One of the widest accepted models for the Moon’s formation states that a renegade, Mars-sized planet, named Theia, slammed into Earth 4.5 billion years ago, and pushed up debris that would eventually coalesce into a satellite. This theory has been able to predict and explain many facts, like the mass of Earth and the Moon, but it also says that most of the lunar-forming debris stemmed from Theia, not proto-Earth. Theia is thought to have originated from a different part of the Solar System, with different elemental isotopes, which conflicts with some of the more sensitive measurements of the past decade showing that rocks from Earth and the Moon have identical isotopic ratios of oxygen, titanium, chromium, and tungsten.
Two teams of researchers report that they have solved this chemical quandary and have been able to uphold this lunar origin theory. The two papers predict different sizes for Theia. One paper states that Theia was smaller than Mars, and another says that Theia was four to five times larger than Mars. Both papers were published in the journal Science (1, 2).
These papers show that there is a wide variety of impact scenarios that were possible. The teams assumed that Earth rotated twice as fast as it does now. Gravitational interaction between the Moon and Sol could have drained the spin of a rapidly rotating Earth.
Robin Canup, one of the originators of the Mars-sized impact model, now considers the possibility that proto-Earth with half of Earth’s current mass collided with an impactor of similar mass. The collision would have mixed the isotopic composition of the debris, making the isotopic composition indistinguishable from either body. It would have also revved up Earth’s spin, which could have drained in a similar fashion to the other study.
The gravitational mechanism known as evection resonance appears when the short axis of the Moon’s elliptical orbit around Earth precesses in sync with Earth’s orbit around the Sun. Some of the angular momentum from a rapidly spinning Earth would be transferred to the Moon, which in turn allows the rotational energy to be absorbed by the Sol.
Other theorists believe making alterations to the impactor’s speed and angle of impact, rather than its size, may result in a plausible Moon forming theory.