NEW YORK -- Scientists have reported finding strong evidence that Earth's inner core is spinning freely and slightly faster than the rest of Earth, making it virtually a planet within a planet.
The scientists, at Columbia University, said the inner core was moving fast enough to lap the surface once every 400 years or so.
Such inner freedom has never been reported before for any rocky body or planet in the cosmos.
The discovery is expected to advance knowledge of how heat flows through Earth, how its interior evolved over the ages, and how its magnetic field forms and periodically reverses.
Earth's inner core is a solid mass of blisteringly hot iron 1,500 miles wide, roughly the distance between Baltimore and Dallas, making it slightly smaller than the Moon. The inner core is very difficult to study because of its remoteness, but recent advances in seismic measurement and computer modeling are beginning to unveil some of its secrets.
Scientists have speculated for more than a decade that the inner core might spin independently of the rest of the planet and recently predicted it in some detail. But evidence was lacking.
The discovery was reported in the journal Nature by Dr. Xiaodong Song and Dr. Paul Richards, seismologists at Columbia's Lamont-Doherty Earth Observatory in Palisades, N.Y.
A team based at Harvard University has also found evidence of the core's independent spin and has submitted a paper to the journal Science.
Dr. Gary Glatzmaier, a geophysicist at the Los Alamos National Laboratory in New Mexico, said: "It's a breakthrough.
"In the past, people thought changes in the interior of the Earth were very slow. Now we can see something changing on a time scale of 10 years. That's remarkable."
The reported speed of the independent rotation is lightning fast compared with many geological movements, which tend to occur over millenniums.
The Columbia team estimates that the speed of the core's independent spin is between 0.4 and 1.8 degrees per year. With 360 degrees in a circle, that equals roughly one lap every 400 years.
Dr. Adam Dziewonski, leader of the Harvard team, which also has a member at the University of California at Berkeley, said it had found evidence of a spin rate up to 3 degrees per year, which yields a lap every 100 years or so.
"If astronomers picked up something like this they would send a satellite," Dziewonski said of the independent spin in an interview. "We need more seismic stations and better computers."
Scientists say the independent motion is most likely caused by the inner core's interaction with powerful magnetic fields generated by moving fluids in Earth's outer core, which is molten. These fields are thought to create a torque that makes the core spin faster, much as the moving magnetic fields in an electric motor cause its armature to turn.
The low viscosity, or low internal friction, of material in the outer core makes it "easy for the inner core to rotate," the Columbia team said in its Nature paper.
"This is just the beginning," Richards of Columbia said in an interview. "There've been hundreds of fine theory papers. But now there's a hard observation to hang the theory on."
By weight, the Earth is mostly iron. Four and a half billion years ago, during the planet's fiery birth, most of that iron sank into the planet's deep interior because of its enormous weight and eventually formed inner and outer cores.
The outer core is molten iron. Deeper still, pressures and densities become so monumental that the iron is solid despite temperatures believed to be above 7,000 degrees Fahrenheit. This solid inner core makes up less than 1 percent of the Earth's volume. Since its discovery 60 years ago, scientists have usually considered it obscure, featureless and having little observable impact on the planet.
The main way of studying this remote domain is with sensors that pick up faint vibrations in the ground, which let scientists map the paths and speeds of shock waves that radiate out from big earthquakes and explosions the way ripples move across a pond.
Fairly recently, after more than a decade of study, geologists who track seismic waves agreed that something odd was going on -- waves traveling through Earth's inner core along a %o north-south path consistently move faster than those traveling east-west. The difference in speed is about four seconds.
Such directional differences, known as anisotropies, are common in everyday materials and often involve such physical properties as elasticity or electrical conductivity. In the case of Earth's inner core, the directional differences of elastic waves were seen as suggesting that the mass of solid iron is actually a single giant crystal.
As seismic work progressed, so did super-computer modeling of the inner core, including predictions that it is moving independently at relatively quick speeds.
That work was published last year by Glatzmaier of Los Alamos and Dr. Paul Roberts of the University of California at Los Angeles, who estimated that the inner core rotated in a complete circle with respect to a point on Earth's surface once every 500 years or so.
The Columbia team's hunt for evidence of a freely rotating core was financed by the National Science Foundation, the federal government's main supporter of basic science.
Song of Columbia received his doctorate in 1994 from Caltech. For his Ph.D. thesis, he studied how echoes from earthquakes in the South Atlantic near Antarctica pass through Earth and can be detected at a seismic station in Alaska, revealing clues to the core's makeup.
In the rotation hunt, he studied such seismic data from the 1980s, which he had acquired for his Ph.D. thesis, and compared it with readings gathered in the 1990s. The first comparison occurred in February.
The travel times for waves traversing the inner core in the 1990s were clearly faster than the travel times for the 1980s. The clear implication was that the inner core had rotated in respect to the surface of the planet.
"I couldn't believe it when we started to get these results," said Richards. "It was so exciting."
Other data were compared and presented in the Nature paper, including seismic readings taken in the Antarctic from large nuclear blasts conducted decades ago by the Soviet Union.
The scientists say much additional research will be needed to confirm and precisely measure the exact rate of inner-core rotation, as well as to explore its ramifications in other fields of science.
Pub Date: 10/06/96