The large low velocity provinces (LLVPs) in the deep Earth mantle may be relics of Theian mantle materials. 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.
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:
Bi Rongxi and Deng Hongping
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 about 1,000 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 2% 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.
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."
by Chinese Academy of Sciences
Source: Massive anomaly within Earth's mantle may be remnant of collision that formed moon (phys.org)
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