ICYMI
The remains of an ancient planet lie deep within Earth (VIDEO)
VIDEO:
VISUALIZATION OF THE EARTH WITH LARGE "BLOBS" OF DENSE MATERIAL NEAR EARTH'S CORE. THESE BLOBS WERE DISCOVERED IN THE 1980S. NOW, RESEARCHERS PROPOSE THAT THEY ARE ACTUALLY THE REMNANTS OF AN ANCIENT PLANET, THEIA, THAT COLLIDED WITH EARTH TO FORM THE MOON.
view moreCREDIT: EDWARD GARNERO
In the 1980s, geophysicists made a startling discovery: two continent-sized blobs of unusual material were found deep near the center of the Earth, one beneath the African continent and one beneath the Pacific Ocean. Each blob is twice the size of the Moon and likely composed of different proportions of elements than the mantle surrounding it.
Where did these strange blobs—formally known as large low-velocity provinces (LLVPs)—come from? A new study led by Caltech researchers suggests that they are remnants of an ancient planet that violently collided with Earth billions of years ago in the same giant impact that created our Moon.
The study, published in the journal Nature on November 1, also proposes an answer to another 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.
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.
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."
Yuan, a geophysicist by training, was attending a seminar about planet formation given 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.
"Right after Mikhail had said that no one knows where the impactor is now, I had a 'eureka moment' and realized that the iron-rich impactor could have transformed into mantle blobs," says Yuan.
Yuan 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. Some of Theia's mantle could have become incorporated into the Earth's own, where it ultimately clumped and crystallized together to form the two distinct blobs detectable today at Earth's core–mantle boundary today; other debris from the collision mixed together to form the Moon.
Given such a violent impact, why did Theia's material clump into the two distinct blobs instead of mixing together with the rest of the forming planet? The researchers' simulations showed that much of the energy delivered by Theia's impact remained in the upper half of the mantle, leaving Earth’s lower mantle cooler than estimated by earlier, lower-resolution impact models. Because the lower mantle was not totally melted by the impact, the blobs of iron-rich material from Theia stayed largely intact as they sifted down to the base of the mantle, like the colored masses of paraffin wax in a turned-off lava lamp. Had the lower mantle been hotter (that is, if it had received more energy from the impact), it would have mixed more thoroughly with the iron-rich material, like the colors in a stirred pot of paints.
The next steps 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.
"A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient," Asimow says. "It makes sense, therefore, to investigate next what consequences they had for Earth's earliest evolution, such as the onset of subduction before conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of the very oldest surviving terrestrial minerals."
The paper is titled "Moon-forming impactor as a source of Earth's basal mantle anomalies." Qian Yuan is the first author. In addition to Yuan and Asimow, the additional Caltech coauthor is Yoshinori Miyazaki, Stanback Postdoctoral Scholar Research Associate in Comparative Planetary Evolution. Additional coauthors are Mingming Li, Steven Desch, and Edward Garnero (PhD '94) of Arizona State University (ASU); Byeongkwan Ko of ASU and Michigan State University; Hongping Deng of the Chinese Academy of Sciences; Travis Gabriel of the U.S. Geological Survey; Jacob Kegerreis of NASA’s Ames Research Center; and Vincent Eke of Durham University. Funding was provided by the National Science Foundation, the O.K. Earl Postdoctoral Fellowship at Caltech, the U.S. Geological Survey, NASA, and the Caltech Center for Comparative Planetary Evolution.
Simulation of Theia's collision with Earth (VIDEO)
A detailed simulation of Theia crashing into Earth. While the collision was violent, it was not energetic enough to melt the Earth's lower mantle -- meaning that remnants of Theia could be preserved, rather than mixed homogenously in with the Earth's material.
CREDIT
Hongping Deng
JOURNAL
Nature
ARTICLE TITLE
Moon-forming impactor as a source of Earth’s basal mantle anomalies
ARTICLE PUBLICATION DATE
1-Nov-2023
Heterogeneity of Earth’s mantle may be relics of Moon formation
IMAGE:
THE LARGE LOW VELOCITY PROVINCES (LLVPS) IN THE DEEP EARTH MANTLE MAY BE RELICS OF THEIAN MANTLE MATERIALS.
view moreCREDIT: PHOTO CREDIT: DENG HONGPING AND HANGZHOU SPHERE STUDIO
An interdisciplinary international research team has recently discovered that a massive anomaly deep within the Earth’s interior may be a remnant of the collision about 4.5 billion years ago that formed the Moon.
This research offers important new insights not only into Earth’s internal structure but also its long-term evolution and the formation of the inner solar system.
The study, which relied on computational fluid dynamics methods pioneered by Prof. DENG Hongping of the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, was published as a featured cover in Nature on Nov. 2.
The formation of the Moon has been a persistent enigma for several generations of scientists. Prevailing theory has suggested that, during the late stages of Earth’s growth approximately 4.5 billion years ago, a massive collision—known as the “giant impact”—occurred between primordial Earth (Gaia) and a Mars-sized proto-planet known as Theia. The Moon is believed to have formed from the debris generated by this collision.
Numerical simulations have indicated that the Moon likely inherited material primarily from Theia, while Gaia, due to its much larger mass, was only mildly contaminated by Theian material.
Since Gaia and Theia were relatively independent formations and composed of different materials, the theory suggested that the Moon—being dominated by Theian material—and the Earth—being dominated by Gaian material—should have distinct compositions. However, high-precision isotope measurements later revealed that the compositions of the Earth and Moon are remarkably similar, thus challenging the conventional theory of Moon formation.
While various refined models of the giant impact have subsequently been proposed, they have all faced challenges.
To further refine the theory of lunar formation, Prof. DENG began conducting research on the Moon’s formation in 2017. He focused on developing a new computational fluid dynamics method called Meshless Finite Mass (MFM), which excels at accurately modeling turbulence and material-mixing.
Using this novel approach and conducting numerous simulations of the giant impact, Prof. DENG discovered that the early Earth exhibited mantle stratification after the impact, with the upper and lower mantle having different compositions and states. Specifically, the upper mantle featured a magma ocean, created through a thorough mixing of material from Gaia and Theia, while the lower mantle remained largely solid and retained the material composition of Gaia.
“Previous research had placed excessive emphasis on the structure of the debris disk (the precursor to the Moon) and had overlooked the impact of the giant collision on the early Earth,” said DENG.
After discussions with geophysicists from the Swiss Federal Institute of Technology in Zurich, Prof. DENG and collaborators realized that this mantle stratification may have persisted to the present day, corresponding to the global seismic reflectors in the mid-mantle (located around 1000 km beneath the Earth’s surface). Specifically, the entire lower mantle of the Earth may still be dominated by pre-impact Gaian material, which has a different elemental composition (including higher silicon content) than the upper mantle, according to Prof. DENG’s previous study.
“Our findings challenge the traditional notion that the giant impact led to the homogenization of the early Earth,” said Prof. DENG. “Instead, the Moon-forming giant impact appears to be the origin of the early mantle’s heterogeneity and marks the starting point for the Earth’s geological evolution over the course of 4.5 billion years.”
Another example of Earth’s mantle heterogeneity is two anomalous regions—called Large Low Velocity Provinces (LLVPs)—that stretch for thousands of kilometers at the base of the mantle. One is located beneath the African tectonic plate and the other under the Pacific tectonic plate. When seismic waves pass through these areas, wave velocity is significantly reduced.
LLVPs have significant implications for the evolution of the mantle, the separation and aggregation of supercontinents, and the Earth’s tectonic plate structures. However, their origins have remained a mystery.
Dr. YUAN Qian from the California Institute of Technology, along with collaborators, proposed that LLVPs could have evolved from a small amount of Theian material that entered Gaia’s lower mantle. They subsequently invited Prof. DENG to explore the distribution and state of Theian material in the deep Earth after the giant impact.
Through in-depth analysis of previous giant-impact simulations and by conducting higher-precision new simulations, the research team found that a significant amount of Theian mantle material, approximately two percent of Earth’s mass, entered the lower mantle of Gaia.
Prof. DENG then invited computational astrophysicist Dr. Jacob Kegerreis to confirm this conclusion using traditional Smoothed Particle Hydrodynamics (SPH) methods.
The research team also calculated that this Theian mantle material, similar to lunar rocks, is enriched with iron, making it denser than the surrounding Gaian material. As a result, it rapidly sank to the bottom of the mantle and, over the course of long-term mantle convection, formed two prominent LLVP regions. These LLVPs have remained stable throughout 4.5 billion years of geological evolution (Fig. 1).
Heterogeneity in the deep mantle, whether in the mid-mantle reflectors or the LLVPs at the base, suggests that the Earth’s interior is far from a uniform and “boring” system. In fact, small amounts of deep-seated heterogeneity can be brought to the surface by mantle plumes—cylindrical upwelling thermal currents caused by mantle convection—such as those that likely formed Hawaii and Iceland.
For example, geochemists studying isotope ratios of rare gases in samples of Icelandic basalt have discovered that these samples contain components different from typical surface materials. These components are remnants of heterogeneity in the deep mantle dating back more than 4.5 billion years and serve as keys to understanding Earth’s initial state and even the formation of nearby planets.
According to Dr. YUAN, “Through precise analysis of a wider range of rock samples, combined with more refined giant impact models and Earth evolution models, we can infer the material composition and orbital dynamics of the primordial Earth, Gaia, and Theia. This allows us to constrain the entire history of the formation of the inner solar system.”
Prof. DENG sees an even broader role for the current study: “This research even provides inspiration for understanding the formation and habitability of exoplanets beyond our solar system.”
MFM simulation of the canonica [VIDEO] |
MFM simulation of the canonical Moon-Forming giant impact. Here different colors trace different components of Gaia and Theia. The lower mantle of Gaia, denoted by the dashed circle with a radius of 0.8 Earth radii (RE), is only marginally contaminated by Theian mantle.
CREDIT
Credit: BI Rongxi and DENG Hongping
JOURNAL
Nature
ARTICLE TITLE
Moon-forming impactor as a source of Earth’s basal mantle anomalies
ARTICLE PUBLICATION DATE
1-Nov-2023
