The impact of the moon on the sea could have spread to the magma ocean of the Earth

The Earth and its moon are unique in our solar system. The Earth is the only rocky planet to have a large moon and only the Pluto dwarf planet has a moon of a size as similar to that of its host planet. The composition of the Moon is also very similar to that of the Earth, suggesting that they were formed from the same reservoir of materials instead of the moon being formed elsewhere and captured.

This collection of properties has given rise to a number of ideas about moon formation, which have not all adjusted the data in different ways. However, scientists eventually came up with an idea that seemed to be right in the overall situation: a collision between the Earth and an object of the size of Mars occurred at the beginning of the system's history solar, creating a cloud of debris that merged into the Moon. .

While this provided a good understanding of the main features of our two-body system, there were still some subtle differences that were not resolved by the impact model. At present, a team of Japanese researchers has stated that it is possible to address some of these difficulties: to ensure that the impact occurs while the Earth was covered with an ocean of magma in fusion.

Near but not quite correct

Breaking an object the size of Mars on a planet the size of the Earth can easily produce something that looks a lot like the Earth-Moon system. If the impact occurs obliquely, you end up with the right amount of angular momentum in the orbits of both bodies. These events dump much of the impactor's iron into the Earth's core and the Moon is left with a large amount of material from the Proto-Earth, explaining their similar composition.

But there are some boring details that do not quite solve. For example, the Moon contains more iron oxide than the Earth, suggesting that the two bodies in collision had different compositions. But the reports of the different isotopes we tested show that the Earth and the Moon are almost identical, suggesting that they must have started with an extremely similar composition. Reconciling these two facts was not easy.

But the researchers behind this new work have decided to rethink the physics of collisions rather than the composition of colliding bodies. At the beginning of the history of our solar system, even after the formation of the big planets, there was still a lot of debris. Many of these objects would eventually be swept by the planets, creating a continuous stream of impacts. The energy released by these impacts could be enough to melt the surface of the planet, creating a magma ocean either on a global scale or near the equator. So what happens if the impact of moon formation occurred while the Earth was in an ocean phase magma?

To find out, the researchers had to update the software used to model these impacts. This tends to treat the two objects involved in the impact as collections of solid materials. Thus, they have modified the existing code so that it can handle the density limits, which appear at the boundary between the ocean magma and the solid mantle below. Once that was put in place, they started breaking things to each other.

A big splash

At a typical impact, the smaller body separates, forming a bow that looks a little like an orange skin that opens (without orange inside). The nucleus of the impactor eventually leaves most of the rest of the planet and forms clusters that fall into the Earth, migrating into the nucleus and leaving debris poor in iron. Meanwhile, much of the magma ocean is peeled off the Earth and eventually forms a bomb that drives and drags the remnants of the impactor. Shortly after, another part of the impactor splashes the remnants of the magma ocean and puts it into orbit around the remnants of the Earth. In the end, almost half of the ocean magma ends up being ejected into space.

The research team conducted several analyzes, with various initial conditions, to get an idea of ​​the range of results that these collisions could produce. To be a realistic solution, the collisions must produce a system with the correct kinetic momentum, the correct mass and a chemistry that corresponds to what we know about the distribution of elements in both bodies. This latter condition includes elements such as a core rich in iron on Earth, an excess of iron oxide in the Moon, and similar isotope ratios.

Two of these problems are not. Most collisions end up with the majority of ferrous metals on Earth and have all produced the correct angular momentum in the final system. This means that a wide range of conditions is consistent with these physical constraints on the model. The mass was less limiting, the results ranging from about half the mass of the moon to 1.4 times the mass of the moon. Generally, you get a larger debris disk if the collision is oblique and at a reduced speed, which slightly limits the conditions of the collision.

But a key difference in the simulations was the presence of an ocean of magma. Simulations showed that the presence of the ocean magma caused a radical change in the warming of shocks during the collision. If the ocean magma is deeper than 500 km, much of the ocean is put into orbit. This is partly because the already hot magma ocean is heated more efficiently by collision. As more and more proto-Earth is put into orbit, an upper fraction of the moon ends up being composed of this material – or, more precisely, of the ocean magma material.

The higher fraction of Proto-Earth materials means that the Moon is formed with an isotopic fraction similar to that of the Earth. In addition, iron oxide has a relatively low melting point, which should place more in the ocean magma. This explains the relative abundance of this material by the moon. Having more of the moon from the proto-Earth means that the stresses on the object that collided with it are not as tight. As a result, the researchers suggest that the impactor could have come from two different classes of starting material, rather than the same material as that of the Earth.

So, the new idea seems to take the dominant explanation of the formation of the Moon – a giant impact – and refines it a bit. In so doing, it corrects some of the inconsistencies between the existing impact models and the data we have developed over the past decades. In many ways, this provides an excellent example of the typical workings of science: rather than rejecting ideas when they do not quite fit the reality, scientists generally refine ideas by tweaking them to better match to the data.

Nature Geoscience, 2019. DOI: 10.1038 / s41561-019-0354-2 (About DOIs).