Molten silicate might solve a long-standing magnetic mystery

Billions of years ago, Earth’s magnetic field may have gotten a jump-start from a turbulent magma ocean swirling around the planet’s core.

Our planet has generated its own magnetism for almost its entire history (SN: 1/28/19). But it’s never been clear how Earth created this magnetic field during the planet’s Archean Eon — an early geologic period roughly 2.5 billion to 4 billion years ago. Now, computer simulations suggest that a deep layer of molten rock-forming minerals known as silicates might have been the culprit.

“There’s a few billion years of Earth’s history where it’s difficult to explain what was driving the magnetic field,” says Joseph O’Rourke, a planetary scientist at Arizona State University in Tempe who was not involved with this study. This new result, he says, is a “vital piece of the puzzle.”

Today, Earth’s magnetism is likely generated in the planet’s outer core, a layer of liquid iron and nickel. Heat escaping from the solid inner core drives flows of fluid that create circulating electric currents in the outer core, turning Earth’s innards into a gigantic electromagnet. The outer core, however, is a fairly recent addition, appearing roughly a billion or so years ago, and ancient rocks preserve evidence of a planetwide magnetic field much earlier than that. So, some other mechanism must have been at work during Earth’s formative years.

One candidate for Earth’s first go at a magnetic field is a sea of liquid rock hypothesized to once have surrounded the young planet’s nascent core. To see if this ocean of molten silicates is a viable option, Lars Stixrude, a geophysicist at UCLA, and colleagues developed computer simulations to estimate the electrical properties of silicates at extreme temperatures and pressures.

The team found that, at pressures more than 10 million times Earth’s surface atmospheric pressure and temperatures comparable to those on the surface of the sun, silicates conduct electricity well enough to produce a planetwide magnetic field. The strength of that field, the team reports February 25 in Nature Communications, roughly matches measurements of fossil magnetic fields in rocks that are about 2 billion to 4 billion years old. Around the end of the Archean, the team suggests, the magma ocean would have cooled and solidified, possibly handing over magnetic field duties to an increasingly turbulent core.

The study is “an extremely important step forward in understanding the history of Earth’s magnetic field,” O’Rourke says. What’s more, it might also be relevant to other worlds today. “It’s not just a curiosity of ancient history,” he says.

Super-Earths, rocky planets a few times as massive as Earth, might retain enough internal heat to sustain a deep silicate ocean for much longer than our planet did. These planets are also the most common worlds found outside the solar system. The mechanism behind Earth’s early magnetic field, the team speculates, may therefore be operating in large rocky planets throughout the universe.