This article is from the WeChat public account: SF Chinese (ID: kexuejiaodian), by SF.
If you are asked about the internal structure of the Earth. Presumably you can answer that the Earth is composed of three spherical shells: the crust, the mantle, and the core, and the core is further divided into the liquid outer core and the solid inner core. Nowadays, it is generally believed that the internal structure of the Earth is like the concentric rings on a tree stump. However, some new research tells you that the structure of the Earth is far from being so simple.
Humans have a history of more than a hundred years of going into space, and a history of more than 60 years of breaking through the atmosphere, and even the first time humans set foot on a planet hundreds of thousands of miles away has passed for more than 50 years, but the record of humans going deep underground has just exceeded 12 kilometers. It can be seen that going deep into the Earth is much more difficult than flying into space.
It can be said that our understanding of the Earth is far less than that of space. For example, for the core that is crucial for the formation of the Earth's magnetic field, in the past, scientists generally believed that the liquid iron alloy that constitutes the outer core of the Earth is well mixed and evenly distributed. But in the past 40 years, some detection results have made this view controversial.
In August 2024, an article published in Science Advances gave a strong evidence of the uneven distribution of the outer core of the Earth - a donut-like structure was found in the outer core.
Scientists from China and Australia selected the data of seismic coda waves (the subsequent part of the clear seismic phase on the seismogram) measured by seismic stations distributed globally. These data were monitored from 2000 to 2021 and all originated from major earthquakes with a moment magnitude greater than or equal to 6.8.
Scientists calculated and processed these seismic data and divided the seismic data into a 'polar group' (latitude higher than 35°) and an 'equatorial group' (latitude lower than 35°) according to the latitude position of the seismic station where they were located. They found that the time for the polar group's seismic coda waves to pass through the interior of the Earth is shorter than that of the equatorial group, and the time difference can be up to 3 to 5 seconds.
There are many reasons for this time difference, mainly including the ellipticity of the Earth, the heterogeneity of the mantle, the topography of the core-mantle boundary, the inhomogeneity of the outer core, and the structure of the inner core. After analysis, scientists excluded the influence of the ellipticity of the Earth, the heterogeneity of the mantle, the topography of the core-mantle boundary, and the structure of the inner core. Then, the only reason left for this time difference is the inhomogeneity of the outer core.
Scientists, through modeling, answered what kind of inhomogeneity can lead to such a time difference. They found that when there is a donut-like annular structure in the low-latitude area of the outer core and this annular structure is a low-velocity area of seismic waves compared to the surrounding outer core area, the model-fitted data is more consistent with the actual detected data.
The results of this study seem well-founded, but for the question of 'what exactly is the area where the mantle and the outer core intersect', different scientists have different explanations. Perhaps they are all right, and it is so complicated here that there may be many landscapes that we can't even imagine.
Geologists from the University of Alabama in the US found through seismic wave research that the outer core is wrapped by a thin 'ocean', which is 5 to 40 kilometer s deep. How did the ancient ocean come to the core-mantle boundary? During the process of plate movement, the bottom of the ancient ocean was continuously pushed into the interior of the Earth, thus forming this slow area that wraps the outer core.
In May 2024, Chinese scientists published their latest research results on this area. They inferred that at the core-mantle boundary, there is a mysterious D layer. This is a magma ocean. Scientists believe that in the early days of the Earth, a Mars-sized celestial body collided with the Earth, forming a magma ocean that covered the entire surface of the Earth, and the D layer is the remains of this magma ocean. Even more unexpectedly, there is a large amount of water in this magma ocean. As for where this water comes from, currently, scientists have not given a definite conclusion.
References: