Earth’s core may have stopped spinning or may even be spinning backwards, a new study reveals.
Our planet’s core is made up of an outer shell of liquid metal and an inner core of solid metal that is about 70% the size of the Moon.
It is generally believed that the core rotates counterclockwise when viewed from the North Pole, like the rest of planet Earth.
But a study carried out by researchers from the University of Peking (China), in which seismic wave data from the last 60 years are analyzed, has come to the conclusion that the rotation of the core stopped around the year 2009 , and then restarted in the opposite direction.
“We believe that the core, relative to the Earth’s surface, rotates in one direction and then in another, like a swing,” Xiaodong Song and Yi Yang, the study’s authors, told AFP.
A complete cycle (in one direction and then the other) of this oscillation lasts about 70 years, they added.
According to the researchers, the last rotation change before 2009 would have occurred in the early 1970s, with the next one taking place in the mid-2040s.
There is still much debate about the nature of the Earth’s core, because gathering information about it is extremely difficult.
The edge of the outer core meets the Earth’s mantle at a depth of about 2,890 km, and this outer core is believed to be made of liquid iron and nickel.
The inner core begins some 5,000 km below the planet’s surface and is believed to be made of solid iron and nickel, due to extreme pressure forcing the metal atoms to pack together.
It lies within the liquid outer core and is believed to have rotated in the same direction as the rest of the Earth, based on analysis of seismic waves caused by earthquakes.
Xiaodong Song and Yi Yang studied these seismic waves and discovered a “gradual retreat of the inner core as part of an oscillation of approximately seven decades.” The article was published in the journal Nature Geoscience.
“This is a very cautious study by excellent scientists who have used a lot of data,” said John Vidale, a seismologist at the University of Southern California who was not involved in the research.
However, he added that “none of the existing models really explains all the available data well.”
How often does the rotation of Earth’s inner core change?
Vidale published a study last year suggesting that the inner core oscillates much faster, changing direction roughly every six years, based on seismic data from two nuclear explosions in the late 1960s and early 1970s.
Another geophysicist, Hrvoje Tkalcic of the Australian National University, believes that the cycle of the inner core is about 20 to 30 years, instead of the 70 years proposed by the study published this week in Nature Geoscience.
The Peking University researchers state in their paper that this oscillation “matches changes in other geophysical observations,” such as the length of days and changes in Earth’s magnetic field.
“These observations provide evidence for dynamic interactions between Earth’s layers, from the deepest interior to the surface, potentially due to gravitational coupling and angular momentum exchange from the core and mantle to the surface,” they write.
“We hope that our research will motivate researchers to design and test models that treat the Earth as an integrated dynamical system,” they explain.