Magnetic field around since Hadean

by Timothy Oleson
Tuesday, December 15, 2015

Earth's magnetic field, which shields the planet from most high-energy solar radiation, could be 4.2 billion years old, far older than previously recognized. Credit: Michael Osadciw/University of Rochester.

Earth’s magnetic field might have been shielding the planet from solar radiation and contributing to its habitability 770 million years earlier than previously recognized, according to a new study in Science.

Based on traces of magnetism recorded in mineral crystals found in South Africa, John Tarduno of the University of Rochester and colleagues reported in 2010 that Earth had a geodynamo of swirling, iron-rich magma deep in its interior as far back as 3.45 billion years ago. The geodynamo is what produces Earth’s magnetic field. In the new study, Tarduno led another team studying zircon crystals that formed between about 3.3 billion and 4.2 billion years ago, and which were later deposited in sediments making up what are now Western Australia’s Jack Hills — home to Earth’s oldest-known mineral grains.

The researchers used an extremely sensitive magnetometer and found that tiny bits of magnetite within the zircons preserved traces of ancient magnetism indicating that the magnetic field from the Late Hadean through the Paleoarchean was 12 to 100 percent as strong as it is today. They also confirmed that the original magnetic signature preserved in the mineral grains had not been reset by later heating or geologic processes, which would have aligned the magnetic fields in the different grains and/or caused lead in the zircons to diffuse in a recognizable pattern.

Tarduno’s team wrote that the thermal convection needed to maintain the early geodynamo and magnetic field was likely driven by “advection [of heat] through heat pipes and/or plate tectonics,” suggesting that plate tectonics might have already been operating in the Hadean.

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