1. The Short Answer: A Mars-Sized Object Hit the Young Earth
Scientists call this the giant impact hypothesis. The object that hit Earth is nicknamed Theia, after a figure from Greek mythology. Nobody has ever seen Theia. It is a name for whatever hit us, and NASA describes it as "perhaps a single object the size of Mars" [1].
The timing is only approximate. NASA says rock samples suggest the Moon formed around 60 million years after the solar system began to form [1]. The young Moon was not the calm gray ball we see now. It was covered by a deep ocean of molten rock, a magma ocean, for tens to hundreds of millions of years [1]. Computer simulations, including one by NASA and Durham University, suggest the Moon could have gathered in a matter of hours after the crash. Treat that as "one model's result," because simulations only show what could happen [1].
It is also not known whether Theia was a single object. "Perhaps a single object" is how NASA puts it, which tells you the question is open [1].
2. The Three Older Ideas About Where the Moon Came From
Before the crash idea, people mostly argued about three other explanations [2]:
- Fission: the Moon spun off from a fast-spinning young Earth. George Darwin, a son of Charles Darwin who studied tides with mathematics, proposed this in 1878 [3].
- Capture: the Moon formed somewhere else in the solar system and was caught by Earth's gravity. A 1952 argument by Harold Urey is described in that timeline as the first serious case for it [3].
- Co-formation: Earth and Moon grew side by side from the same cloud of material. Laplace's 1795 explanation for how the solar system began is described in that timeline as the first scientific theory of the Moon's origin, known today as the co-accretion hypothesis [3].
Each idea seemed possible. But when scientists studied Moon rocks, some clues did not fit.
3. How Moon Rocks and a 1984 Conference Changed the Debate
The Apollo missions, NASA's trips that took astronauts to the Moon, brought back 842 pounds (382 kilograms) of Moon rocks and soil. NASA says that after scientists studied them, capture and co-formation fell out of favor [1].
In 1975, William Hartmann and Donald Davis suggested that a huge planetesimal (a small building block of a planet) hitting the early Earth could have thrown out the material that became the Moon [3]. It did not win at once. A review by the planetary scientist David Stevenson says the impact idea became a strong contender after a conference in Kona, Hawaii, on October 13 to 16, 1984. The reason was not a dramatic new discovery. It was that the idea was, for the first time, taken seriously and discussed at length [2]. The same review adds a caution: it was still unclear whether the impact idea fit all the evidence [2].
So the idea won in two steps. First, rock evidence weakened its rivals. Then researchers gave the impact idea real attention and tested it.
4. The Moon-Rock Clues That Fit a Giant Impact
NASA lists several clues from Apollo samples and meteorites [1]:
- Moon rocks look like Earth rocks. Oxygen comes in slightly different forms, called isotopes. Scientists compared how much of each form was in Moon rocks and in Earth rocks, and the match was "too precisely" close to be a coincidence.
- Little of what turns to gas easily. Moon rocks have only small amounts of elements that turn into gas when heated. That fits a violent, very hot event.
- A tiny iron core. Earth's core is about 30 percent of its mass. The Moon's is only about 1.6 to 1.8 percent.
- A magma ocean. Pale rock called anorthosite covers much of the Moon. NASA says it points to a molten ocean hundreds to thousands of kilometers deep.
One simplified picture is that the Moon formed mostly from rocky material near the outsides of Earth and Theia. Such material would look like Earth's rock and hold little iron, because the heavy iron mostly sank into the planets' cores. That is only a way of picturing the idea, and the real reasons are more complicated. NASA still lists the Moon's low supply of easily vaporized elements as a question to be answered [1].
5. Why the Moon Looks So Much Like Earth, and Why That Is a Puzzle
Here is the catch. Different bodies in the solar system usually have different oxygen isotope mixes. If Theia came from elsewhere, and models say the Moon should contain more of Theia than Earth does, the Moon should look somewhat different from Earth. Instead the match is very close [4].
In 2014, Daniel Herwartz and colleagues measured Apollo 11, 12 and 16 samples very carefully and reported a tiny but measurable difference: 12 plus or minus 3 parts per million. They took it as support for the giant impact idea. They also noted that Theia might have resembled enstatite chondrites, a rare kind of meteorite, but said another explanation could produce the small difference [4].
In 2018, Simon Lock, Sarah Stewart and colleagues offered another route. They argued that an even more energetic impact could turn Earth into a new kind of spinning, partly vaporized structure they named a synestia. A Moon formed inside it could share the chemistry of the surrounding vapor, which would explain the match. Their paper says the standard giant impact models "struggle to explain" the Moon's composition and its isotopic similarity to Earth [5]. No synestia has been observed, so this is a model, not a finding.
6. Where Did Theia Go? A 2023 Idea About Earth's Deep Interior
If Theia hit Earth, where did the rest of it end up? In 2023, Qian Yuan and colleagues suggested a surprising place. Deep inside Earth, there are two huge regions, each about the size of a continent, where earthquake waves travel more slowly. They are called large low-velocity provinces. The authors propose these might be buried relics of Theia's mantle. Their simulations show that part of Theia's mantle could have reached Earth's lower mantle, and they estimate it was about 2.0 to 3.5 percent denser than its surroundings, so it could have sunk and collected above the core [6].
This is a hypothesis. Nobody has sampled these regions.
7. What Scientists Still Do Not Know About the Moon's Origin
NASA says it is still not known which formation scenario is correct, and it expects data from the Artemis program to help narrow the choices. It also lists open questions about why the Moon lacks easily vaporized elements, how its two very different faces relate to its birth, and whether models can reproduce the present orbits and spins of Earth and the Moon [1]. Lock and colleagues also say the standard model has trouble with the matching isotopes [5].
So the honest summary is: the giant impact idea explains a lot, but the details, such as the size, speed and number of impactors, are not settled.
8. How You Can Look at the Moon With This Story in Mind
On a clear night, look at the Moon and remember that its surface may once have been a sea of molten rock. If you want to dig deeper, read the NASA page and the abstracts listed below. Ask yourself which clue you find most convincing, and which gap you would try to fill.