New computational models suggest Earth’s Moon may have formed in as little as five hours following the catastrophic impact of a Mars-sized body named Theia 4.5 billion years ago. Researchers at the Southwest Research Institute and University of Arizona found that incorporating material strength and pre-impact temperature changes traditional collision outcomes.
Earth’s unusually large companion has defined our planet’s global tides, stabilized its climate, and shaped the trajectory of life since its inception. Yet the exact mechanics of how the lunar body came to exist have remained a central puzzle in planetary science. For decades, the leading framework has been the giant impact hypothesis, which posits that a protoplanet roughly the size of Mars struck the proto-Earth about 4.5 billion years ago, dispersing a fiery ring of debris that slowly coalesced into the Moon over millennia or millions of years.
That conventional picture relies on a simplifying assumption: that the colliding worlds were hot and energetic enough to behave essentially as fluids. Because the collision was violent enough to vaporize and melt vast portions of both planetary mantles, researchers previously assumed that internal rock and metal strength played no meaningful role in the outcome.
Reconsidering Rock and Metal Strength in Planetary Impacts
A team of researchers from the Southwest Research Institute (SwRI) and the University of Arizona used state-of-the-art computational techniques to test whether treating the colliding worlds as realistic geological bodies changes the aftermath. The findings were published in The Astrophysical Journal Letters.
Adeene Denton, a planetary scientist who worked on the modeling as a postdoctoral researcher at the University of Arizona’s Lunar and Planetary Laboratory before moving to SwRI, noted that accounting for geological properties fundamentally alters lunar origins.
To test this, the research team employed advanced smoothed particle hydrodynamics (SPH) simulations developed at the University of Arizona and the University of Bern. Unlike earlier fluid-approximation models, this version incorporates temperature-dependent strength models, giving the rocky mantles and metallic cores realistic resistance to deformation that weakens as temperatures approach melting points.
Broader Implications for Planetary Science and Exomoons
Co-author Erik Asphaug of the University of Arizona noted that while previous models treated the immense energy of the collision as a justification for ignoring structural integrity, the new data demands a reassessment across planetary collision science.
“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.”
Despite solving the timeline puzzle for how quickly a moon can assemble, the simulations do not fully resolve all geochemical mysteries. Earth and the Moon share a remarkably similar chemical composition, a similarity that standard impact models still struggle to account for entirely.
Plot Twist: Moon Formed Within Hours Instead of Millions of Years
