The Moon may have been created twice
Our understanding of the Moon’s formation has long been founded on the idea of debris accreting in the wake of a collision between the Earth and Theia. New simulations now suggest, however, that in the period after the impact there was a much more complex, two-step mechanism.
A new view of the Moon, during the Artemis 2 mission in April 2026. | Photo: NASA
A new scenario is offering a fresh perspective on the origins of Earth’s moon. The classic model has it that a major impact between the Earth and the planet Theia happened four billion years ago, creating a debris disk that quickly accreted into a single mass. According to a study by the astrophysicist Jérémy Couturier, carried out between the universities of Geneva and Rochester (United States), the Moon may actually have undergone a two-stage process of creation.
To demonstrate this, Couturier simulated the trajectory of 100,000 pieces of debris, i.e., one hundred times more than previous models, and he integrated realistic fragmentation. During violent collisions, debris can also break as well as fuse and ricochet. The result is that after the collision, for a few simulated decades, several massive sub-moons emerge in the cloud of debris. Then, between 10,000 to 100,000 simulated years later, tidal forces move these bodies away from the Earth, pushing them towards orbital resonances. It’s a mechanism that then synchronises their movements causing the collisions and ejections of superficial masses. Following this process of orbital cleansing, only the Moon would remain.
Due to the chaotic nature of this process, Couturier remains cautious. Each of his four simulations represents only “one possibility among an infinite number of others”. Shifting the position of a piece of debris by one centimetre can radically change its outcome. “We still can’t tell whether the Moon is an obvious or an exceptional outcome. To know that, it would take thousands of simulations”, he says. Only statistical studies will tell us if the birth of our Moon was a logical process or a cosmic miracle.
