More than four decades ago, geophysicists made an intriguing discovery. In the 1980s, they found two continent-sized blobs of unusual material deep near the center of the Earth. One beneath the African continent and one beneath the Pacific Ocean. According to the California Institute of Technology (Caltech), each blob is twice the size of the Moon and likely composed of different proportions of elements than the mantle surrounding it.
Now, in a new study, led by Caltech researchers and published in the journal Nature on 1 November, scientists suggest that these blobs (formally known as large low-velocity provinces or LLVPs) are remnants of an ancient planet that violently collided with Earth billions of years ago in the same giant impact that created our Moon.
For decades, there have been lingering questions about Theia, a hypothesized ancient planet, and the true origins of the Moon – some of the greatest mysteries that have captured scientific minds for years. The new study, in fact, proposes an answer to this planetary science mystery. Researchers have long hypothesized that the Moon was created in the aftermath of a giant impact between Earth and a smaller planet dubbed Theia, but no trace of Theia has ever been found in the asteroid belt or in meteorites. This new study suggests that most of Theia was absorbed into the young Earth, forming the LLVPs, while residual debris from the impact coalesced into the Moon, a news release from Caltech explains.
The research was led by Qian Yuan, O.K. Earl Postdoctoral Scholar Research Associate in the laboratories of both Paul Asimow (MS '93, PhD '97), the Eleanor and John R. McMillan Professor of Geology and Geochemistry; and Michael Gurnis, the John E. And Hazel S. Smits Professor of Geophysics and Clarence R. Allen Leadership Chair, director of Caltech’s Seismological Laboratory, and director of the Schmidt Academy for Software Engineering at Caltech.
According to Caltech, scientists first discovered the LLVPs by measuring seismic waves traveling through the earth. “Seismic waves travel at different speeds through different materials, and in the 1980s, the first hints emerged of large-scale three-dimensional variations deep within the structure of Earth. In the deepest mantle, the seismic wave pattern is dominated by the signatures of two large structures near the Earth's core that researchers believe possess an unusually high level of iron. This high iron content means the regions are denser than their surroundings, causing seismic waves passing through them to slow down and leading to the name 'large low velocity provinces,'” the release explains.
In 2019, Yuan attended a seminar about planet formation by Mikhail Zolotov, a professor at Arizona State University, in 2019. Zolotov presented the giant-impact hypothesis, while Qian noted that the Moon is relatively rich in iron. Zolotov added that no trace had been found of the impactor that must have collided with the Earth, the Caltech release explains.
That is when Yuan had a "eureka moment" and “realized that the iron-rich impactor could have transformed into mantle blob.”
Yuan then worked with multidisciplinary collaborators to model different scenarios for Theia's chemical composition and its impact with Earth. The simulations confirmed that the physics of the collision could have led to the formation of both the LLVPs and the Moon, the release adds.
The next steps, the researchers say, are to examine how the early presence of Theia's heterogeneous material deep within the earth might have influenced our planet's interior processes, such as plate tectonics.