Measurements of seismic waves traveling through the Earth’s core suggest that its rotation may be slowing down, changing its orientation relative to the rest of the Earth’s rotation
January 23, 2023
Our planet’s solid inner core may be slowing down its spin and preparing to change the direction of its spin relative to the rest of the planet. This appears to be part of a cycle lasting about 60 years in which the core periodically speeds up and slows down again.
under mantle A layer of churning molten iron and nickel, the dense iron core remains solid due to the enormous pressure at the planet’s center. The motion of the inner core relative to the mantle and surface has been debated for decades, and seismic measurements are now helping researchers better understand it.
Yi Yang and Song Xiaodong Peking University, China, and colleagues analyzed seismic wave From nearly identical earthquakes that have passed through the Earth’s core over the past 60 years or so. If Earth’s solid core were perfectly spherical and had the same structure throughout, we would expect each set of waves to look exactly the same no matter when they pass. This isn’t the case, though, so we can use the difference between the waves to measure changes deep underground.
The researchers found that until about 2009, the planet’s core seemed to be spinning slightly faster than the mantle and surface — meaning that if you could stand on the surface and look down at the core, you’d see it slowly spinning forward . But around 2009, this rotation started to slow down. If you could look down into the core right now, their measurements showed that you couldn’t see it spinning at all, because it was spinning at roughly the same rate as the surface.
“This means it’s not spinning as steadily as it was originally reported about 20 years ago, but it’s actually more complicated,” said bruce buffett at UC Berkeley. According to Yang and Song’s measurements, the last turning point of the inner core’s rotation was in the early 1970s, so the rotation rate seems to oscillate regularly.
“We have several different ideas about how the inner core moves, and this idea of steady motion and then slowing down at the beginning and end of about 50 years is probably the main idea, but it doesn’t explain everything,” said John Vidale at USC. Notably, it doesn’t take into account the period from 2001 to 2003, during which the rate of change in core spin appears to have been much higher than we’ve seen at other times, he said. “But my guess is that other things are going on, so it’s really not a serious flaw if all the data isn’t explained by one model.”
The oscillations are likely caused by interactions between the solid mantle and the inner core. Because neither is perfectly spherical, the gravitational pull of the lumps and bumps in them pulls on each other.that may change rotation rate It’s a bit of both – although the mantle is much heavier than the inner core and therefore has much less of an effect on the outer layers of the planet.
This is confirmed by measurements of small changes in the length of the day at the Earth’s surface, which fluctuate slightly.Variations in core rotation are also expected to affect planet’s magnetic fieldbut relatively small.
“People are horrified by the idea that Earth’s magnetic field is about to reverse, and it’s not that kind of thing, and it has very little effect,” Buffett said. “Flows in the core alter the magnetic field slightly and change the length of the day by about a tenth a year. One millisecond.”
But we’re not sure what’s going on center of the earthmainly because it is very difficult to measure these very small changes in seismic waves, magnetic fields, and the length of a day.
“I wish I could say this is final, but I think we still have some work to do before we can come to a final interpretation,” Vidale said. “We’re having trouble simulating these waves because they’re so high in frequency across the planet, and some of the measurements are very uncertain and conflicting.” More observations in the coming decades will help researchers resolve this issue .
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