The Earth and its Moon are very similar in composition, a fact that's neatly explained by the idea that the two formed from the shared debris of a collision between the early Earth and a Mars-sized body. But there are some elements that show a distinctly uneven distribution between the two. If everything else is so similar between the two bodies, why are these exceptional?

It turns out that these elements are more volatile—they remain in a gaseous form at lower temperatures. And a new model of the Moon-forming collision suggests that, in the high temperatures of the collision debris, they remained gaseous until after the Moon moved out of the debris field.

The elements in question are metals that are relatively light—things like potassium, sodium, and zinc. Scientists noticed that the Earth had more of these elements than lunar samples; they suggested that it might have something to do with the fact that they remain gasses at lower temperatures than elements that were more evenly distributed. And there's a simple way to explain this: the Moon formed while the temperatures were too high.

The only problem with this explanation is that we had few ideas of how the Moon could have experience different temperatures than the Earth during its formation. After all, the two condensed from the same debris disk in the aftermath of the collision.

Now, a team of US-based scientists has built a model that shows how this can happen. Or rather, the team put together a small collection of models, which combined to handle the chemistry, heat exchange, and physical movement of all the debris left over from the collision.

Their model shows that the debris disk initially extended on both sides of the Roche limit, the distance at which the Earth's gravity is sufficiently strong that it will tear apart any solid bodies. Obviously, the Moon starts forming outside the Roche limit; nearly half its mass condenses quickly from the outer portion of the debris disk. But smaller objects end up condensing and being ripped apart within the Roche limit. The friction of this process heats up the inner region of the debris disk sufficiently that elements like sodium and zinc remain gaseous and don't condense onto the Moon.

By around 100 years after the collision, several things are happening. Due to gravitational interactions, the Moon has started moving further away from the Earth, stretching out the debris disk behind it. At this point, most of the disk is cool enough (less than 1,100K) that sodium, potassium, and zinc can start to condense. But gravitational interactions between these condensing objects, the Earth, and the Moon send most of them into eccentric orbits. As a consequence of this, the majority of them end up contributing to the Earth.

This creates a switch or, as the authors call it, a "relatively abrupt transition." Prior to 100 years, when much of the Moon's mass condensed, large areas of the debris disk were too hot for these three metals to do so. After 100 years when more of the metals could condense, most of the material ended up falling into the Earth rather than contributing to the Moon.

As a result of this switch, their model produces a situation where the Earth has about a 2.5-fold enrichment of these metals. This is roughly in line with what we see for sodium and potassium, though the figure is too low for zinc. The authors suggest several possible explanations for this. One is that volcanic activity could have caused more zinc to be lost into space long after the Moon had formed. Alternately, the zinc could have been preferentially retained in the core of the Moon. Finally, they note that their chemical model assumes there was no water in the debris disk, which might have influenced the final results.

The model has to be considered incomplete at this point. But the key thing is that it's a big step forward given that it can account nicely for the differences in composition between the Earth and the Moon. It's possible that further iterations of the model could bring it in closer agreement with our data. In the meantime, it could help inform our ideas of the formation of not only our home but of the billions of planets and moons that are present in our galaxy.