What is the exact origin of the Moon? What key evidence supports the currently most accepted 'giant impact hypothesis' in the scientific community?

Okay, no problem! The origin of the Moon is a truly classic and fascinating topic in astronomy. I'll try to explain it clearly in plain language.

What is the exact origin of the Moon? What key evidence supports the currently most accepted 'Giant-Impact Hypothesis' in the scientific community?

Hey there! Your question touches on one of the most captivating and classic topics in astronomy. Scientists have proposed several theories about the Moon's origin, like the "sister theory" (formed alongside Earth), the "capture theory" (snatched by Earth's gravity), and the "fission theory" (spun off from Earth). But over the decades, one truly "earth-shattering" theory has emerged as the dominant view: the Giant-Impact Hypothesis.

Simply put, this hypothesis paints a picture of an incredibly spectacular cosmic "car crash."

Here's the story:

About 4.5 billion years ago, the Solar System was young, and Earth itself was newly formed, still a molten ball of fire. At that time, a "reckless" protoplanet roughly the size of Mars, named Theia, slammed into the primordial Earth at an oblique angle.

The force of this impact was unimaginable! It instantly shattered Theia and also blasted off a huge chunk of Earth's outer layers – its mantle and crust. These shattered, molten rock fragments were flung into orbit around Earth, forming a "debris ring" (similar to Saturn's rings, but made of scorching magma). Over thousands to tens of thousands of years, pulled together by gravity, these fragments gradually coalesced, cooled, and solidified, eventually forming the Moon we see today.

(A simple animation to help visualize the process)

This story sounds cool, but it's not just a story. Scientists have found several "smoking guns" supporting it.

Four Key Pieces of Evidence Supporting the Giant-Impact Hypothesis

This hypothesis became the leading theory because it elegantly explains several long-standing puzzles all at once.

Key Evidence 1: The Moon's "Anemia" – Its Iron-Deficient Core

Observation: Scientific measurements show the Moon is much less dense than Earth. Specifically, the Moon's iron core (the heaviest central part of a planet) makes up a very small proportion of its mass, only about 1-2%. In contrast, Earth's iron core accounts for roughly 30% of its mass. The Moon is severely "anemic."
Why this matters: If the Moon formed alongside Earth as a "sister," their compositions should be similar, and the Moon should have a large iron core. If the Moon was "captured" by Earth, as an independent planet, it should also have a standard-sized iron core.
How the impact explains it: This hypothesis solves the puzzle perfectly! Imagine the impact: the heavy metallic cores (iron) of both Earth and Theia, being the densest material, had already sunk to their centers. The collision primarily ejected the lighter "shells" and "mantles" (mostly rock) of both bodies. These "iron-poor" materials recombined in space to form the Moon. It's like baking a cake: you fling out the flour and butter (mantle and crust), but most of the egg yolk (iron core) stays stuck at the bottom of the bowl.

Key Evidence 2: Earth and Moon's "DNA" – Nearly Identical Oxygen Isotopes

Observation: Scientists analyzing lunar rock samples brought back by the Apollo missions found their "oxygen isotope ratios" are almost identical to those of Earth rocks. You can think of this ratio as a planet's "chemical fingerprint" or "DNA." Within the Solar System, bodies from different places (like Mars or asteroids) have distinctly different "chemical fingerprints."
Why this matters: Earth and the Moon sharing the same "DNA" strongly suggests the Moon's material primarily came from Earth.
How the impact explains it: After the impact, material from Earth's mantle and Theia was vaporized and thoroughly mixed, forming a uniform disk of magma vapor. The Moon, born from this "well-stirred" disk, naturally inherited an oxygen isotope fingerprint almost identical to Earth's.

Key Evidence 3: The Moon's "Dryness" – Lack of Water and Volatiles

Observation: Lunar rocks are extremely "dry," containing almost no water and lacking other easily evaporated "volatile substances" (like sodium, potassium, zinc).
Why this matters: These substances are actually quite common in the universe.
How the impact explains it: The immense heat generated by the cataclysmic impact was enough to "bake off" almost all the water and other volatiles, causing them to evaporate and escape before the Moon formed. It's like using a blowtorch on a wet piece of wood: the water and resins get burned away first, leaving only dry charcoal behind. Only the heat-resistant, refractory materials (mainly silicate rock) could survive and eventually form the Moon.

Key Evidence 4: The Earth-Moon System's "Waltz" – Unique Angular Momentum

Observation: Earth spins relatively fast (though slower now, it was faster early on), and the Moon orbits Earth. The total angular momentum (think of it as the total rotational energy) of the entire Earth-Moon system is unusually large.
Why this matters: The "capture theory" struggles to explain such a large angular momentum, as a gravitationally captured body is unlikely to impart that much rotational energy. The "sister" and "fission" theories also fail to explain it perfectly.
How the impact explains it: A massive, oblique, sideways impact is an excellent way to inject huge amounts of angular momentum into a system. It's like hitting a "side shot" in pool: the cue ball doesn't just move forward, it spins rapidly. Theia's glancing blow precisely set the Earth and the newly forming Moon system spinning in this high-speed "cosmic waltz."

To Summarize

So, while we can't travel back 4.5 billion years to witness it firsthand, the Giant-Impact Hypothesis acts like a brilliant detective. It uses one core event to perfectly connect the seemingly separate clues of the Moon's "iron-deficient core," "shared origin with Earth," "extreme dryness," and "high angular momentum," providing the most logically consistent and well-supported explanation.

Of course, science is always advancing. Researchers continue to refine the model's details using more powerful computer simulations (e.g., How big was Theia? What was the exact impact angle?). But the core framework of the "Giant Impact" remains the most solid foundation we currently have for understanding the Moon's origin.