What is Earth? Poets say it's a celestial sapphire, a cerulean orb. Astronomers say it's a medium-size planet orbiting an average star. Some environmentalists say it's Mother. Biologists say it's life's only known home. But the most scientifically precise definition may prove to be the one that no one suspected. Earth, says geophysicist J. Marvin Herndon, is a gigantic natural nuclear power plant. We live on its thick shield, while 4,000 miles below our feet a five-mile-wide ball of uranium burns, churns, and reacts, creating the planet's magnetic field as well as the heat that powers volcanoes and continental-plate movements. Herndon's theory boldly contradicts the view that has dominated geophysics since the 1940s: that Earth's inner core is a huge ball of partially crystallized iron and nickel, slowly cooling and growing as it surrenders heat into a fluid core. Radioactivity, in this model, is just a supplementary heat source, with widely dispersed isotopes decaying on their own, not concentrated.
If Herndon's theory is true, it would be the biggest news in geophysics in decades. "I would rank it right up there with plate tectonics as one of the truly great discoveries," says Hatten Yoder, director emeritus of the Geophysical Laboratory of the Carnegie Institution of Washington. The idea also has immediate implications for human beings and all other living things on Earth. While it leaves open the question of whether nuclear fission is a sensible way to make power, it does at least mean that fission is a natural, even essential process. "We owe our very lives to it," Herndon says. That subterranean nuclear reaction, he says, is the dynamo that powers Earth's magnetic field, which protects us from the ravages of the sun.
"Solar radiation would have stripped off our atmosphere long ago without the repulsion provided by the field," says nuclear engineer Daniel Hollenbach, Herndon's collaborator at the Oak Ridge National Laboratory. "We absolutely depend on it."
Herndon recently put forth what he sees as the most compelling argument for his theory in a paper published in the Proceedings of the National Academy of Sciences. Using computer simulations that Hollenbach helped him run at Oak Ridge, Herndon showed how software that tracks fuel usage at nuclear power plants indicates that a "planetary-scale geo-reactor" could indeed have been blazing away for 4.5 billion years, the widely accepted estimate of Earth's age, at heat levels that match Earth's actual output of roughly four terawatts. Moreover, such a reactor would vary in intensity— sometimes strong, sometimes weak, sometimes shutting down altogether— which could explain why Earth's magnetic field has periodically waxed, waned, and reversed through the millennia.
Herndon contends that not only does Earth probably have a reactor boiling at its core but so do Jupiter, Saturn, and Neptune. Natural nuclear reactors could explain a lot of mysteries, from how stars ignite to the nature of dark matter, the mysterious, elusive stuff that astronomers say is 10 times more common than the ordinary matter they can observe. Indeed, Herndon's theory, if correct, would require nothing less than revamping our view of how much of the material universe operates.
