Earth has an unusually large moon compared to other rocky planets in the solar system, but scientists still don’t fully understand how it formed. A new study by researchers at the Southwest Research Institute and the University of Arizona suggests that one long-overlooked factor may have played an important role in the birth of the Moon: the physical force of the colliding worlds.
Using advanced computer modeling, researchers studied a huge collision believed to have occurred about 4.5 billion years ago between the young Earth and a Mars-sized object. Their results show strikingly different results depending on the temperature and structural properties of the two bodies.
The results were published in Letters from an Astrophysical Journal. For the first time, the simulation takes into account the material strength of the two ancient worlds. The addition could change scientists’ understanding of the impact that formed the Moon and could also help narrow down questions about when it happened.
“We found that the pre-existing geology of a Mars-sized protomoon matters,” said Adine Denton, a former Lunar and Planetary Laboratory scientist now at SwRI. “When you model the Earth and Moon as colliding bodies with geological properties, it changes the way the Moon is formed by that impact—something we previously thought was unnecessary.”
Returning to the giant impact theory
One of the leading explanations for the origin of the Moon 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 her remains into orbit around the Earth, creating a disk of debris that eventually collected on the Moon.
Fundamental research into this scenario was conducted in 2001 by Robin Kanup, vice president of SwRI’s Division of Solar System Science and Exploration in Boulder, Colorado, and Eric Asphaug, a professor at the Lunar and Planetary Laboratory and co-author of the new study.
These early models, like many later models, did not take into account the strength of the material. Scientists theorized that the collision was so energetic that the rocky bodies could be treated as liquids.
Denton and her colleagues decided to test whether this assumption was justified. Using modern computational techniques, they incorporated temperature-dependent geological strength into modeling of Moon formation for the first time.
“Because the impact was thought to be strong enough to melt and vaporize large parts of Earth and Theia, previous papers suggested that they could be approximated as liquids,” Asphaug said. “However, based on our new findings, we believe it is time to reconsider this.”
Adding Realistic Geology to a Collision
To take another look at the giant impact, Denton used a more advanced form of smoothed particle hydrodynamics, or SPH, modeling. These simulations take into account the structural strength of the objects involved rather than treating them as completely fluid.
“It turns out that material strength is really important when you study collisions between smaller bodies like asteroids, dwarf planets and moons,” said Denton, who got the idea to apply the concept to Earth’s moon while working on a previous paper on the formation of the Pluto-Charon system. “We weren’t sure whether this would make a difference to our Moon or not. When we did the modeling, we found that it actually made quite a difference.”
The version of SPH used in the study was developed at the University of Australia and the University of Bern in Switzerland. His strength model allows simulated planetary material to resist deformation in ways more consistent with real geological materials, including the rocks and metals that likely made up Theia and the proto-Earth, or solid ice.
The moon could have formed in different ways
Temperature is especially important. Hotter planetary bodies are mechanically weaker than cooler ones, and simulations have shown that this difference can significantly affect what happens after an impact.
Under certain conditions, the collision destroys Theia and creates a wide proto-lunar disk around the Earth. The material in this disk then gradually coalesces to form the Moon.
But other simulations produced much more impressive results. Instead of slowly forming from the debris, a completely intact Moon emerged in just a few hours.
Because young protoplanets typically start out hot and gradually cool as they age, this result creates a potentially important link between the timing of the giant impact and how the Moon originally formed.
“Depending on how hot the Earth and Moon were before the collision, the impact could destroy Theia and create a huge disk of debris that eventually forms the Moon,” Denton said. “But when we used the same parameters as in the original impact simulation – right down to the same temperature structures inside both bodies – an intact moon emerged after about five hours.”
Previous simulations have also produced intact moons, but the new work is the first to demonstrate that material strength and temperature can be critical in determining whether the Moon forms largely intact or gradually assembles from processed debris in the protolunar disk.
New clues about the moon’s birth time
The results could potentially link the Moon’s current properties to the physical state of Earth and Theia billions of years ago.
“These surprising and exciting new results suggest a potential link between the physical properties of the modern Moon, including perhaps its volatile composition, and the thermal state of Earth and Theia during the giant impact,” said Kanup, who was not involved in the study. “This, in turn, could help scientists better control how the Moon is formed.”
However, one main mystery remains unsolved.
The Earth and Moon have remarkably similar compositions, something that giant impact models have long struggled to fully explain. New simulations do not fix this problem.
“Because Earth and Mars formed in the same region of the solar system, they are like brothers and sisters,” Denton said. “The Moon and Earth are more like twins.”
One possibility is that Theia and the proto-Earth formed from material in the same region of the early Solar System. Mars, which has a different composition from both Earth and the Moon, may have formed further away.
A New Window on the Origin of the Moon
By showing that the internal state of Earth and Theia can greatly influence the consequences of their collision, the study gives researchers another way to probe one of planetary science’s greatest mysteries.
“We now know that the geophysical state of Earth and Theia plays a fundamental role in shaping the outcome of the collision,” said study co-author Namiya Baijal, a doctoral student in Asphaug’s group. “This gives us a new way to study impact conditions and what they can reveal about the origin of the Moon.”