The Earth formed from local building blocks
Planetary scientists have long debated where the material that formed our Earth comes from. Despite its location in the inner Solar System, they consider it likely that 6–40 per cent of this material must have come from the outer Solar System, i.e., beyond Jupiter.
For a long time, material from the outer Solar System was considered necessary to bring volatile components such as water to Earth. Accordingly, there must also have been an exchange of material between the outer and inner Solar Systems during the formation of the Earth. But is that really true?
“We were truly astonished”
Planetary scientists Paolo Sossi and Dan Bower, from ETH Zurich, compared existing data on the isotopic ratios of a wide range of meteorites, including those from Mars and the asteroid Vesta, with those of Earth. Isotopes are sibling atoms of the same element (same number of protons) that have a different mass (different number of neutrons).
The researchers analysed this data in a new way and arrived at a surprising conclusion: the material that makes up Earth originates entirely from the inner region of the Solar System.
Material from the outer Solar System, by contrast, is likely to account for less than two per cent of Earth’s mass, or even nothing at all. The corresponding study has just been published in the journal Nature Astronomy.
“Our calculations make it clear: the building material of the Earth originates from a single material reservoir,” says Sossi. His colleague Bower adds: “We were truly astonished to find that the Earth is composed entirely of material from the inner Solar System distinct from any combination of existing meteorites.”
For their study, the ETH researchers used existing data on ten different isotopic systems from meteorites, and analysed them using a specialised statistical method. Previous studies have mostly considered only two isotopic systems.
“Our studies are actually data science experiments,” says Sossi. ‘We carried out statistical calculations that are rarely used in geochemistry, even though they are a powerful tool.’
Isotope signature reveals origin
Isotopes in meteorites have long been used by researchers to determine the origin of celestial bodies, i.e. which part of the Solar System they come from. Historically, however, only the various isotopes of the element oxygen could be used to determine their provenance.
It was not until the early 2010s that an American researcher discovered that other isotopes, such as of chromium and titanium, could also be used for this purpose. This has enabled researchers to classify meteorites into two categories: non-carbonaceous ones, which form exclusively in the inner Solar System, and carbonaceous ones, which contain more water and carbon and originate in the outer Solar System.
The new analysis reveals that the Earth is composed entirely of non-carbonaceous material. No evidence for the previously suspected exchange between the outer- and inner solar system reservoirs was found.
Therefore, the Earth grew within a relatively static system, incorporating its smaller neighbouring planets as it grew. This also implies that most volatile elements, such as water, must have already been present in the inner Solar System.
Jupiter acts as a material barrier
But why are there two distinct material reservoirs in our Solar System? Researchers assume that our Solar System split into two reservoirs during its formation due to Jupiter’s rapid growth and size. The gravity of the gas giant tore a gap in the protoplanetary disc orbiting the young Sun. These discs are ring-shaped and consist of gas and dust; they are the birthplace of planets. Jupiter prevented material from the outer solar system from entering the inner region. However, the extent to which this barrier was permeable remained unclear until now.
In their new analysis, the two ETH researchers demonstrate that almost no material from beyond Jupiter flowed towards Earth. “Our calculations are very robust and rely solely on the data itself, not on physical assumptions, as these are not yet fully understood,” Bower emphasises. The analysis also shows that Earth's material composition is similar to that of Vesta and Mars.
The researchers also suspect that Venus and Mercury lie on the same line. “Based on our analysis, we can theoretically predict the composition of these two planets,” says Paolo Sossi. However, he cannot verify this analytically, as no rock samples from Mercury and Venus, which are the two innermost planets in the Solar System, are currently available to the researchers.
New light on the formation history
“Our results shed new light on the formation history of our Earth and the other rocky planets,” says Sossi.
Sossi and his team intend to follow up by investigating why there was sufficient water in the hot, inner Solar System to form the Earth’s oceans. Furthermore, they will examine whether these processes can be applied to exoplanetary systems.
“Until then, however, Dan and I will have to engage in many heated debates about the material composition of Earth and its neighbouring planets, because the scientific discourse over the building blocks of Earth is far from over, despite the new findings,” says Sossi.
IMAGE: This is roughly what the formation of the Earth in our solar system might have looked like. The birth of two planets (light brown dots) in a protoplanetary disc around the young star WISPIT 2 Credit ESO / Lawlor C et al.
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