks of the Earth and, as a recent paper points out, Theia as well. Moreover, the symbols are a bit questionable and Theia may have come from a more local origin.
This gives the impression that it is a long-lost couple on our planet. For now, scientists have to rely on models to solve Theia’s puzzle, but if a meteorite is found straying from the crash, it may provide the final clues needed to unravel the mystery.
William K. Hartmann
Chief Scientist Emeritus, Institute of Planning Science, Tucson, Arizona