New Moon Formation Simulation Challenges Everything We Thought We Knew About Earth’s Origins

New Simulations Challenge Long-Held Theories on the Moon’s Violent Origins

For decades, the prevailing narrative of the Moon’s birth has centered on a cataclysmic collision between the early Earth and a Mars-sized protoplanet dubbed Theia. While this giant impact hypothesis has provided a foundational framework for understanding the Earth-Moon system, a new study published in The Astrophysical Journal Letters suggests that our current models may have been missing a critical variable: the structural integrity of the colliding worlds.

Historically, researchers treated both the Earth and Theia as fluid-like bodies in their simulations, operating under the assumption that the immense energy of the collision would render the material strength of the planets negligible. However, by applying advanced smoothed particle hydrodynamics (SPH) simulations that incorporate temperature-dependent geologic strength, a team of scientists from the Southwest Research Institute (SwRI) and the University of Arizona has discovered that the physical state of these ancient bodies significantly alters the aftermath of the impact.

“We discovered that the preexisting geology of the Mars-sized proto-moon matters,” says Adeene Denton, a former postdoctoral researcher at the University of Arizona’s Lunar and Planetary Laboratory, who led the investigation. “When you simulate the Earth and the moon as colliding bodies with geologic properties, it changes how the moon forms out of that impact—that’s something we considered unnecessary before.”

The research team employed a sophisticated strength model that accounts for the resistance to deformation inherent in rock, metal, and ice. Their findings demonstrate that the internal temperature of these protoplanets is a decisive factor in whether the resulting Moon forms rapidly as an intact body or accumulates slowly from a vast ring of orbiting debris. Because planets cool over time, this discovery creates a potential link between the timing of the collision and the final chemical and physical composition of the lunar surface.

Erik Asphaug, a professor at the Lunar and Planetary Laboratory and a coauthor of the study, notes that while the “fluid approximation” was a useful simplification in early research, the new results mandate a shift in perspective. “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,” Asphaug explains. “Based on our new results, however, we think that it is time to reconsider that.”

The simulations produced a wide range of outcomes. In some scenarios, an intact moon emerged within just five hours of the collision, while others generated a massive protolunar disk. This sensitivity to initial thermal conditions provides scientists with a powerful new tool to “backtrack” the event, potentially uncovering when exactly the collision occurred by analyzing the specific geochemical markers found on the Moon today.

Despite these advancements, the “sibling” mystery remains: the Earth and the Moon share a remarkably similar chemical makeup, a characteristic that remains difficult to fully reconcile with standard impact models. Denton suggests that the answer may lie in their shared regional origin in the early solar system, noting that while the Earth and Moon are akin to fraternal twins, Mars—which bears a distinct chemical signature—likely formed in a different, more distant part of the solar nebula.

As the team continues to refine these models, they emphasize that the geophysical state of the early Earth and Theia is no longer an optional variable. “We now know that the geophysical state of Earth and Theia play a fundamental role in shaping the outcome of the collision,” says study coauthor Namya Baijal. “This gives us a new way to explore the conditions of the impact and what they might reveal about the moon’s origin.”