For a long time, planetary scientists have told a simple story: the Moon formed in a single, enormous collision between the early Earth and a Mars-sized body. It’s a compelling idea, and for decades it has shaped how we think about lunar origins. But growing evidence suggests that this picture may be too neat. Join us in today’s astrobito to learn how a sequence of collisions could have built the Moon we see today!
Paper details:
Title: A new hope for Moon formation: Presenting a multiple impact pathway
Authors: Harrison Davies, Phillip J. Carter, Louise Eddershaw, et. al.
First-author institution: H.H. Wills Physics Laboratory, University of Bristol, BS8 1TL, UK
Status: Open access arXiv.
For decades, the story of the Moon’s birth has sounded almost cinematic: a colossal collision between the young Earth and a Mars-sized body, blasting molten rock into orbit and assembling our satellite in one dramatic event. It’s a powerful idea, but what if the Moon didn’t form in one night? What if it grew, piece by piece, over many impacts? New research suggests exactly that. Instead of one giant collision, the Moon may be the end result of a chain of more modest impacts, each contributing a little mass and a little momentum.
Why the classic giant impact struggles
The giant impact hypothesis became dominant because it solves a major dynamical puzzle: the Earth-Moon system contains a lot of angular momentum, and a large collision can naturally supply it. Simulations show that a single oblique impact can place enough material into orbit to form a Moon-sized body.
However, chemistry complicates the picture. Samples from the Moon are nearly indistinguishable from Earth rocks in their isotopic composition for oxygen, silicon, titanium, calcium, and even tungsten. That’s surprising. In the canonical giant impact scenario, most of the Moon’s material comes from the impactor, not the Earth. Unless the Earth and the impactor formed from almost identical material their isotopic fingerprints should differ. This tension has pushed planetary scientists to look for alternative ways to build a Moon that looks chemically like Earth without requiring improbable initial conditions.
A Moon built in stages
The study discussed in today’s astrobito explores a different idea: that the Moon formed through a sequence of impacts as the Earth itself was still growing. Instead of one giant collision, the researchers simulated chains of four moderate impacts onto a rapidly rotating proto-Earth, as it grew from about half an Earth mass to its present size.
Each impact produces a disk of debris and a small “moonlet”, sometimes well below the Moon’s current mass. Over time, these moonlets merge and accrete additional material from subsequent impacts, gradually building a larger satellite. Crucially, both the Earth and the Moon grow from the same set of impactors. With each collision, their compositions become more similar.
Multiple impacts make sense
Using thousands of particles and smoothed-particle hydrodynamics simulations, the authors tested 12 different impact chains. The results are quietly striking.
All of the chains produced moonlets at least half as massive as the Moon. Nine formed moons comparable in size to today’s Moon while leaving behind Earth-sized planets. Two of them satisfied all the major constraints of the real Earth-Moon system: Moon mass, low iron content, acceptable angular momentum, and a close compositional match.
Early impacts tend to produce moonlets that are compositionally distinct from the Earth, much like in the canonical giant impact. But as impacts accumulate, both bodies sample the same incoming material. The compositional “distance” between Earth and Moon shrinks naturally, without requiring any fine-tuned starting conditions. In other words, similarity emerges because of repetition.
Multiple-impact Moon formation: A series of collisions gradually builds the Moon. First, an impact onto the young Earth throws debris into orbit (a), forming a disk from which a small “seed” Moon emerges (b). Later impacts create new debris disks, each spawning additional moonlets (c, d, e). Over time, these moonlets merge with the existing Moon, steadily adding to its mass. Repeated many times, this process could ultimately produce the Moon we see today.
This cumulative picture aligns well with what we know about planet formation. Earth didn’t grow from one final collision; it experienced many moderate impacts over tens of millions of years. If those same impacts can also grow a Moon, then lunar formation becomes a natural extension of terrestrial planet growth.
Rethinking the Moon’s origin
This work doesn’t claim to replace the giant impact model outright. Instead, it reframes the problem. The Moon may not be the product of a single, rare catastrophe, but rather the cumulative outcome of common planetary processes.
If that’s true, then our Moon is a record of Earth’s long and chaotic growth. And elsewhere in the universe, large moons may form not in “one go”, but through patience, repetition, and time… lots of time.