On a crucial day about 4.5 billion years ago, a Mars-sized object known as… Thea With the primordial Earth, turning them into a molten mass of rocks and minerals. After the collision debris is collected, two distinct bodies remain bound together in orbit: the Earth and its moon.
But where did it come from? Thea? It appears to have originated here in the warmer, more comfortable inner solar system. In fact, Theia and Primordial Earth might be neighbors!
More precisely, Theia may have originated closer to the Sun than we are today, and may have been closer than most of the matter that gathered to form our nascent planet.
These insights come from a recent study led by researchers from the Max Planck Institute for Solar System Research and the University of Chicago.
Researchers analyzed samples from Earth, the Moon, and meteorites to examine the ratios of several isotopes, which are lighter or heavier forms of a particular element that contain fewer or more neutrons in their nuclei.
“The composition of any object preserves the entire history of its formation, including its place of origin,” explains Max Planck Institute cosmochemist Torsten Kleine.
Over time, the materials that make up the planet as it cools are distributed in a way that depends on their different masses, melting points, solubility, and tendencies to react with other minerals. For example, iron and zirconium are observed in different concentrations across the Earth’s layers.
The iron, along with the iron-loving metal molybdenum, would have quickly sunk deep into Earth’s primordial core, collecting as if they were precious gems. As for zirconium, it remained in the mantle throughout the Earth’s existence and did not sink into the core.
Therefore, it seems logical that most of the iron now in the Earth’s mantle arrived after the planet was reconstituted, perhaps brought in by a massive, world-destroying cosmic impact.
But where did this body that carried the same iron come from?
Comparing isotope ratios in different parts of the solar system allowed researchers to derive a possible list of Theia’s components and trace their origin.
Variations within the giant molecular cloud that formed the Sun and its planetary disks billions of years ago also helped accumulate different elements and isotopes.
These ratios stayed in place, effectively providing a chemical fingerprint for any object born from matter at that location.
The Moon’s chemical signature of iron, chromium, calcium, titanium and zirconium matches those found on Earth, prompting researchers to look elsewhere in the solar system for distinct isotope ratios.
Meteorites are of specific origin and, thanks to their pristine state, serve as cosmic time capsules.
As for meteorites coming from the inner solar system, or from the planetary disk that forms the planets, they are known as non-carbonaceous (NC) meteorites. It is rocky and carbon and other volatiles have been removed from it as a result of its proximity to the sun.
As for meteorites coming from the outer solar system, they are known as carbonaceous chondrites (CC). They formed in cooler environments, making them rich in carbon, while retaining water within them.
Generally, the isotope ratios in the Earth’s mantle match those of meteorites from the inner solar system. However, the isotopes that the researchers assigned to Theia have ratios that were not previously known and do not match the building blocks of Earth.
“The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner solar system,” concludes lead author and Max Planck Institute geoscientist Timo Hoppe.
Earth and Theia were likely neighbors. The rest, as they say, is history: The great collision between the two neighbors gave us our Moon, which has been moving away from Earth ever since, and is currently moving away from our planet at a slow rate of about 3.8 centimeters per year.