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Leftovers from the moon鈥檚 formation may have tunnelled to the Earth鈥檚 core, and make up far more of our planet than we once thought.

According to earlier estimates about 0.5 per cent of our planet鈥檚 mass came from swallowing parts of planetesimals, 1000-kilometres across, just after the 鈥淏ig Splash鈥� that formed the moon. Now, simulations by researchers at the Southwest Research Institute (SWRI) in Boulder and the University of Maryland have upped that percentage to between 1 and 2.5 per cent.

That may not sound like much, but it helps explain something odd about the mantle, the part of Earth鈥檚 interior between the crust and the core. Earth鈥檚 mantle has a lot of siderophile elements, metals with a chemical affinity for iron. That is unexpected since iron should sink to the Earth鈥檚 core.

鈥淲e realised that there are many cases where the core of the impactor does not end up being well mixed with mantle of the Earth,鈥� says at the SWRI, who led the study. Previously, many scientists assumed the siderophile elements would stay suspended and evenly distributed in the mantle, instead of ending up deep in the core.

Core breaches

The story begins 4.5 billion years ago, when Earth had barely cooled off from its initial formation. A world the size of Mars, called Theia, collided with Earth and sent debris mostly from the mantle into space. This formed the moon.

For the next few hundred million years after that, until between 3.9 and 3.7 billion years ago, other large bodies kept smacking into the Earth. Some of those impactors were so large they had their own iron cores.

Using known models of impacts Marchi and his team simulated the after-effects of large impacts on Earth and found that much of the time, the core material of the body smashing into the planet was either flung into space or tunnelled straight to the Earth鈥檚 core. Not much ended up in the mantle.

Mix and match

Earth鈥檚 mantle doesn鈥檛 have much of this material, there would need to have been many more impactors than we previously thought to deliver those elements to get the concentrations we see today.

The study also found that previous assumptions about materials mixing thoroughly in the mantle are wrong.

鈥淚f you accept the idea that large collisions would result in heterogeneous mixing, and there are chunks contaminated with more impactor material than other parts of the Earth, then maybe this heterogeneous mix could have an effect in terms of tungsten isotopes,鈥� Marchi says.

A lack of even distribution in tungsten isotopes is clearly visible in Greenland, where some 3.7 billion year old rocks were found to have a higher ratio of tungsten-182 to tungsten-184 than Earth鈥檚 average.

at Arizona State University in Tucson says Marchi鈥檚 model 鈥� good as it is 鈥� won鈥檛 be the last word. 鈥淚鈥檓 not ready to hang my hat on any of these models,鈥� as there are assumptions built into the simulations about the behaviour of iron cores that might not be completely correct, he says.

Read more: The moon has hundreds more craters than we thought