The simple story for the public: a dynamo creates the Earth’s magnetic field. The real story: complex scenario needed to start it and keep it running.
Science is often like the two-headed Roman god Janus: one face for the insiders, a different face for the outsiders. Geophysicists have locked themselves into an explanation for the Earth’s magnetic field—the only one, in fact, that offers any hope for keeping it going for billions of years. That theory is the geodynamo theory. Since electromagnetic theory is sufficiently abstruse to keep all but certain college graduates able to fathom its intricacies, only a select few are able to see the problems. But since the public understands what a dynamo is if they know enough to say that a hydroelectric power plant turns a turbine that creates electricity, all the outside-facing face of Janus has to do is smile and say, “a dynamo creates the Earth’s magnetic field.” If asked what runs the dynamo, they have a stock answer: “convection in the Earth’s core.” Since everyone has seen water boil, they can feel satisfied with that explanation.
The public deserves better. When journal articles report problems with commonly-assumed notions like the geodynamo, they should hear about it in language they can understand. Here’s an excerpt from a recent paper in Nature by Anke Wohlers and Bernard Wood, followed by a layman’s translation:
An additional question in the context of heat production is that of the energy source for the Earth’s magnetic field. Arising from convection in the core, Earth has had a magnetic field for at least 3.5 billion years. The crystallization of the inner core is an important source of energy for the geodynamo but most attempts to construct histories of core cooling indicate that the inner core cannot be much older than 1–1.5 billion years unless a source of radioactive heating is present. Numerous studies have focused on 40K as a potential core heat source, because K, in common with all moderately volatile elements, is depleted in the silicate Earth relative to the chondritic abundance.
Furthermore, high-pressure experiments indicate that K enters sulfide under oxidizing conditions and sulfur (S) is believed to be a major component of the core’s complement of approximately 10% of elements of low atomic number. It appears, however that the maximum possible K content of the core is insufficient to generate more than a small fraction of the 2–5 TW required to generate reasonable core thermal histories. The alternative explanation—that U [uranium] and/or Th [thorium] provide the energy for core convection—has some support from early experiments on sulphide–silicate partitioning but more recent results indicate very little partitioning of U into S-bearing metals even under extreme conditions.
Translation: the geodynamo cannot be billions of years old. There aren’t enough heat sources to run the convection that theoretically runs the dynamo.
Faced with a dilemma that would spell doom for “deep time” and the Darwinian evolution that needs all that time, the geophysicists were forced to concoct a highly-improbable scenario: that a Mercury-sized object loaded with uranium was “added” to the Earth way back in unobserved prehistory when it was forming. This is known in the philosophy of science as an “ad hoc theory rescue device.” But even then, according to the Editor’s Summary of the paper, the added uranium was not enough. It could only contribute a “substantial part of the heat source to power the geodynamo.”
Richard W. Carlson tries to put a happy face on this ad hoc idea. In Nature, he says, with a very big if, “[Wohler and Wood’s] results lead to the intriguing conclusion that if Earth formation started with highly chemically reduced building blocks, the planet’s metallic core might contain enough uranium to power the convection that creates, and has maintained, Earth’s magnetic field for more than 3 billion years.” Notice the angry face of Janus on the insider’s view:
The energy source that drives the generation of Earth’s magnetic field in the core has long been a topic of discussion, as has the suggestion that one such energy source could be a moderate concentration of uranium or potassium in the core. However, before Wohlers and Wood’s experiments, there was only limited (and controversial) experimental evidence that either uranium or potassium can be incorporated in iron metal at the high temperatures and pressures of core formation.
That’s why the addition of a Mercury-like body might bring in more magic pixie dust (if you’ll pardon the expression for unobserved uranium) into the equation to save the theory. Even so, Carlson does not seem ready to jump on this new ad hoc platform. To make it work, he notes, Wohlers and Wood need their Magic Mercury (if you’ll pardon the expression for an unobserved planetesimal bringing pixie dust to the Earth) to be finely balanced between oxidized and reducing elements. “Although possible, this carefully balanced ratio must also satisfy other potential geochemical consequences of involving highly reduced materials in Earth-formation models — not least, how Earth ended up in its present oxidized state, which it has apparently retained for more than 3 billion years.”
Space.com contains the improbable scenario “could” rescue long ages. Notice the words could and if propping up the solution to a “mystery”—
These experiments could also help solve a mystery concerning Earth’s magnetic field.
Prior research suggests that Earth has possessed a magnetic field for at least 3.5 billion years. Earth’s magnetic field results from churning metal in the planet’s outer core, but it was uncertain how Earth’s core could have remained molten for so long.
The new experiments revealed that if the early Earth engulfed a sulfur-rich Mercury-like body, uranium could have dissolved better in iron sulfide. This in turn would help uranium sink toward Earth’s core. Uranium is a radioactive element that generates heat, which could have kept Earth’s core molten.
Update 4/19/15: This news just in from Rosetta: the comet on which the Philae lander descended contains no magnetic field. This is surprising, the BBC News and New Scientist reports, because magnetic fields are assumed to have helped assemble comets in the early solar system. At least for Comet 67P, there was none. “If this comet is representative of all the others, there is no chance you can have accretion with magnetic forces,” the principal investigator for the magnetometers says. That means the modelers cannot invoke that force in their theories; they would have to rely on collisions between particles, which are far more likely to disrupt small bodies than assemble them. “We can say nothing about millimetre of centimetre-scale particles, but for these larger particles we can exclude that magnetic forces have played a role in the accumulation of planetary building blocks.”
You deserve to know these things. The moyboys are unable and unwilling to reconsider their dogmatic adherence to deep time. So what do they do? They add magical elements to fairy tales [“scenarios” that are highly improbable] to get them to work. Instead of science (despite the jargon about uranium, neodymium, samarium et al), the “scenario” morphs into fantasyland, wishing upon a star to fill in the gaps (like a Magic Mercury coming special delivery with just the right balance of conditions). When you wish upon a star, your dreams come true. Your perhapsimaybecouldness index skyrockets, but why let a little epistemic modesty spoil the fun?
In the limit, this becomes no less supernatural than any given theological origins story. We suggest they save a step. Believe what the Eyewitness (the Creator of the heavens and the Earth) says about how He did it. No fairies, no pixie dust, no magic. The Creator had the desire, power, and will to create a habitable Earth for creatures He desired to create in His image. As a being of infinite intelligence, wisdom and love, the Creator also had the ability to tell his creatures what He did. It’s simple cause and effect. That’s science. Magical ad hoc planetesimals, finely balanced with the right ingredients, arriving in the nick of time to support a dogmatic belief in deep time is not science. It is unbelief masquerading as science through bluffing with highfalutin jargon. What is their “cause” for the observed effect? Chance! It’s the Stuff Happens Law at work, the same law that Darwin relied on so magnificently.
The Creator explanation permits sentient beings like Wohlers, Wood and Carlson to live safely inside our magnetic field’s protective bubble where they can employ the capacity for reason with which they were endowed. We hope they will try it.