Earth and its moon are unique in our solar system. The Earth is the only rocky planet with a large moon, and only the dwarf planet Pluto has a moon that is so similar in size to its host planet. The Moon is also remarkably similar to the Earth in terms of its composition, suggesting that they formed from the same set of materials instead of the Moon forming elsewhere and having been captured.
This collection of properties led to a series of ideas about how the Moon formed, and all failed to tweak the data in several ways. Eventually, however, the scientists came up with an idea that seemed to have most of the correct picture: a collision between Earth and an object the size of Mars happened early in the history of the Solar System, creating a cloud of debris that merged on the moon. .
Although this had the main features of our two-body system, there were still some subtle differences that were not resolved by the impact model. Now a team of Japanese researchers says there is a way to sort out some of these loose ends: to have the impact occurring while the Earth was covered by an ocean of melted magma.
Close, but not right
Crushing an object the size of Mars on a planet the size of the Earth can produce something that closely resembles the Earth-Moon system. If the impact occurs obliquely, you end up with the right amount of angular momentum in the orbits of the two bodies. These events dump much of the impactor's iron into the Earth's core, and the Moon ends up with a lot of proto Earth material explaining its similar composition.
But there are some irritating details that do not work well. For example, the Moon has more iron oxide than Earth, suggesting that the two bodies that collided had different compositions. But the proportions of the various isotopes we tested show that the Earth and Moon are almost identical, suggesting that they must have begun with an extremely similar composition. Reconciling these two facts was not easy.
But the researchers behind the new paper 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 main planets, there were still many debris left. Many of these objects would eventually be swept away by the planets, creating a steady stream of impacts. The energy released by these impacts could be enough to melt the surface of the planet, creating an ocean of magma, either globally or near the equator. So what if the impact of Moon formation occurred while the Earth was in a magma ocean phase?
To find out, the researchers first had to update the software used to model those impacts. This tended to treat the two objects involved in the impact as collections of solid material. So they modified the existing code so that it could deal with density limits, which occur at the boundary between the magma ocean and the solid mantle below it. With that in place, they began to crush things to each other.
A big splash
In a typical impact, the smaller body collapses, forming an arch that is a bit like an orange peel opening (without an orange inside). The core of the impactor eventually leaves most of the rest of the planet and forms clumps that fall to Earth, migrating to the core and leaving the debris poor in iron. Meanwhile, much of the magma ocean is plucked from Earth, resulting in a jet that carries and trails the remains of the impactor. Shortly after, another cluster of impactor splashes into the remains of the magma ocean, putting more of it into orbit around the remains of the Earth. In the end, almost half the ocean of magma ends up being ejected into space.
The research team ran several executions, with varying initial conditions, to get an idea of the variety of outcomes these collisions could produce. To be a realistic solution, collisions need to produce a system with the right angular momentum, the right mass, and a chemistry that matches what we know of the distribution of elements in the two bodies. The latter condition includes things like an iron-rich core on Earth, excess iron oxide on the moon, and similar isotope ratios.
Two of these are no problems. Most collisions wipe out most of the metal iron on Earth, and all of them produced the correct angular momentum in the final system. This means that a huge variety of conditions is consistent with these physical constraints in the model. Less limiting was the mass, where the results ranged from about half the mass of the Moon to 1.4 times the mass of the Moon. Typically, you get a larger disk of debris if the collision is oblique and with a slower speed, which limits the conditions of the collision.
But a fundamental difference in the simulations was the presence of an ocean of magma. The simulations showed that the presence of the magma ocean causes a dramatic change in the warming of the shock that occurs in the collision. If the magma ocean is deeper than 500 km, much of the ocean is put into orbit. This is partly because the hot magma ocean is heated most efficiently by the collision. Once more of the proto Earth is placed in orbit, a higher fraction of the Moon ends up being composed of this material – or, more specifically, the material of the magma ocean.
The largest fraction of material on the proto Earth means that the Moon formed with an isotopic fraction similar to that of Earth. In addition, iron oxide has a relatively low melting point, which should place more in the magmatic ocean. This explains the Moon's relative abundance of this material. Having more of the Moon originated in the proto Earth means that the constraints on the object that collided with it are not so rigid. As a result, the researchers suggest that the impactor could have originated from two different classes of starting material, rather than the same material that the Earth formed.
So the new idea seems to take the dominant explanation for the Moon's formation – a gigantic impact – and refines it a bit. In doing so, it corrects some of the inconsistencies between existing impact models and the data we have developed over the past few decades. In many ways this provides a prime example of how science normally operates: instead of discarding ideas when they do not match reality, scientists often refine ideas by adjusting them to better match the data.
Natural Geosciences, 2019. DOI: 10.1038 / s41561-019-0354-2 (About DOIs).