The inner core at the center of the Earth, a ball of iron and nickel about 1,500 miles wide, may not be completely solid.
New research finds evidence that the outer boundary of the inner core has changed significantly over the past decades.
“The most likely thing is that the outer core can pull the inner core and move a little,” said John Vidale, a professor of geoscience at the University of Southern California.
Dr. Vidale and his colleagues reported their findings in the magazine Nature Geoscience on Monday.
It adds to the mystery about the planet's centre. Geophysicists have previously reported that the inner core does not rotate at exactly the same speed as the rest of the Earth. They also showed that the pace of rotation changes – the inner core appears to be spinning slightly faster than the outer layer decades ago, and now it spins slightly slower.
The inner core is the deepest of the Earth's geological layers. The crust (the layer we live in) is a few miles thick. Below that, it is a 1,800-mile-thick mantle that fills 84% of the planet. This is soft enough to flow up and down and generate force pushing the continent. Between the mantle and the inner core is the liquid outer core.
Of course, scientists cannot cut into the earth, and cannot directly observe its inside. Instead, their knowledge is inferred from the vibrations produced by earthquakes passing through the planet. The velocity and direction of earthquake vibrations vary depending on the density and elasticity of the rock.
In this study, Dr. Videle and his colleagues saw an earthquake in the South Sandwich Islands, a volcanic chain in the South Atlantic.
There are so many earthquakes there that it is almost in the same event and position as the new events that occurred a few years ago.
Scientists identified more than 100 such “earthquake pairs” and analyzed measurements from two seismometers over 8,000 miles apart from 1991 to 2004.
The analysis was originally intended to improve previous tasks suggesting slowing down the spin of the inner core. However, scientists did not understand the signal aspects of the yellow knife array.
“Essentially, the Wiggles are different,” Dr. Vidale said.
Incidentally, in some pairs, the inner core was in the same direction between both earthquakes.
Identifying seismic vibrations passing through the same parts of the globe should generate the same seismic signals at Fairbanks and Yellowknife. In Fairbanks, that was true, but in Yellowknife the signal was different.
Because Yellowknife is slightly closer to the Southern Sandwich Islands than Fairbanks, seismic waves from the island's earthquakes did not move deeper into the inner nucleus as deep as those who reached Fairbanks. It suggests something has changed near the outer boundary of the inner core.
Turbulence of the outer core or gravitational attraction from the dense portion of the mantle can deform the inner core boundary, which could explain changes in seismic signals, Dr. Vidale said.
“I think it's soft because it's close to the melting point,” he said. “So it's not surprising that it transforms.”
New discoveries are not the last of this theme. “The interpretations provided are sound,” said Hrvoje Tkalcic, professor of geophysics at the Australian National University.
In recent years, geophysicists have debated whether differences in seismic signals are caused by changes in rotational speed or by changes in the shape of the inner core. “This study reconciliates the final argument by proposing a combination of both causes,” Dr. Tkalcic said.
Lianxing Wen, a professor of geoscience at Stony Brook University in New York, reported in 2006 that the inner core may change in shape at the boundary of the inner core, but the inner core rotates at a different speed than the rest of the Earth. I'm not convinced to do it.
Dr. Wen said the Yellowknife data contradicts that hypothesis. “Usually such contradictions should lead to abandonment of the original inconsistent interpretation,” he said.
Dr. Wen said the shape change without any change in the rotational shape was sufficient to explain the seismic data.
Even Dr. Vidale is not sure he is right. “We're sure we're right, but this is not a bulletproof paper,” he said. “What are you sure? I'm 90%.”
Dr. Tkalcic said that more data is needed to solve the questions. This can be achieved by building seismic infrastructure in remote areas of the planet, including the seabed.”
The song by Xiaodong, a professor at Peking University in China in the mid-1990s, was one of the first to propose that the inner core rotates at a different speed than the surface of the Earth.
“This new study,” Dr. Song said, “we should motivate a new exploration of strange behaviors at the heart of the planet.”
