The Earth-Moon system is one of the unique features of the solar system. Earth is the only rocky planet with such a large moon, and the only planet besides Pluto to have a moon so fundamentally similar to itself in composition. The Pluto-Charon system is considerably different otherwise from the Earth-Moon system, however, leaving our planet in a category all its own.
The problem is, we’ve never had a completely satisfactory model for how the Moon came into existence in the first place. The Giant Impact Hypothesis has dominated for decades. The theory is that not long after the Earth was formed, a protoplanet roughly the size of Mars slammed into it. That mixed material between the impactor, named Theia (mother of the moon, Selene, in Greek myth), and the Earth.
As we’ve learned more about the Earth-Moon system, researchers have struggled to use this idea to explain every feature of both. Certain features, such as high angular momentum and near-identical isotopic ratios between the Earth and the Moon when most models suggest the Moon should contain a significant amount of material from Theia, have been difficult to resolve, requiring narrow preconditions to explain the relationship we see today. A new theory for how to do so argues that the Moon makes a lot more sense if it hit Earth while the Earth was still partially molten.
One of the difficulties in explaining similarities between the Earth and Moon is that rock is intrinsically hard to mix. It takes a great deal of energy to liquefy it and it’s tough to make certain that the liquefied rock mixes evenly between two impactors. But as researchers are now proposing, if the Earth still had a magma ocean when the impactor hit, some of these issues are resolved. Still-liquid rock is far easier to mix and less overall energy is required to transform the system.
The new theorized impact event timeline with a magma ocean assumed to exist. Click to enlarge.
Running the model with a liquid magma ocean on Earth produces new results in comparison with the old, solid model. Roughly half of the magma motion is ultimately ejected into space, along with some of the original impactor. Iron left over from the collision still sinks to the core of the Earth, as expected, but a greater fraction of Earth material is slung into orbit to fuse with what’s left over from Theia.
The net effect of this model is to widen the total number of collision possibilities that would still create a Moon with the characteristics we see today. Since we obviously can’t travel back in time to observe what actually happened, one important question for the models we use to examine what might have happened is just how narrow they are. If there are many different ways for a given system to have been created, that’s generally regarded as better evidence than betting on a single, highly specific set of circumstances that would lead to a given outcome, not because this is impossible — highly improbable things happen all the time — but because it’s intrinsically harder to validate.
As for where the Earth’s magma ocean would have come from, the Japanese researchers hypothesize that there was a great deal of rock still floating around in the immediate aftermath of the solar system’s formation. If Earth had been struck by smaller-but-still-massive impactors prior to Theia, it could well have possessed a magma ocean. This model isn’t the final word on the Earth-Moon system, but it opens up more possibilities for how it came to exist in the first place.