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Collision may have formed the moon in mere hours

NASA originally published this article on October 4, 2022. Edits by EarthSky.

Collision may have formed the moon in mere hours

Billions of years ago, an object about the size of Mars – called Theia – smashed into the early Earth, forming the moon. How exactly that formation occurred is a scientific puzzle researchers have studied for decades, without a conclusive answer. Most theories claim the moon formed out of the debris of this collision, coalescing in orbit over months or years. But on October 4, 2022, NASA announced a new simulation that puts forth a different theory. It shows the moon may have formed immediately, in a matter of hours, when material from the Earth and Theia launched directly into orbit after the impact.

Jacob Kegerreis is the lead author at NASA’s Ames Research Center. His team published their study in the peer-reviewed journal The Astrophysical Journal Letters on October 4, 2022. Kegerreis said:

This opens up a whole new range of possible starting places for the moon’s evolution. We went into this project not knowing exactly what the outcomes of these high-resolution simulations would be. So, on top of the big eye-opener that standard resolutions can give you misleading answers, it was extra exciting that the new results could include a tantalizingly moon-like satellite in orbit.

The simulations used in this research are some of the most detailed of their kind. They operate at the highest resolution of any simulation run to study the moon’s origins or other giant impacts. This extra computational power showed that lower-resolution simulations can miss out on important aspects of these kinds of collisions. The new simulations allowed researchers to see new behaviors emerge in a way previous studies just couldn’t see.

The Earth and moon have similar makeup

Understanding the moon’s origins requires using what we know about the moon. Our knowledge of its mass, orbit, and the precise analysis of lunar rock samples are key to creating scenarios for what we see today.

Previously prevailing theories could explain some aspects of the moon’s properties quite well, such as its mass and orbit, but with some major caveats. One outstanding mystery has been why the composition of the moon is so similar to Earth’s. Scientists can study the composition of a material based on its isotopic signature. An isotopic signature is a chemical clue to how and where an object formed. The lunar samples scientists have been able to study in labs show very similar isotopic signatures to rocks from Earth, unlike rocks from Mars or elsewhere in the solar system. This makes it likely that much of the moon’s material originally came from Earth.

In previous scenarios, Theia sprayed out into orbit and mixed with only a little material from Earth. These scenarios showed it’s less likely we’d see such strong isotopic similarities. That is, unless Theia was also isotopically similar to Earth, which would be an unlikely coincidence. The new theory uses more Earth material to create the moon, particularly its outer layers, which could help explain the similarity in composition.

There have been other theories proposed to explain these similarities in composition. One is the synestia model. In this model, the moon formed inside a swirl of vaporized rock from the collision. But theories such as these struggle to explain the moon’s current orbit.

A speedier theory

The new faster, single-stage formation theory offers a cleaner and more elegant explanation for both these outstanding issues. Also, it could give new ways to find answers for other unsolved mysteries. This scenario can put the moon into a wide orbit with an interior that isn’t fully molten. And that could potentially explain properties like the moon’s tilted orbit and thin crust. Which is why it’s one of the most enticing explanations for the moon’s origins yet.

Getting closer to confirming which of these theories is correct requires analysis of future lunar samples. This is something NASA’s future Artemis missions can provide. Samples from other parts of the moon and from deeper beneath the moon’s surface will allow scientists to compare how real-world data matches up to these simulated scenarios. In addition, it will help explain how the moon evolved over billions of years.

Collision may have formed the moon: Large blob with trail of material and smaller blob at end.
View larger. | New simulations show that the moon could have formed within hours during a massive collision with Earth. Image via NASA.

A shared origin

On the one hand, these simulations help us learn more about the moon. And on the other hand, they bring us closer to understanding how our own Earth became the life-harboring world it is today.

Vincent Eke, a researcher at Durham University and a co-author on the paper, said:

The more we learn about how the moon came to be, the more we discover about the evolution of our own Earth. Their histories are intertwined and could be echoed in the stories of other planets changed by similar or very different collisions.

Collisions are common in the cosmos. Impacts are an essential part of how planetary bodies form and evolve. On Earth, we know that the impact with Theia and other changes throughout its history are part of how it was able to gather the materials necessary for life. The better scientists can simulate and analyze what’s at play in these collisions, the more prepared we are to understand how a planet could evolve to be habitable like our own Earth.

Bottom line: New simulations from NASA show that a collision may have formed the moon in mere hours. Watch a video of the simulation and read more.

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