The planet inside

Paul Voosen in Science:

Earth’s magnetic field, nearly as old as the planet itself, protects life from damaging space radiation. But 565 million years ago, the field was sputtering, dropping to 10% of today’s strength, according to a recent discovery. Then, almost miraculously, over the course of just a few tens of millions of years, it regained its strength—just in time for the sudden profusion of complex multicellular life known as the Cambrian explosion.

What could have caused the rapid revival? Increasingly, scientists believe it was the birth of Earth’s inner core, a sphere of solid iron that sits within the molten outer core, where churning metal generates the planet’s magnetic field. Once the inner core was born, possibly 4 billion years after the planet itself, its treelike growth—accreting a few millimeters per year at its surface—would have turbocharged motions in the outer core, reviving the faltering magnetic field and renewing the protective shield for life. “The inner core regenerated Earth’s magnetic field at a really interesting time in evolution,” says John Tarduno, a geophysicist at the University of Rochester. “What would have happened if it didn’t form?”

Just why and how the inner core was born at that moment is one of many lingering puzzles about the Pluto-size orb 5000 kilo meters underfoot. “The inner core is a planet within a planet,” says Hrvoje Tkalčić, a seismologist at Australian National University (ANU)—with its own topography, its own spin rate, its own structure. “It’s beneath our feet and yet we still don’t understand some big questions,” Tkalčić says.

But researchers are beginning to chip away at those questions. Using the rare seismic waves from earthquakes or nuclear tests that penetrate or reflect off the inner core, seismologists have discovered it spins independently from the rest of the planet. Armed with complex computer models, theorists have predicted the structure and weird behavior of iron alloys crushed by the weight of the world. And experimentalists are close to confirming some of those predictions in the lab by re-creating the extreme temperatures and pressures of the inner core.

More here.