A Bold New Take on the Moon’s Origins: Could It Be More Than One Collision?
The question of how the Moon formed has long centered on a single dramatic event. For years, the prevailing idea has been that Earth slammed into a Mars-sized world called Theia, flinging enough debris into orbit to assemble our Moon. Yet fresh studies suggest the story might be more intricate. New models propose that the Moon’s birth could have involved not one, but three major impacts in the early days of the solar system, each shaping the satellite’s composition in subtle and meaningful ways.
This multi-impact idea aims to address lingering inconsistencies that the single-impact theory strains to explain, especially the chemical quirks that set Earth and the Moon slightly apart. By considering multiple collisions, scientists hope to better account for the Moon’s unique isotopic fingerprints and its overall makeup, which are hard to reconcile with a single-source debris disk.
The Classic Explanation: A Single Giant Impact
The giant-impact scenario remains the most widely taught origin story. In this view, Earth collided with Theia, a body roughly the size of Mars, about 4.5 billion years ago. The collision ejected a vast plume of material that eventually coalesced into the Moon. This model has strong explanatory power because Earth and the Moon share many similarities in their composition, suggesting a common origin.
But no theory is perfect. Subtle chemical discrepancies—such as slight differences in oxygen isotopes between Earth and Moon rocks—pose questions that a sole-source debris model struggles to answer. These mismatches have encouraged researchers to consider alternative possibilities, including the idea that multiple impacts could have contributed material to form the Moon.
A Fresh Perspective: Three Impacts, Not One
Recent work highlighted in Monthly Notices of the Royal Astronomical Society proposes a new twist: Earth may have experienced several significant impacts, with objects of different sizes—including a Theia-like body—hitting Earth at different times. The net effect of these successive collisions could accumulate enough material in the right orbit to birth the Moon.
If correct, this multi-impact framework could naturally explain the Moon’s somewhat lower density and its distinct isotopic signature, since the Moon would have incorporated material from a variety of sources, not just Earth-derived debris.
As Bristol University researcher Philip Carter notes, after three substantial collisions, there would be sufficient mass in orbit to form a full Moon.
Reframing the Early Solar System
A three-impact scenario paints a picture of Earth’s early environment as even more tumultuous than previously imagined, with repeated large-body encounters shaping planet formation. Understanding the Moon’s origin in this context could also shed light on how Earth’s climate-stabilizing features—partly linked to the Moon’s gravitational influence on Earth’s axial tilt—developed and persisted through time. If the Moon’s birth involved multiple sources, it could influence how we think about the conditions that allowed life to emerge on our planet.
How scientists test it
The three-impact hypothesis remains a working idea, pending further validation. Researchers are running sophisticated computer simulations and examining lunar samples to see if the data align with a multi-collision history. If successful, this model could rewrite chapters of solar-system formation and provide new clues about the early dynamics of our neighborhood in space.
As Robert Citron of the Southwest Research Institute puts it, fully calculating every detail is a formidable challenge, but he personally favors the multiple-impact concept over the traditional single-impact narrative. The remaining hurdle is uncovering more direct evidence from Earth’s distant past; if found, it could finally fill a long-standing gap in our understanding of Earth’s companion in the sky.
