The atomic makeup of Earth’s iron may shatter previously-held theories about how the planet formed.

Geologists from UT and several other institutions discovered that the unusual ratio of Earth’s different varieties of iron is not due to formation of Earth’s core in its early years. These findings may discredit one previous theory on how Earth was formed. The researchers published their findings in Nature Communications on Feb. 20. 

Iron is common in the solar system and plays an important role in the formation of planets. Researchers often study the variation of iron isotopes, which are iron atoms with different atomic masses, to better understand planetary formation. In contrast with meteors and other planets, the crust under Earth’s oceans contains more heavy iron isotopes than light isotopes by 0.1 percent. Although the margin may seem small, this is enough to distinguish Earth from other rocky planetary bodies such as Mercury, Mars, the moon and asteroids, said Jin Liu, first author of the study, postdoctoral researcher at Stanford University and UT geological sciences alumnus.

“The Earth’s iron composition in particular is very different from the iron isotopic composition of other planets (in the solar system) and meteorites,” said Jung-Fu Lin, UT associate professor of geosciences and last author of the study. “The difference is actually very, very significant.” 

Several theories exist that could explain why Earth has more heavy iron isotopes, Lin said. One popular theory says Earth’s large size, along with certain temperature and pressure conditions, resulted in more heavy iron isotopes in the mantle. The core was thought to be made up of lighter iron isotopes bonded to each other as well as other metals.

“A popular hypothesis suggests that isotopes (were divided) between the core and the mantle during the core-forming event,” Liu said. “In this context, the large size of the Earth, implying high-pressure equilibration between the metallic and the rocky reservoirs, would be the origin of its unique signature.”

Researchers used a small diamond anvil to place iron-bearing materials and silicate rocks under the pressure of up to one million times the Earth’s atmosphere, effectively simulating the environment of Earth’s interior. Unexpectedly, they found the bonds between iron and other metals strengthened and the iron isotopes did not separate. Heavy isotopes did not rebond with mantle elements and light isotopes did not rebond with core elements. Theories on early Earth formation cannot explain the planet’s heavy iron isotopes, Lin said.

“The origin of the iron isotope anomaly in our planet remains a mystery,” Liu said. “The nature of its building blocks that accreted 4.5 billion years ago may play a major role, but more studies are needed on that.”

Lin said the diamond anvils used were a particularly interesting part of the study.

“People are always really intrigued about the use of diamonds to study Earth’s interiors,” Lin said. “They’re called nature’s gift because natural diamonds were formed deep inside the earth’s interior in the upper mantle and brought up to Earth’s surface. We found them and made them into certain shapes for our research here to reveal the nature of earth’s interior.”

Researchers will work on investigating other elements found in Earth and finding ways to simulate the environment of the interior, as well as learning more about Earth’s iron, Lin said.

“Our research really calls for a new answer to the iron composition anomaly,” Lin said.