Earth has an unusually large moon compared with other rocky planets in the solar system, but scientists still do not fully understand how it formed. A new study from researchers at the Southwest Research Institute and the University of Arizona suggests that one long-overlooked factor may have played a major role in the Moon's birth: the physical strength of the worlds that collided. Using advanced computer simulations, the researchers examined the enormous impact thought to have occurred about 4.5 billion years ago between the young Earth and a Mars-sized object.
Their results reveal strikingly different outcomes depending on the temperature and structural properties of the two bodies. The findings were published in The Astrophysical Journal Letters. For the first time, the simulations account for the material strength of the two ancient worlds.
That addition could reshape scientists' understanding of the Moon-forming collision and may also help narrow down when it happened. "We discovered that the preexisting geology of the Mars-sized proto-moon matters," said Adeene Denton, a former postdoctoral researcher at Lunar and Planetary Laboratory who is now at SwRI. "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." One of the leading explanations for the Moon's origin is the giant impact scenario.
According to this idea, a Mars-sized body called Theia collided with the early Earth. The impact destroyed Theia and sent its remains into orbit around Earth, creating a disk of debris that eventually assembled into the Moon. A foundational 2001 study of this scenario was led by Robin Canup, vice president of SwRI's Solar System Science and Exploration Division in Boulder, Colorado, and Erik Asphaug, a professor at the Lunar and Planetary Laboratory and co-author of the new study.
Those early simulations, along with many later models, did not account for material strength. Scientists assumed that the collision was so energetic that the rocky bodies could effectively be treated like fluids. Denton and her colleagues decided to test whether that assumption was justified.
Using modern computational methods, they incorporated temperature-dependent geologic strength into Moon formation simulations for the first time. "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 said. "Based on our new results, however, we think that it is time to reconsider that." Adding Realistic Geology to the Collision To revisit the giant impact, Denton used a more advanced form of smoothed particle hydrodynamics simulations, or SPH.
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