The formation of the Moon had many hypothesis over the years, most of them involving a Mars-sized body called Theia, which presumably collided with the Earth. But even within the Theia hypothesis, is not very clear the conditions of that collision. So, what is the most plausible hypothesis about Theia today?

  1. Theia fully disintegrates and the remnants debris form the Moon in a head-on collision(the Big Impact hypothesis)

  2. Theia-Earth suffer a glazing collision and Theia manages to escape to space, but some of the remnants of the collision form the Moon (Reufer, 2012)

  3. Other hypothesis, such a double Theia system

All the hypothesis have problems, but to me the most believable is 2, because it solves the majority of the problems of the other ones, although there's not much consensus on that one, the only who defends it is Reufer.

For example, some research shows that only 2% of Theia actually contributed to the formation of the Moon (I don't remember the reference), so 1 doesn't seem too much feasible because according to that one Theia fully disintegrates.

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    $\begingroup$ I believe the commonly accepted hypothesis most closely aligns with 1, although you appear to have a misconception about it. Theia would not have "fully disintegrated". What does that even mean? The mass from Theia would have had to go somewhere. It can't just disintegrate into nothing. The Giant Impact Hypothesis posits that a large amount of the mass from Theia simply combined with the Earth with the rest either being ejected from Earth's gravity or else orbiting the combined Earth/Theia and eventually coalescing into the Moon or impacting the Earth again. What's not believable about that? $\endgroup$
    – zephyr
    Commented Jul 17, 2017 at 13:50
  • $\begingroup$ It's not that Theia disintegrated into nothing, but fully disintegrates into debris, that it's to say, all Theia turned into debris, some evaporated and some not. But 2 says a "fragment" of Theia is pushed out into space, therefore is not entirely torn apart. $\endgroup$ Commented Jul 17, 2017 at 18:26
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    $\begingroup$ But that's not what the Giant Impact hypothesis says. The Giant Impact hypothesis says most of Theia immediately combined with and became a part of the Earth. Only a small amount of the debris would've been available to form the Moon. $\endgroup$
    – zephyr
    Commented Jul 17, 2017 at 18:58
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    $\begingroup$ You have to be careful with #1 when you say head on in #1. The more common assumption is that Theia hit at an oblique angle, which would explain Earth's rapid rotation after the impact. Between your 1 (head-on) and 2 (grazing) there's hitting at an angle and while that's not the only possibility it may still be the leading theory. $\endgroup$
    – userLTK
    Commented Jul 17, 2017 at 19:24
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    $\begingroup$ Why does it feel like you are not here to learn the answer but to try to push the answer you want to hear? Theia and proto-Earth would have both been one hot mess after the collision, why are you so caught up on the debris that came from Theia? $\endgroup$ Commented Dec 1, 2017 at 0:05

2 Answers 2


According to new simulations, a better model seems to be one where the Earth had a magma ocean at the time of impact, see here:

We perform density-independent smoothed particle hydrodynamic collision simulations with an equation of state appropriate for molten silicates. These calculations demonstrate that, because of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean, even in a highly oblique collision. We show that this model reconciles the compositional similarities and differences between the Moon and Earth while satisfying the angular momentum constraint.


As I recall, when Theia collided with Earth it created oceans, and the modern atmosphere, which led to life. As a side-effect, a ring was created around the Earth, with colliding rocks forming two moons. That's right; Earth had two moons 4 billion years ago. Eventually, they collided to form the modern moon.

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    $\begingroup$ The two moon theory came about because the Moon, like Mars but even more so, has a thicker crust on one side than the other. There's a more recent theory that the crustal variation was due to heat from the very hot planet earth after impact. The side of the moon closer to the Earth was partly vaporized by the Earth's heat. earthsky.org/space/dark-side-of-the-moon-mystery-solved $\endgroup$
    – userLTK
    Commented Oct 2, 2017 at 1:32
  • $\begingroup$ What? Immediately after the collision the Earth was way too hot to have a liquid ocean. And I suspect it would have lost a lot of atmosphere too. $\endgroup$
    – PM 2Ring
    Commented Dec 31, 2017 at 6:48
  • $\begingroup$ Perhaps both ideas have equal probability for the moment? I thought that the earth's surface cooled faster than the Theia collision ejecta took to merge into a moon? $\endgroup$ Commented Jan 1, 2018 at 21:38

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