For its size and location in the solar system, Earth has a surprisingly large moon. But how exactly the blue planet got its companion has vexed researchers for a long time. Researchers at the Southwest Research Institute and the University of Arizona used state-of-the-art computational techniques that revealed fundamental differences in how the moon may have formed from the collision between a Mars-sized object and Earth roughly 4.5 billion years ago.
Published in The Astrophysical Journal Letters, the results – for the first time – factor in the material strength of the two ancestral bodies and may change how researchers think the planetary collision happened. These impact simulations could change how researchers understand moon formation and may help constrain the timing of the event.
"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."
Earlier studies of the giant impact scenario include a foundational 2001 paper 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 current study. In the giant impact scenario, the debris from the collision of a Mars-sized body known as Theia with the early Earth obliterated Theia, scattering its remains into a disk around Earth. However, those simulations and subsequent giant impact modeling ignored material strength, which was thought to be insignificant for such high-energy events. Denton and her team revisited this hypothesis, using modern computational methods that incorporate temperature-dependent geologic strength 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."
Denton revisited the hypothesis, for the first time using more advanced smoothed particle hydrodynamics simulations, or SPH, that take into account the structural strength of the bodies involved in the collision.
"It turns out material strength is really important when you're studying collisions between smaller bodies like asteroids, dwarf planets and moons," said Denton, who got the idea of applying the concept to Earth's moon while working on a previous paper about the formation of the Pluto-Charon system . "We weren't sure if it would matter for our moon or not. When we did the simulations, we found it actually matters quite a bit."
This version of SPH, developed at the U of A and the University of Bern in Switzerland, uses a strength model that gives the bodies the kind of resistance to deformation that is expected for realistic geologic materials such as the rock and metals that would have made up Theia and the proto-Earth, or solid ice.
Hotter bodies are weaker than colder ones, and the team found that the different outcomes of Moon formation scenarios are sensitive to the temperatures of the colliding bodies. Some scenarios produce a fully intact moon within hours of the impact, while others result in a protolunar disk around the Earth that ultimately forms the moon over time. Because protoplanets generally start off hot and cool with age, this establishes an important new connection between the timing of the giant impact and the nature of the moon's initial state and assembly.
"Depending on how hot the Earth and moon are prior to the collision, the impact can destroy Theia and produce this massive disk of debris that eventually forms the moon," Denton said. "But when we 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."
While intact moon outcomes have been seen in prior simulations, this work is the first to show that material strength and temperature play a central role in whether the moon forms intact or is assembled from material processed within a protolunar disk.
"These surprising and exciting new results imply a potential connection between the physical properties of the moon today, including perhaps its volatile content, and the thermal state of the Earth and Theia at the time of the giant impact," said Canup, who was not involved in the study. "This in turn might help scientists better constrain when the moon-forming event occurred."
The other conundrum remains the similarity of materials making up the Earth and the moon. As with prior models, explaining the close compositional makeup of the Earth and Moon remains an open scientific question.
"Because Earth and Mars formed in the same neighborhood of the solar system, they are like siblings," Denton said. "The moon and Earth are more like fraternal twins."
A possible explanation is that Theia and the proto-Earth formed from a common region of the solar system, while Mars, which is compositionally distinct from the Earth and Moon, formed farther away.
"We now know that the geophysical state of Earth and Theia play a fundamental role in shaping the outcome of the collision," said study co-author Namya Baijal, a doctoral student in Asphaug's group. "This gives us a new way to explore the conditions of the impact and what they might reveal about the moon's origin."
Watch the Moon formation simulations: https://www.youtube.com/watch?v=QAIRqc9dFHg
