According to the leading theory for the formation of the Earth-Moon system (the Giant Impact Hypothesis), a Mars-sized object (called Theia) collided with a proto-Earth 4.5 billion years ago. This caused both objects to turn into molten lava, which eventually flowed together and cooled to form the Earth and the Moon. Over time, the moon migrated outward, eventually reaching its current, tidally locked orbit around Earth, with one side constantly facing us. For decades, scientists have debated where Theia might have originated, whether it formed in the inner or outer solar system.
According to new research from the Max Planck Institute for Solar System Research (MPS), Theia and Earth were actually “neighbors” (in the cosmic sense). As they argue in a recent paper, Theia likely originated in the inner solar system. Their analysis is based on studying the ratios of iron isotopes in lunar rocks returned by Apollo astronauts compared to those found on Earth. This allowed the team to narrow down Theia’s composition and trace its origins.
The team was led by Timo Hopp, a geoscientist and laboratory leader at MPS and the Department of Geophysical Sciences at the University of Chicago. He was joined by colleagues from both institutions as well as researchers from the Department of Earth and Planetary Sciences at the University of Hong Kong, the CNRS Laboratoire Magmas et Volcans at the Université Clermont Auvergne, and the Department of Earth and Environmental Sciences at Michigan State University. Their study, “The moon-forming impactor Theia came from the inner solar system,” appeared Nov. 20 in the journal Science.
Scientists first began to suspect that the Earth and the Moon formed together when Apollo astronauts brought lunar rock samples back to Earth. These rocks showed that the Moon’s crust was very similar to the Earth’s crust and was composed predominantly of silicate minerals and metals. Further experiments with seismometers showed that the moon also had a similar structure, consisting of a silicate crust and mantle and an iron-nickel core. This led to the Giant Impact hypothesis, but questions remained about Theia’s size, composition, and its place of origin in the solar system.
Given Theia’s catastrophic fate and the passage of time, answering these questions is a major challenge. Fortunately, traces of Theia can still be found in the rocks of today’s Earth and the Moon, which scientists compare to determine whether they come from Theia. That’s exactly what Hopp and his colleagues did, studying iron isotopes in 15 terrestrial rocks and six lunar samples and comparing them with each other and with several meteorites. The ratios of these isotopes in a body can reveal where it formed, as scientists believe isotopes of different elements were likely not evenly distributed in the early solar system.
In short, it is theorized that billions of years ago, when the planets were still forming, isotopes of iron, silicon, carbon and other building blocks in the outer solar system occurred in different ratios than those found closer to the Sun. In addition to iron isotopes, the team also considered those of chromium, molybdenum and zirconium. Co-author Nicolas Dauphas from the University of Chicago and the University of Hong Kong said:
These elements have different affinities for metal and are therefore distributed in different proportions in planetary mantles. That’s why gold is so rare and valuable! They give us access to different phases of planet formation.
Using mass balance calculations with unprecedented accuracy, their results showed that Earth and the Moon have indistinguishable mass-independent iron isotope compositions. This supports previous isotope ratio measurements for other elements, including chromium, calcium, titanium and zirconium, which had already shown that the Earth and the Moon were indistinguishable in this regard. Unfortunately, these similarities do not allow direct conclusions to be drawn about Theia, as there are too many possible collision scenarios and unanswered questions about how the collision redistributed its material.
While most models assume the moon was formed almost entirely from material from Theia, it may be composed primarily of material from the mantle of proto-Earth. There is also the possibility that material from Earth and Theia has mixed to the point where it becomes inseparable. However, the team’s results allowed them to consider scenarios based on different compositions of Proto-Earth and Theia, as well as different sizes of Theia. This allowed them to get a clearer picture of the planets and the impact event based on how it shaped the Earth-Moon system we see today.
The best-fitting scenario was that both Earth and Theia came from the inner solar system. This was supported by the team’s study of meteorites. Because they are essentially material left over from the formation of the solar system (and from different classes formed in different regions), meteorites can shed light on what building materials were available in the early solar system. Based on the team’s analysis, they determined that Theia likely formed closer to the sun than our planet.
“The most compelling scenario is that most of Earth and Theia’s building blocks come from the inner solar system,” Hopp said. “Earth and Theia were probably neighbors.”
These results provide additional clues about what the early solar system looked like. Between the giant impact that created the Earth-Moon system, the late heavy bombardment, and the way objects from the outer solar system migrated inward (causing collisions along the way), it’s safe to say that our solar environment was a harsh and violent place! But from this destruction and recombination came our planet and its only natural satellite, which eventually gave rise to life as we know it. The famous words of Pablo Picasso come to mind: “Every act of creation is first an act of destruction.”
Further reading: Max Planck Institute for Solar System Research
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