Half a billion years ago, Earth was on track to become just like Mars. Ancient rocks now show what saved our magnetic field

A depiction of Earth, first without an inner core; second, with an inner core beginning to grow, around 550 million years ago; third, with an outermost and innermost inner core, around 450 million years ago. Credit: University of Rochester illustration / Michael Osadciw.

Thank Earth’s magnetic field for all the abundant riches and diversity of life this planet has to offer. Without it, solar radiation would strip away the atmosphere and oceans, leaving behind a barren landscape. That’s in fact what happened to Mars, once believed to be a “blue” water world covered in a planetary ocean, which eventually turned “red” after it mysteriously lost its magnetic field some four billion years ago. What’s truly scary is that, at some point in geological history, Earth wasn’t lagging too far behind.

About 565 million years ago, scientists found evidence that the strength of Earth’s magnetic field plummeted to just 10% of today’s strength. If this trend had continued, all life on our planet would have been surely doomed. But right before the so-called Cambrian explosion of multicellular life on Earth, the magnetic field bounced back — and, according to a new study by geologists at the University of Rochester, this is all thanks to some fundamental changes in the inner core.

Ancient rocks reveal how Earth avoided a Mars-like fate

Earth is made up of many layers, like an onion. There’s the crust, on top of which we all live, followed by Earth’s thickest layer, known as the mantle, the molten outer core, and the solid inner core, in this order. The inner core is, in turn, now made of an outermost core and an innermost core.

It’s the movement of the outer core that is responsible for the magnetic field. It is here that swirling liquid metals — in motion due to convection currents and the planet’s rotation — act like a “geodynamo” that generates the magnetic field which shields us from harmful blasts of solar radiation. The metals are in a constant churning motion due to the planet’s interior releasing heat — and this flow is aided by plate tectonics.

According to a 2015 study published in Science, the best estimate of the age of Earth’s magnetic field is 4.2 billion years. In comparison, Earth is about 4.6 billion years old. However, the strength of the magnetic field has not always been constant.

Scientists are able to know how Earth’s magnetic field varied even billions of years in the past by studying ancient iron-rich minerals. Initially, these minerals were fluid lava whose iron atoms aligned themselves with Earth’s magnetic field just like a compass needle. Once the lava cools, the resulting minerals serve as a direct record of the strength and orientation of Earth’s magnetic field at that time.

By studying this paleomagnetism record, scientists were able to tell that Earth’s magnetic field went through many episodes of reversal, in which the north and south poles of the dipole swapped, as well as important changes in strength.

The lowest point in Earth’s magnetic field probably occurred some 565 million years ago, when it almost disappeared. But in just 15 million years — almost a flash in geological time — the magnetosphere bounced back. According to a new study led by John Tarduno, Professor of Geophysics in the Department of Earth and Environmental Sciences at the University of Rochester, this rapid renewal in magnetic field strength can be explained by the formation of a solid inner core about 550 million years ago, which essentially recharged the molten outer core. Then, some 450 million years ago, the growing inner core’s structure changed yet again, cementing a boundary between the innermost and outermost inner core.

Quite importantly, these changes to the inner core coincide with the shifts in the structure of the overlying mantle owed to plate tectonics at the surface.

“Because we constrained the inner core’s age more accurately, we could explore the fact that the present-day inner core is actually composed of two parts,” Tarduno says. “Plate tectonic movements on Earth’s surface indirectly affected the inner core, and the history of these movements is imprinted deep within Earth in the inner core’s structure.”

To reach these conclusions, Tarduno and colleagues used a CO2 laser and a superconducting quantum interference device (SQUID) magnetometer to analyze feldspar crystals from the rock anorthosite. The geophysicists were able to determine what that paleointensity was by heating single crystals to demagnetize them, and then reheating the samples in the presence of a magnetic field to impart magnetization.

By better understanding how the magnetosphere’s strength waxed and waned in time, researchers can not only uncover Earth’s past and predict its future, but they may also be able to tell how other planets might form their magnetic fields and what this means for alien life. Again, Mars serves as a cautionary tale.

“Earth certainly would’ve lost much more water if Earth’s magnetic field had not been regenerated,” Tarduno says. “The planet would be much drier and very different than the planet today.”

“This research really highlights the need to have something like a growing inner core that sustains a magnetic field over the entire lifetime—many billions of years—of a planet.”

The findings appeared in the journal Nature Communications.

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