New Signs of a Shielding Magnetic Field Found in Earth's Oldest Rock Crystals

Colin Barras | Science Magazine
Tuesday, January 21, 2020

Read the full story at Science Magazine

A new study published in PNAS led by John Tarduno, a geophysicist at the University of Rochester, finds "evidence that our planet had a strong magnetic field 4.2 billion years ago, three-quarters of a billion years earlier than previously thought and just 350 million years after Earth formed. The field would have shielded Earth, protecting its atmosphere from being stripped away by high-energy particles from the Sun—and perhaps helping life gain a foothold," Colin Barras writes in Science Magazine. The group found these clues in rocks, which originated from Jack Hills of Western Australia.

If true, these rocks, this would have happened during the Hadean period of Earth's history, from which very few rocks survived. Evidence would have been trapped in crystals within minerals called zirons as they cooled, preserving like a bar magnet a record of the Earth's magnetic field. But potential subsequent heating of the magnetite grains within the zircons could affect this history and/or record a later magnetic field.

EAPS Professor Ben Weiss, who studies paleomagnetism, questions this recent finding.

Tarduno’s team says the magnetization was imprinted 4.2 billion years ago, when the original zircon-containing rock first cooled. However, if the magnetite grains, at any point in their history, became hot enough—above about 600°C—they would have lost the magnetic alignment and gained a new one as they cooled down again. And just such a heating event may have occurred about 2.6 billion years ago, says Benjamin Weiss, a geologist at the Massachusetts Institute of Technology (MIT) and leader of a group that has challenged Tarduno’s claims. “These zircons have mind-bogglingly long and largely unknown histories,” he says.

Tarduno's group offered new geologic evidence to support their conclusion. If heating had occurred chemical diffusion of lithium across zircon boundaries would have happened, but this didn't occur. Others contest contest this.

MIT geologist Claire Nichols hasn’t been involved in any of the Jack Hills studies but she did recently argue that Earth’s magnetic field was in place at least 3.7 billion years ago after analyzing rocks in Greenland. “It’s great to see different research groups pushing each other to use more and more advanced techniques,” she says of the new study. “It gives us the best chance of understanding the earliest record of the geodynamo.”

Story Image: Earth’s magnetic field produces an aurora when disturbed by the solar wind. (Credit: Tiziano Valeno/IStock)