Earth's Geodynamo: An Energy Crisis

A new theory to maintain Earth’s magnetic field looks like a case of special pleading to rescue a dogma.

Keeping Earth’s magnetic field going for billions of years is a major problem, because its strength is steadily decaying. Geophysicists know this but rarely mention it, because they already “know” in their hearts that the earth is billions of years old. A new paper by Caltech scientists O’Rourke and Stevenson appeared in Nature trying to patch up the age problem with what looks like an ad hoc theory rescue device. It involves adding magnesium at the core-mantle boundary so that it precipitates into the core.

Bruce Buffett explains this new idea in his summary, “Another energy source for the geodynamo,” also in Nature. The details of the new theory are not as interesting as the admissions in both articles about how inadequate the received wisdom is.

Magnesium is not usually considered to be a constituent of Earth’s core, but its presence there has now been proposed to explain an ongoing enigma — the identity of the energy sources that drive our planet’s magnetic field.

Buffett does not question the geodynamo theory; he just recognizes the “energy crisis” of keeping it running. “Sustaining a magnetic field is difficult for a terrestrial planet,” he says. If the new rescue device becomes accepted, there will be ripple effects elsewhere, because it “warrants a serious reassessment of magnetic-field generation in other rocky (terrestrial) planets.”

But can they justify putting a bunch of magnesium down there, out of sight, just to keep their favored theory from being falsified? Look at the dilemma their view of Earth’s antiquity puts them in:

A problem with this conventional view of the geodynamo’s energy sources emerges when we extrapolate back in time. Before the inner core formed (possibly less than 1 billion years ago), the only energy source was thermal convection. But current estimates of iron’s thermal conductivity suggest that the heat flow required to sustain such convection at that time was extremely high. Even if this heat flow was feasible, an implausibly high core temperature would be needed to sustain it over geological time. Despite these difficulties, Earth has somehow maintained a magnetic field for at least the past 3.4 billion years.

How many people know that? The “conventional view” is usually presented without any reference to this problem. From there, Buffett considers the pros and cons of the magnesium precipitation theory. The amount of magnesium and its precipitation rate are uncertain, but  “early and abundant precipitation is required to provide an effective solution to the geodynamo energy crisis,” he worries. Even with the new suggestion, there are complications that will be likely to send geophysicists “back to the drawing board.” The paper is not a good rescue device yet. “More work is clearly required to address these uncertainties, but the potential contribution of magnesium precipitation to the geodynamo provides plenty of motivation to improve our current knowledge.”

As usual, Live Science glosses over the worries and presents the new theory cheerfully, other than to note that it is a break with tradition: “Magnesium is the fourth most common element in the Earth’s outer layers, but previously, scientists thought there was almost no magnesium in the core,” Becky Oskin writes. “Iron and magnesium don’t easily mix, and researchers thought that the Earth’s core was mostly iron.” She gives one of the authors of the paper time to announce his triumph: “We think we now understand why the Earth has had a magnetic field for the last 4 billion years, and that the process will keep happening into the foreseeable future.” But others thought they understood things before now. The only hint of objectivity is quoting a couple of other scientists who think the new model needs additional evidence and work. The article does recognize the importance of having a magnetic field:

A magnetic field is important for life because it shields the planet and the atmosphere from the solar wind. Knowing how ancient Earth’s magnetic field kicked into gear may help improve estimates of when life first appeared and inform the search for life on other planets.

The statement implies that they don’t know how it “kicked into gear.” Kicking assumes a kicker.

For those who can handle the jargon, O’Rourke and Stevens’ final paragraph in their Nature paper is interesting for the amount of gesticulation required to make their new theory work. Did you know, for instance, that radiogenic heating is implausible as a heat source? Watch as they invoke one of planetary scientists’ favorite rescue devices, giant impacts:

Precipitation of Mg from the core has profound implications for the evolution of Earth’s deep interior. Most importantly, it eliminates the need to invoke a geochemically dubious magnitude of radiogenic heating or enhanced heat flux across the CMB [core-mantle boundary] into a basal magma ocean. High thermal conductivity and slow core cooling are consistent with inner-core nucleation in the Mesoproterozoic. Models that include only the inner core as a source of compositional buoyancy predict that stable layers hundreds of kilometres thick should develop near the CMB, which may be disrupted by precipitation. However, precipitation may actually occur at depth if the solubility of Mg is strongly pressure dependent. The real situation is even more complicated if the CMB is undersaturated in Si and O, meaning that material from the mantle tends to dissolve in the core. Elemental transport in both directions is potentially permissible because the Mg-rich precipitate differs in composition from the CMB. The effect of giant impacts on core formation should motivate additional experiments on metal–silicate partitioning at temperatures above 5,000 K. Non-standard evolutionary scenarios featuring precipitation are perhaps applicable to the cores of other terrestrial planets.

The overarching question that none of these articles addresses is the decay of the magnetic field’s strength over time. That is an observational fact going back 186 years since first measured in 1829. Even with reversals, that decay cannot be extrapolated back for billions of years; it would have been impossibly strong in even 100,000 years (ICR). The fact that old-age geologists continue to look for creative ways to power the field shows that they really don’t know “how ancient Earth’s magnetic field kicked into gear.”

Update 1/28/16: A new paper in Nature Communications admits that “direct observations” show that the magnetic field has been decaying steadily since 1840, and that if trends continue, its energy will reach zero in 1,900 years! No proposal for reversing the trend is offered. The paper is open-access so that everyone can read it.

Here’s what’s going on. It’s the pattern we see in all of evolutionary theory. (1) Assume long ages and evolution. Believe in it so strongly, no amount of evidence will disrupt your faith. (2) Observe an anomaly. (3) Make up a story to rescue your faith from the evidence. Whatever you do, never, ever think outside the box of billions of years. That’s the moyboys‘ Law of the Misdeeds and Perversions that cannot be altered.

Open-minded people can see this evidence and consider the implications rationally; “Wow, this implies the Earth is young.”