The Enigmatic Core

It was a bold plan: blast open the Earth’s crust using an atomic bomb, then build a probe off-loaded with measuring instruments, and send it down to the Earth’s core. This was the “modest proposal”–not to be taken too seriously, of course–suggested four years ago by David Stevenson, a researcher at the California Institute of Technology.¹
But the truth is that in the end, we know very little about the inaccessible center of our planet. Yet researchers have found all sorts of clever ways to try and better understand the Earth’s inner structure. They study meteorites to learn more about the primitive materials that gave rise to the successive layers of our planet; they use lasers to heat matter to extreme temperatures and then crush it in diamond anvils to reproduce the level of temperatures and pressures found at its core–around 5000°C and 3.5 million times Earth’s atmospheric pressure; and they repeat numerous computer simulations. But above all, they draw up and verify their hypotheses using earthquakes. In fact, the biggest earthquakes are the ones that can be used to “see” the core: the seismic waves they generate are to geophysics what light is to astronomy. Using tools such as seismographs and seismic tomography, scientists are able to map the interior of the planet by observing the waves’ propagation, speed, and changes in direction or amplitude.

A Heart of Iron
Ever since the mid-1930s, we’ve known that there was a solid inner core caused by the massive amounts of pressure at the center of the Earth’s liquid core,” explains Annie Souriau, from the DTP laboratory in Toulouse.² If Stevenson’s probe were not a mere figment of the imagination, it would travel down into the liquid core to a depth of around 3000 kilometers, before hitting the inner core a further 2200 kilometers below. Inside the liquid core it would witness matter shifting around in swirls, giving rise to the Earth’s magnetic field. As it continued its journey, it would tell us more about the precise ingredients that make up the liquid core, about which we have much to learn. What we know is that it mainly consists of iron, that it contains some nickel, probably some sulfur and oxygen, and a very small proportion of other elements.

The Puzzling Inner Core
Researchers now also know that the inner core did not solidify at random, based on their detection of P-waves, “which travel faster parallel to the Earth’s rotation axis than perpendicular to it,” Souriau explains. “But we can’t fully understand the reasons for this anisotropy.³” According to the most recent avenue of research, the crystals of iron present in the inner core are aligned with the Earth’s axis of rotation. Yet the investigation is made more difficult by the presence of impurities like sulfur, which change the form and properties of the crystals. “We can’t reproduce everything in the lab,” Souriau says regretfully. “We use numerical models that describe a core made of iron, we then add some sulfur, or some nickel, and see what they predict.” The structure of the inner core may well be complex, it might perhaps even contain a sub-core, although Souriau thinks that this is “probably only a variation in anisotropy at the center,” and talks about “Russian dolls nested within each other that we know increasingly less about the closer we get to the center.”
One of the stumbling blocks is the presence of an especially puzzling layer at the base of the lower mantle that distorts seismic waves. “Located 2900 kilometers under the surface, this layer–dubbed the D” layer–acts like a ground-glass screen.”
Although everyone agrees about the existence of the inner core and its anisotropy, other aspects are subject to debate: Does gravity force the inner core to follow the Earth’s rotation or does it have a life of its own, its movement influenced by the Earth’s magnetic field? “In principle, we tend to think that the force of gravity predominates and that the inner core does not have its own rotation, which is also what the models say,” Souriau explains. “However, some teams believe that they’ve observed a rotation.” Figuring out which theory is right won’t be easy.

Denis Delbecq