A new study suggests that the Moon’s formation may have depended not only on the size and speed of the impact that created it, but also on the temperature and physical strength of the young worlds involved. Researchers from the Southwest Research Institute and the University of Arizona found that the geology of the early Earth and the Mars-sized body Theia could have dramatically changed the outcome of the Moon-forming collision.
Scientists widely believe that the Moon formed about 4.5 billion years ago when a Mars-sized object, known as Theia, collided with the young Earth. The impact likely sent molten and vaporized material into orbit, creating a debris disk that eventually came together to form the Moon.
However, a new study published in The Astrophysical Journal Letters indicates that earlier models may have overlooked an important factor: the material strength of the colliding planetary bodies. By including temperature-dependent geological strength in computer simulations, researchers found that the same impact could produce very different results.
“We discovered that the pre-existing geology of a Mars-sized protoplanet is important,” said Adeen Denton, a former postdoctoral fellow at the Lunar and Planetary Institute and now a researcher at SwRI. “Simulating Earth and Theia as geological bodies changes how the Moon forms after the collision.”
Reexamining the giant impact theory
The giant impact theory is the leading explanation for the Moon’s origin. Under this model, Theia struck the early Earth and was largely destroyed. Debris from both bodies entered orbit around Earth, forming a protolunar disk that gradually assembled into the Moon.
Early computer simulations of the Moon-forming impact, including influential work led by Robin Canup and Erik Asphaug, generally treated Earth and Theia as fluids. Researchers assumed the collision was energetic enough to melt or vaporize much of both worlds, making their physical strength less important.
Denton and colleagues revisited that assumption using advanced smoothed particle hydrodynamics simulations, or SPH. Unlike traditional fluid-based models, the new approach accounts for the strength and resistance of planetary materials as they deform during a collision.
“This collision was thought to be violent enough to melt and vaporize large parts of Earth and Theia, so previous studies assumed that it was reasonable to approximate both bodies as fluids,” Asphaug said. “Based on the new results, we think that assumption deserves to be reconsidered.”
Temperature can change how the Moon forms
The researchers found that temperature plays a central role in determining the outcome of a giant planetary collision. Hot planetary bodies are mechanically weaker than cooler bodies, meaning they deform and break apart more easily during an impact.
In some simulations, the collision destroyed Theia and produced a broad disk of debris around Earth. Over time, that material could have gathered to form the Moon.
In other simulations, however, a nearly intact Moon emerged within only a few hours of the impact. Rather than forming slowly from a disk of processed debris, the satellite formed directly from material launched into orbit during the collision.
“Depending on the temperatures of Earth and Theia before the impact, the collision could destroy Theia and create a large debris disk that eventually forms the Moon,” Denton said. “But when we used conditions similar to those in earlier impact models, an intact Moon emerged in about five hours.”
Because young protoplanets are expected to begin hot and gradually cool, the findings may provide clues about when the Moon-forming impact occurred. The physical condition of Earth and Theia at the time of the collision could have influenced whether the Moon formed rapidly or assembled over a longer period.
Previous simulations have also produced intact satellites, but this study highlights the possibility that material strength and temperature are major factors controlling the Moon’s formation process.
New clues about the Moon’s formation date
The study could help scientists connect the Moon’s present-day properties with the conditions that existed on Earth and Theia billions of years ago. Researchers say the Moon’s composition, internal structure, and abundance of volatile materials may preserve evidence of the thermal environment during the giant impact.
“These surprising results suggest a potential connection between the Moon’s current physical properties, including its volatile materials, and the thermal conditions of Earth and Theia at the time of the impact,” said Robin Canup, who was not involved in the study. “This may help scientists better determine when the Moon-forming collision occurred.”
Despite the new findings, one major mystery remains: Earth and the Moon have remarkably similar chemical compositions. Many giant impact models have struggled to explain why the Moon resembles Earth so closely.
“Earth and Mars are somewhat like siblings because they formed in the same region of the solar system,” Denton said. “The Moon and Earth are more like fraternal twins.”
One possibility is that Earth and Theia formed from similar material in the same region of the early solar system. Mars has a different composition and may have formed farther from the Sun before migrating inward.
A new way to study the Moon’s origin
By showing that the internal temperature and geological strength of Earth and Theia can strongly influence the aftermath of a collision, the study offers scientists a new way to investigate the Moon’s origin.
“We now know that the geophysical conditions of Earth and Theia played a fundamental role in shaping the collision’s outcome,” said study co-author Namya Baijal, a doctoral student in Asphaug’s group. “This gives us a new way to investigate the circumstances of the impact and what it can reveal about the Moon’s formation.”
Source: www.sciencedaily.com