Earth’s moon may have formed intact in just five hours following a colossal collision around 4.5 billion years ago, according to new computer simulations that challenge assumptions about one of the solar system’s most important events.

Researchers from the Southwest Research Institute and University of Arizona found that the strength and temperature of rock inside the bodies involved in the collision can dramatically alter what happens after impact. The findings, published in The Astrophysical Journal Letters , suggest that scientists will need to consider whether the worlds that collided were soft and hot or cold and solid.

Giant Impact May Have Created The Moon Within Hours

The widely accepted explanation for the moon’s origin involves a Mars-sized protoplanet known as Theia colliding with the young Earth. Traditional giant-impact models suggest the enormous collision destroyed Theia and blasted material into orbit around Earth. This debris subsequently formed a disk from which the moon gradually coalesced. However, these new simulations suggest another possibility—that an intact moon could have emerged just hours after the massive impact.

“Depending on how hot the Earth and Theia are prior to the collision, the impact can destroy Theia and produce this massive disk of debris that eventually forms the moon,” said Dr. Adeene Denton, formerly a NASA Postdoctoral Program fellow at SwRI and now a postdoctoral researcher in SwRI’s Solar System Science and Exploration Division. “But when I 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.”

Rock Strength Changes Moon Formation Models

Earlier simulations generally treated the proto-Earth and Theia almost like fluids because the impact was assumed to be energetic enough to melt and vaporize huge quantities of material.

Challenging that assumption, this new research used computational simulations of the two colliding planets’ temperature-dependent geological strength. The simulations showed that hotter planetary bodies are weaker, while colder bodies have greater material strength. Those differences can fundamentally change the result of a giant impact.

“Models have evolved to include material strength, something that’s really important when you’re studying collisions between smaller bodies,” said Denton. “We weren’t sure if it would matter for the moon or not. When we did the simulations, we found it actually matters quite a bit.”

Earth And Theia Temperature Could Hold A Crucial Clue

Depending on Earth and Theia’s initial temperatures, researchers found the collision could follow markedly different paths. In some scenarios, Theia is destroyed, creating a huge debris disk that eventually produces the moon. In others, an intact moon forms within hours.

Because young protoplanets generally start hot and cool as they age, the findings suggest a potential link between the collision timing and how the moon initially formed.

Mystery Of Earth And Moon Similarity Remains

The simulations do not solve every mystery surrounding the moon’s origin. One major question that remains is why rocks from Earth and the moon are compositionally so similar despite the moon supposedly containing significant material from another planetary body. “Because Earth and Mars formed in the same neighborhood of the solar system, they are like siblings,” said Denton. “The Moon and Earth are more like fraternal twins.”

It could be that Theia and the proto-Earth formed in a similar region of the protoplanetary disk — the ring of gas and dust that forms around a young star like the sun. Meanwhile, Mars — which is compositionally distinct from the Earth and moon — formed farther away in a different section of that disk.

Wishing you clear skies and wide eyes.