The Earth's "two-faced" inner core rotates faster than the surface

What causes the differential rotation of the Earth's inner core? Experts point out that under the action of external torque, the Earth's inner core will rotate relative to the mantle, making the inner core rotate at a different rate from the Earth's rotation. This external force may come from the coupling of the Earth's magnetic field with the inner core and the gravitational coupling of the mantle and the inner core.

Recently, a research team from the University of Southern California re-analyzed seismic wave data from nearly six years after two underground nuclear tests in 1969 and 1971, and found that the rotation rate of the Earth's inner core was 0.1 degrees slower than the Earth as a whole from 1969 to 1971, and 0.29 degrees faster than the Earth from 1971 to 1974. This marks the first time that scientists have verified the theory of the six-year reciprocating oscillation of the Earth's inner core through seismological observations.

Why is the rotation speed of the Earth's inner core different from that of the Earth? Does this phenomenon also exist in other celestial bodies in the universe? How do scientists "see through" the internal conditions of celestial bodies?

The Earth's inner core is called a "planet within a planet"

Many people believe that the Earth should be a whole, and the rotation speed and direction of the inner core should be consistent with that of the Earth. However, this is not the case. "Since its birth, the Earth has undergone a variety of dynamic processes, such as the aggregation of planetesimals, sedimentation differentiation, cooling of the magma ocean and crust, which has led to the formation of a layered structure of crust, mantle, liquid outer core, solid inner core, etc. from the outside to the inside of the Earth, similar to an onion or a boiled egg," said Ping Jinsong, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences.

Song Xiaodong, a geophysicist and professor at Peking University, told Science and Technology Daily that the Earth, as a celestial body, rotates as a whole; however, according to previous observations, the rotation of the Earth's inner core is slightly different from that of the Earth, which is the "differential rotation" of the inner core. Therefore, the Earth's inner core is also called a "planet within a planet." The latest data shows that the rotation rate of the Earth and the Earth's inner core differs by about 0.1 degrees per year, but this rate is likely to change over time.

What causes the differential rotation of the Earth's inner core? Song Xiaodong pointed out that under the action of external torque, the Earth's inner core will rotate relative to the mantle, making the inner core rotate at a different rate from the Earth's rotation. This external force may come from the coupling of the Earth's magnetic field and the inner core and the gravitational coupling of the mantle and the inner core. In theory, the inner core can rotate faster or slower than the Earth.

The reporter learned that the mainstream theoretical model of the current core research believes that the rotation speed of the Earth's inner core is constant, and the Earth's rotation has a periodic oscillation of about 6 years (5.9 years). In explaining the mechanism of the 6-year periodic oscillation, scientists have given a variety of speculations, including geomagnetic anomalies inside the Earth, core-mantle coupling oscillations, and core oscillations.

However, at the beginning of this century, Ping Jinsong's team studying the Earth's rotation solid tides used long-term solid tide monitoring data to discover an 11.5-12 year oscillation period of the Earth's inner core, which is consistent with the orbital period of Jupiter. "The 5.9-year period of the Earth's rotation is exactly half of Jupiter's revolution period." Ping Jinsong believes that it is the periodic changes in the torque magnitude of the Earth's crust and mantle caused by the conjunction of Jupiter and the Sun that cause the changes in the rotation rate of the Earth's inner core.

"The rotation speed of the Earth's inner core may be slightly faster than that of the mantle, but the rate will always remain constant. The rotation of the crust and mantle is affected by internal and external torques, loads and other factors, and the rotation speed may appear fast or slow or the relative direction may change." Ping Jinsong said that due to the influence of multiple factors, the changes in the Earth's rotation cannot be simply predicted or forecasted.

Ping Jinsong pointed out that in addition to the Earth, companion planets with internal structures similar to that of the Earth, such as the Moon, as well as Venus, Mars, and satellites of gas planets, dwarf planets, etc., may also rotate faster or slower or change direction.

Among them, humans have used the Earth-Moon laser ranging technology to continuously observe the changes in the lunar rotation period for about 50 years; in recent years, the rotation changes of Mars have also been confirmed by the satellite-to-ground microwave link on the "Insight" spacecraft.

Li Gang, a postdoctoral fellow at the Institute of Astrophysics and Planetology in Toulouse, France, added that there are also differences in rotation speed between the layers inside stars, which is called "differential rotation." Astronomers have found that differential rotation exists in everything from white dwarfs and neutron stars to star clusters and galaxies. Taking the sun as an example, at a depth of 0.7 solar radii, there is an area called the "differential rotation layer" with a thickness of 0.04 solar radii. With this area as the boundary, the sun's rotation changes dramatically: within the differential rotation layer, the sun's differential rotation is not very obvious; outside the differential rotation layer, the sun exhibits very obvious differential rotation at different depths and latitudes.

The "temperament" of a celestial body is closely related to its core

Do all planetary cores have a differential rotation mechanism? Ping Jinsong introduced that there are three main types of planetary cores, namely solid core, liquid core, and a combination of liquid outer core and solid inner core. The Earth's inner core belongs to the third type, which includes a huge liquid outer core and a solid inner core rich in iron and nickel.

The "temperaments" of planets with solid cores and liquid cores are different. Ping Jinsong pointed out that for planets with solid cores, the rotation of the core is synchronized with the planet as a whole. However, for planets with liquid outer cores, liquid or semi-liquid core-mantle boundaries, the rotation of their solid inner cores or solid cores can be out of sync with the mantle.

A planet with a liquid core is most likely in the early stages of planetary formation. Its rotation is a bit like a spinning raw egg, with a fast rotation speed and unstable direction and speed. As the energy dissipates and decays and the layers solidify, the rotation speed gradually slows down and stabilizes.

So, what does the core of a star look like? Li Gang said that a star usually has a core that is burning in nuclear reactions. However, the cores of stars of different ages are also very different. For example, a star in its prime, such as the sun, has a core that burns hydrogen; while for old stars, some cores may have stopped burning, but there is a layer of burning envelope surrounding the core; some may have their cores ignited again and continue to burn helium or heavier elements; and some may eventually die, leaving only a core that burns out and slowly cools down.

"The properties of the star's core directly determine the star's appearance." Li Gang explained that compared to the sun, most old stars are cooler but brighter; while dead stars, such as white dwarfs and neutron stars, appear dim but have higher temperatures.

Using shock waves to see into the core of celestial bodies

How do scientists detect the rotation direction of the core of a celestial body through the surface of the celestial body?

Song Xiaodong told reporters that scientists have been using seismic waves to detect various physical properties of the Earth's inner core. He explained that after an earthquake occurs, seismic waves are generated, and seismic waves propagate deep into the Earth. By studying the propagation speed and other properties of seismic waves, we can understand the structural stratification and movement conditions deep inside the Earth.

Song Xiaodong and his team first observed changes in the timing and speed of seismic waves passing through the Earth's inner core in 1996, and thus inferred that the Earth's inner core is rotating relative to the mantle. However, there is still uncertainty about the rotation rate of the Earth's inner core, because the rotation rate itself is also likely to change.

Ping Jinsong pointed out that the method of using seismic waves to detect the interior of celestial bodies is very common. Scientists used it to detect the layered structure of the earth in the 19th century. In the 1970s, the US lunar exploration mission used seismic waves to detect the internal layered structure of the moon. In recent years, the European Mars exploration mission has used it to detect the internal structure of Mars.

For stars, it is more difficult for astronomers to directly observe the internal structure of stars, so they use "asteroseismology" to detect the interior of stars.

Li Gang said that asteroseismology is very similar to seismology, both of which infer the internal structure of celestial bodies by studying the vibration waves inside celestial bodies, a bit like in daily life we ​​judge whether the watermelon is ripe by "patting it". However, the difference between asteroseismology and earthquakes is that asteroseismology is a continuous vibration, while earthquakes only occur occasionally. The vibration waves of stars will locally compress the gas, causing changes in the temperature and brightness of the stars. Based on this characteristic, astronomers observe the vibration of stars by monitoring the changes in the brightness of stars for a long time and measure the frequency of the vibration.

The frequency of a star's vibrations will vary due to disturbances caused by different physical processes inside it. "For example, if you tap a ripe watermelon and an unripe watermelon, the sound you hear is different," said Li Gang. Astronomers use high-precision frequency measurements and detailed physical model deductions to calculate different physical properties of stars, such as the rotation rate and element distribution inside the star.

For example, for red giants formed when stars grow old, astronomers discovered through analyzing the vibration signals of some red giants that red giants have a very strong differential rotation phenomenon, and the rotation rate of their inner cores is generally dozens of times faster than that of the surface.