Scientists have long debated where the raw material that formed Earth originally came from. Although our planet sits in the inner Solar System, many researchers have proposed that 6-40 percent of its building material arrived from the outer Solar System, beyond Jupiter.
That idea has been especially important for explaining how Earth acquired volatile substances such as water. If those materials came from farther out, then matter must have moved between the outer and inner regions of the Solar System while Earth was forming.
New research now challenges that picture.
“We were truly astonished”
Planetary scientists Paolo Sossi and Dan Bower of ETH Zurich compared existing measurements of isotope ratios from a wide range of meteorites, including samples associated with Mars and the asteroid Vesta, with the isotopic composition of Earth.
Isotopes are sibling atoms of the same element (same number of protons) that have a different mass (different number of neutrons).
Using a new approach to analyze those data, the researchers reached a striking conclusion. Their results indicate that the material that formed Earth came entirely from the inner Solar System.
Material from the outer Solar System appears to account for less than two percent of Earth’s total mass, and possibly none at all. The study was published in 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.”
The ETH Zurich team based its analysis on existing measurements from ten different isotope systems found in meteorites. Earlier studies typically focused on only two isotope 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.”
Meteorite Isotopes Reveal Earth’s Origins
Scientists have used isotopes in meteorites for decades to trace where celestial objects formed, including which region of the Solar System they came from. For many years, however, oxygen isotopes were the main tool available for identifying that origin.
That changed in the early 2010s, when an American researcher showed that isotopes of other elements, including chromium and titanium, could also reveal where meteorites formed.
This discovery allowed scientists to divide meteorites into two broad groups. Non-carbonaceous meteorites formed exclusively in the inner Solar System. Carbonaceous meteorites contain more water and carbon and originated in the outer Solar System.
According to the new analysis, Earth is made entirely from non-carbonaceous material. The researchers found no evidence for the previously proposed exchange of material between the inner and outer Solar System reservoirs.
That suggests Earth grew in a relatively stable environment, gradually incorporating smaller nearby planetary bodies as it developed. It also means that most volatile substances, including water, must already have existed in the inner Solar System.
Jupiter May Have Divided the Solar System
Why did the young Solar System end up with two distinct reservoirs of material?
Scientists think Jupiter played a major role. As the gas giant grew rapidly, its enormous gravity carved a gap in the protoplanetary disc surrounding the young Sun. These discs are ring-shaped and consist of gas and dust; they are the birthplace of planets.
Jupiter appears to have acted as a barrier, limiting the movement of material from the outer Solar System into the inner region. Until now, however, scientists did not know how effective that barrier really was.
The new analysis suggests that very little material from beyond Jupiter ever reached the region where Earth formed.
“Our calculations are very robust and rely solely on the data itself, not on physical assumptions, as these are not yet fully understood,” Bower emphasizes.
The results also show that Earth’s material composition resembles that of Mars and Vesta.
Mercury and Venus May Share the Same Pattern
The researchers suspect that Mercury and Venus could fit the same pattern as Earth, Mars, and Vesta.
“Based on our analysis, we can theoretically predict the composition of these two planets,” says Paolo Sossi.
For now, however, that prediction cannot be directly tested. Scientists do not currently have rock samples from Mercury or Venus, the two planets closest to the Sun, that could be analyzed in the same way.
A New Mystery About Earth’s Water
“Our results shed new light on the formation history of our Earth and the other rocky planets,” says Sossi.
The findings also raise a major new question. If Earth did not receive large amounts of water-rich material from the outer Solar System, scientists must explain how enough water could have existed in the hot inner Solar System to eventually produce Earth’s oceans.
Sossi and his team plan to investigate that question next. They also want to determine whether similar processes could occur in planetary systems around other stars.
“Until then, however, Dan and I will have to engage in many heated debates about the material composition of Earth and its neighboring planets, because the scientific discourse over the building blocks of Earth is far from over, despite the new findings,” says Sossi.
