Today's geoscientists have a working theory of the deep Earth. What that means, in essence, is that it yields fruitful questionsproblems to work on using supercomputers, high-pressure equipment, and analyses of rocks from everywhere including outer space and other planets. Is a dazzling new theory like Marvin Herndon's what we need?
Herndon, an independent geophysicist, made a splash in the August 2002 issue of Discover magazine with his idea that a ball of uranium minerals sits deep in the Earth's core, a gigantic natural nuclear reactor.
The Discover article is a fair introduction, but Herndon's own Web site nuclearplanet.com reproduces earlier papers that Herndon has published. These are indispensable for understanding his hypothesis. It has several plausible elements, and the state of current knowledge permits it as far as I can tell. But I have my doubts as to whether his hypothesis is a fruitful one.
The Herndon Thesis
Briefly Herndon's story is this: the Earth's iron core formed with a goodly share of radioactive uranium, which combined with sulfur to make an ultra-dense compound. The uranium settled to the center of the core, where it eventually formed a mass big enough to sustain a supercritical nuclear reactor. Not just that, but it functions as a breeder reactor, creating more nuclear fuel than it burns. This provides the energy needed to generate the geomagnetic field, a problem that has challenged scientists since the 17th century when Edmond Halley proposed the first theory of geomagnetism.
What do I like about it? One thing: the uranium sulfide angle. Uranium is classified as a lithophile element, which like calcium, magnesium and other elements dislikes iron. The composition of the core is almost entirely iron, with an unknown lighter element mixed in. The lighter element could be sulfur or oxygen or both, so it's plausible that the lithophile elements in the young core would form sulfides and separate out as an immiscible liquid. The sulfide droplets would rise and settle in the D'' layer at the top of the core. But apparently uranium monosulfide, unlike other lithophile sulfides, is denser than iron. So Herndon argues that it would form a ball in the core's center.
Once you accept Herndon's hypothesis, a host of tempting consequences follow, and the bulk of his papers (not to mention the articles about his papers) follow that temptation to entertaining effect.
Unanswered Questions and Unlikely Answers
But I have two quick questions before I, for one, would take it further. First, would the uranium sulfide separate from other sulfides? I don't see why it would, if they are liquids. Second, would the uranium sulfide sink? Gravity is essentially zero in the inner core, and we know the inner core is solid.
Herndon is captivated by a real examplethe actual natural reactor that once existed at Oklo, in the African nation of Gabon, some 1800 million years ago. I don't blame him; it's a geologic wonder. The Oklo example proves that a natural fission reactor can exist, under certain unusual conditions near the Earth's surface. But it is not evidence for anything in the Earth's core.
It's one thing to look at that example and try to imagine other ways such a thing could occur. But it's another for Herndon to imagine reactors everywhere he sees a mysteryfor instance, in the excess energy output of Jupiter and Saturn and even in the lukewarm core of the Moon. It is his universal solution, in search of a problem.
An Unneeded Theory
So I return to the opening question: Do we need this dazzling new theory? Not really. We are already making computer models of the geomagnetic dynamo (Gary Glatzmaier currently leads the pack) that don't need a big ball of uranium to make them work. There is enough uncertainty in our picture of Earth's internal heat that Herndon's solution isn't superior. And our notions of how the Earth and solar system formed don't require, or even favor, his scheme.
For a summary of the mainstream view, see a summary of the talk that David Stevenson gave to the 2002 SEDI Symposium in July. Also at that meeting, William McDonough specifically addressed the problem of uranium in the core.
The great conversation of science is on a different page. Everything can change, it's truebut not today.