The Moon may have appeared almost fully formed just a few hours after the collision that gave birth to it.
The origin of our satellite is generally linked to an impact that occurred early in the history of the Solar System. A Mars-sized planet, called Theia, is thought to have struck the young Earth. In the classic scenario, the collision scatters a large amount of rock around our planet. This debris then gradually comes together to form the Moon.
The collision of Theia and the young Earth
Credit: Hernán Cañellas
A new study nevertheless shows that one detail can profoundly change the outcome: the strength of the rocks. Earlier simulations often treated both bodies as materials capable of deforming very easily during the impact. The energy released seemed so great that the strength of the materials appeared negligible.
Researchers from the Southwest Research Institute and the University of Arizona reran these simulations, giving the rocks more realistic properties. Their strength depends in particular on their temperature. Very hot rock deforms more easily than cooler rock, which changes how Theia responds during the collision.
The result is spectacular. Under certain conditions already used in historical models, a Moon-sized body remains about five hours after the impact. The calculations therefore establish that such a scenario is physically possible.
Temperature plays a major role here. When the primitive Earth and Theia are very hot, their materials resist the impact less. Theia can then be largely destroyed and contribute to a vast ring of debris. If the two bodies are cooler and stronger, a large mass can remain assembled and directly become a satellite.
This difference could also provide clues about the date of the collision. Young planets begin their existence very hot, then cool over time. If the Moon's current properties eventually make it possible to determine which scenario best matches its formation, they could therefore help pinpoint when the impact occurred.
One difficulty remains: Earth and the Moon have remarkably similar compositions. The new simulations do not resolve this question. One possibility is that Theia and Earth formed in the same region of the disk of matter surrounding the young Sun, with materials that were already quite similar before their encounter.
Researchers can now compare these rapid scenarios with other clues preserved in lunar rocks. The quantity of substances that evaporate easily during intense heating could in particular retain evidence of the impact conditions. These measurements will help test whether a largely intact Moon is compatible with what lunar missions have brought back to Earth.