New simulations suggest moon formed intact within hours of giant impact

Scientists at the Southwest Research Institute have run a new set of computer simulations of the giant impact that created the moon, and the results suggest something unexpected: the moon may have formed intact, fully assembled, within just five hours of the collision. The finding challenges decades of modeling work by introducing a factor that previous researchers had dismissed as negligible—the material strength of the two colliding bodies, and how that strength changes with temperature.

The standard account of lunar origin, refined over the past quarter-century, holds that a Mars-sized object called Theia struck the early Earth in a cataclysmic collision. The impact vaporized Theia and ejected vast quantities of material into orbit around Earth. Over time, this debris disk coalesced into the moon. That model, first developed in detail by Robin Canup and Erik Asphaug in 2001, has held up well against observations. But it rested on a simplifying assumption: that the extreme energies involved in such a collision made the physical properties of the colliding bodies irrelevant. Heat and pressure would overwhelm any structural differences between hot rock and cold rock.

Adeene Denton, a postdoctoral researcher now at SwRI, decided to test that assumption. Working with Asphaug and others at the University of Arizona, she built simulations that treated the colliding bodies as materials with temperature-dependent strength—the way rock actually behaves. Hotter rock is weaker and more deformable; colder rock is stronger and more brittle. When she ran the models with these properties included, something changed. The outcome depended critically on how hot Earth and Theia were at the moment of impact.

In some scenarios, the collision destroyed Theia completely, scattering its material into a disk around Earth that gradually assembled into the moon over time—the traditional picture. But when Denton used the same initial conditions as the original 2001 models, including the same internal temperature structures, the simulations produced something different: an intact moon, already formed, within about five hours. The difference was not in the impact energy or the masses involved. It was in the material properties of the bodies being struck.

This opens a new line of inquiry. The thermal state of the proto-Earth and Theia at the moment of collision may have been the deciding factor in how the moon formed. Protoplanets begin their existence hot and cool over time. If the giant impact happened early, when both bodies were still molten or near-molten, the collision might have produced an intact moon. If it happened later, after the bodies had cooled and solidified, the debris-disk scenario would have prevailed. The physical properties of the moon today—including perhaps the amount of volatile compounds it contains—might therefore carry a signature of how hot the colliding bodies were.

Robin Canup, who developed the original giant impact model and was not involved in this study, noted that the new results could help scientists narrow down when the moon-forming event actually occurred. The research also touches on a long-standing puzzle: Earth and the moon have nearly identical isotopic compositions, suggesting they formed from material in the same region of the early solar system. Mars, by contrast, is compositionally distinct, having formed farther away. Denton described the relationship this way: Earth and Mars are like siblings, born in the same neighborhood. The moon and Earth are more like fraternal twins, sharing a common origin but not identical. The new simulations may eventually help explain why.

The work, published in The Astrophysical Journal Letters, represents a shift in how planetary scientists model catastrophic collisions. For decades, the assumption that material strength could be ignored in giant impacts went largely unquestioned. Denton's team showed that for smaller bodies—asteroids, or the Pluto-Charon system—material strength clearly mattered. The question was whether it mattered at the scale of planetary collisions. The answer, it turns out, is yes. The moon's origin may have hinged on physics that earlier models left out.