The Moon was born from a cosmic collision. It may have happened shockingly fast

Earth’s Moon is unusually large, standing out prominently compared with the satellites of other planets in the Solar System. This unusually large moon has profoundly shaped our blue planet since its birth: it has stirred up monstrous global tides, stabilized Earth’s climate, and dictated the trajectory of life’s evolution. Yet to this day, exactly how the Moon came to exist remains an unsolved mystery.

A new study from the Southwest Research Institute (SwRI) and the University of Arizona adds a surprising twist. For the first time, researchers have shown that the physical strength and temperature of the colliding worlds, the young Earth and a Mars-sized body called Theia, may have determined how the Moon was born.

According to the giant impact hypothesis, Theia collided with early Earth about 4.5 billion years ago. The collision destroyed Theia, sending an iron-poor debris into orbit that eventually coalesced into the Moon.

Early simulations treated both Earth and Theia as fluids, assuming the impact was so violent that geology didn’t matter.

Water likely accumulated on the Moon slowly over billions of years

Prof Erik Asphaug from the Lunar and Planetary Laboratory said, “Because the collision was considered violent enough to melt and vaporize large portions of Earth and Theia, those previous papers assumed that it is okay to approximate them as fluids. Based on our new results, however, we think that it is time to reconsider that.”

But Adeene Denton, a former postdoctoral researcher at the Lunar and Planetary Laboratory now at SwRI, wondered whether that assumption was too simplistic. The team revisited that hypothesis, using advanced smoothed particle hydrodynamics (SPH) simulations that incorporate material strength. They discovered that geology and temperature could dramatically change the outcome.

Denton thought of applying the concept to Earth’s Moon while working on a previous paper about the formation of the Pluto-Charon system. He said, “We weren’t sure if it would matter for our Moon or not. When we did the simulations, we found it actually matters quite a bit.”

Even though the stresses deep inside Theia were much greater, the strength of its outer layers still mattered because it reduced Theia’s deformation when it was hit. This condition changed the momentum-transfer process and ultimately altered the logic of how the Moon came into being.

The strength model in the SPH simulation gives colliding planets the resistance to deformation expected for realistic geologic materials such as the rock and metals that would have made up Theia and the proto-Earth, or solid ice. They found that the temperatures of Earth and Theia before the collision play a crucial role in shaping the Moon’s birth.

Earth was born dry, then a cosmic crash turned it into a blue world

Hotter planets are weaker, while cooler ones are stronger; the impact could therefore either break up Theia into a disk of debris that slowly gives rise to the Moon, or, in some cases, form a completely intact Moon within just a few hours. The fact that young planets begin life hot and then cool as they age establishes a new connection between when the giant impact occurred and the Moon’s initial state.

“Depending on how hot the Earth and Moon are before the collision, the impact can destroy Theia and produce this massive disk of debris that eventually forms the Moon,” Denton said. “But when we used the same parameters as original impact modeling, down to the equal temperature structures inside both bodies, within around five hours, an intact Moon emerged.”

For the first time, this work shows that material strength and temperature are central to whether the Moon formed directly or assembled from debris, offering fresh insight into one of planetary science’s biggest mysteries.

Robin Canup, a pioneer of giant impact modeling, noted: “These surprising and exciting new results imply a potential connection between the physical properties of the Moon today and the thermal state of the Earth and Theia at the time of the giant impact.”

There are still some mysteries. One such puzzle is that Earth and the Moon have remarkably similar compositions, which giant impact theories still cannot fully explain. Denton proposes that Theia and Earth might have formed from the same part of the solar system, whereas Mars, with its different composition, could have come from a more distant area.

Moon formation brought water to the Earth

The study offers a fresh way to look at one of the greatest cosmic mysteries by showing that both material strength and temperature determine whether the Moon forms from a disk or appears as a whole object. Rather than viewing the Moon’s formation as a simple fluid-like impact, it presents it as a collision in which the internal geology of the planets involved was crucial.

As co-author Namya Baijal put it: “We now know that the geophysical state of Earth and Theia play a fundamental role in shaping the outcome of the collision.”

The Moon’s origin story is far from settled. But with each new simulation, scientists peel back another layer of the puzzle. Was our Moon assembled grain by grain from a disk of debris, or did it emerge whole in the fiery aftermath of a planetary collision?

The answer may lie in the hidden strength of worlds long gone, and in the enduring clues preserved in the Moon that lights our night sky.

Journal Reference:

1. C. Adeene Denton, Erik Asphaug, Namya Baijal, and Robert E. Melikyan. Collisional Capture of an Intact Moon Depends on Strength. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae91e9