Researchers at the Earth-­Life Science Institute (ELSI) at the Tokyo Institute of Technology (Tokyo Tech) have made some exciting discoveries about the processes in the Earth’s core that drive the geomagnetic field. Their findings hint at a previously unknown source of energy that drives the Earth's magnetic field, as well as insights into the planet’s core – how it cooled, its chemical composition and the conditions that existed when it formed.

The research dealt with the mechanism that drives the Earth’s magnetic field, which is generated as currents of molten iron-rich metal in the outer core swirl around the solid iron inner core, producing a dynamo effect. The conventional view is that the swirling is the result of thermal convection currents driven by heat escaping from the core.

But the mechanism of the early field has been a mystery, in particular the energy source that powered the long-running dynamo that created the field. Judging by the high thermal conductivity of the Earth’s core, the temperature of the core, for example 3 billion years ago, must have been unrealistically high if convection in the core has been induced by thermal convection. There must be something else going on, some other energy source, to keep the ball rolling.

Crystal energy

Research by the ELSI team and others indicates that this additional energy source could be “compositional convection.” Under the extreme conditions of the core, crystals of silicon dioxide form and precipitate out of the liquid metal. As the silicon and oxygen are removed, the buoyancy of the liquid changes, and this induces convection.

The team at ELSI, led by Prof. Kei Hirose, recently reported in Nature on their investigations into this mechanism. They examined what happens to cooling liquid alloys spanning a range of compositions under conditions that mimic those in the Earth’s core.

At Hirose's lab at ELSI, the scientists used precision-cut diamonds as an “anvil” to squeeze tiny dust-­size samples to the same pressures that exist at the Earth's core, and a laser to simulate the high temperatures. They then examined the textural and chemical characteristics of the resulting samples under an electron microscope.

The researchers were surprised to find that, when they examined the samples under an electron microscope, the small amounts of silicon and oxygen in the starting sample had combined together to form silicon dioxide crystals (Fig. 2), the same composition as mineral quartz found at the surface of the Earth.

The search of alloys began to yield even more useful results when Hirose and his collaborators began mixing more than one alloy. “In the past, most research on iron alloys in the core has focused only on the iron and a single alloy,” Hirose said. “But in these experiments we decided to combine two different alloys containing silicon and oxygen, which we strongly believe exist in the core.”

In addition to core cooling and energy sources for the geomagnetic field, the research gives some indications of the chemical composition of the core. “The core is mostly iron and some nickel, but also contains about 10% light alloys such as silicon, oxygen, sulfur, carbon, hydrogen and other compounds,” Hirose said. “We think that many alloys are simultaneously present, but we don't know the proportion of each candidate element.”

John Hernlund of ELSI, a co­author of the study, said, “This result proved important for understanding the energetics and evolution of the core.” He added, “We were excited because our calculations showed that crystallization of silicon dioxide crystals from the core could provide an immense new energy source for powering the Earth's magnetic field.”

The team has also explored the implications of these results for the formation of the Earth and conditions in the early Solar System. As the crystals form, the composition of the entire core changes as silicon and oxygen are gradually removed over time.

Eventually, some time long in the future, the crystallization process will stop when the core runs out of its ancient inventory of either silicon or oxygen.

The ELSI team’s experiments go a long way toward resolving the mystery of what powered the long-running geodynamo in the early Earth. In addition, the model is potentially as applicable to magnetic fields operating elsewhere in the Solar System as it is to the one operating within our own Earth.