The Earth has finite resources of matter. But thanks to its own internal heat and the light of the sun it has almost unlimited supplies of energy with which to remake itself over a vast range of timescales. Water lasts in the atmosphere for a fortnight or so; carbon dioxide stays in the oceans for thousands of years. Mountains rise and fall over tens of millions of years; oceans open and close at rates even slower than that.
And for some things, in some places, there is a sort of stillness. The argon in the atmosphere just sits there, inert. The crystalline cratons at the centres of continents get neither buried nor torn apart by plate tectonics, though they may sometimes be submerged in shallow seas and sediments as they drift from place to place. Not everything, everywhere is in flux. But it feels as though the harder scientists look at the world, the fewer islands of stability they find.
A study published this week in Nature bears out that trend in a spectacular way. At the centre of the Earth, below the mountains and the oceans and the thin, brittle crust, below the stony, slow-flowing mantle and the roiling outer core of liquid iron, is a solid inner core. If anything about the planet looked unlikely to partake in a process of endless recycling, you might think this ball of metal, 1,200 kilometres across, squeezed from every direction by a planet's worth of weight, would be it—a dense static hub about which all else turns.
Scientists have known for some time that this inner core is not unchanging. But they had thought that it changed in only one direction—that it simply grew bigger. The Earth is growing cooler as it loses the heat trapped in its creation and generated by radioactive elements within it. It is in fact this cooling which powers the slow circulation of the mantle, and through that the endless remaking of the surface through plate tectonics. As things cool down, the liquid outer core freezes into the solid inner core. It is thought that this process leads the inner core to grow larger at a rate of roughly 30 centimetres a century.
The remarkable new idea floated by Thierry Alboussière, Renaud Deguen and Mickaël Melzani of the Joseph Fourier University in Grenoble is that this slow growth is a net effect, the residual left over when a greater rate of freezing is offset by a rate of melting almost as large. This notion follows from the hitherto unexplored idea that the spherical inner core is very slightly offset from the planet's centre of mass, so that one side—the western side, as it happens—is slightly lower than the other. On the lower side the pressure is greater, and liquid iron freezes solid. On the higher side the pressure is less, and solid iron melts.
The net effect of this asymmetry, should it persist, would be what the authors call a “convective translation”: iron that joins the core in the west will slowly move through it until it melts off in the east. At the rate the authors suggest for this process, it would take about 80 million years for iron to pass all the way through and back into the outer core, though the deformation that this flow would impose on the solid core would undoubtedly complicate matters in ways that have yet to be addressed.
This model may be able to explain various oddities about the inner core—such as the fact that seismic waves pass through it differently when headed north-south than when going east-west—and its surroundings, including the existence of a peculiarly dense fluid layer just above it. It is possible that this new behaviour may have implications beyond the core; that it might explain details of the way that the outer core circulates, and thus the ways in which the Earth's magnetic field changes over time. Once the world is seen as a set of cycles rather than of things it is easier to imagine interesting ways for them to mesh like cogs. The carbon cycle influences the rate at which mountains weather down into seas, the deep circulation of ocean waters helps govern the ebb and flow of ice sheets, and so on.
That said, even if further evidence backs it up, the idea that the inner core is in a continuous cycle of self recreation probably won't matter that much to the landscapes and ecosystems doing similar things 5,000 kilometres further out. The effect is more one of underlining an aesthetic, or even an ideology, of the planet as an engine of ceaseless self-stabilising change. Such an ideology may serve as a useful guide to dealing with the unavoidable impacts that a large technological civilisation must have on the planet it inhabits: while caution counsels minimising such impacts, a sense of how the planet works suggests that making sure its natural systems can deal with them, that they can become part of the flow, could matter just as much.
That may seem too farfetched. Sufficient, perhaps, just to stop and think how strange it is that the inner core, imperviously locked away since the creation of the world, may yet be added to the long list of other solid-looking things, such as the Himalayas and the Atlantic Ocean and the planet itself, that are in some ways better understood not as places, but as processes.