News in Science

Earth's magnetic field much older than previously thought

Ancient dynamo Earth's magnetic field is 800 million years older than previously thought, new research suggests.

A new analysis of Western Australian zircon minerals has found the engine that generates the field started not long after the planet formed.

Earth's so-called "geodynamo", involving the movement of molten iron in the Earth's outer core, began 4.22 billion years ago, say researchers today in the journal Science.

"This opens a window into a period that we know almost nothing about," says co-author, Professor Francis Nimmo of the University of California, Santa Cruz.

"Before this study we knew that the dynamo had existed for around three and a half billion years. What this study has done is push back the age of the dynamo by another 800 million years."

Earth's magnetic field acts as a shield protecting the planet's atmosphere and water, which make life on Earth possible.

Without the magnetic field Earth's atmosphere would have been eroded away by the solar wind, a stream of charged particles flowing from the Sun.

The magnetic field was particularly important in Earth's early history when solar winds were about 100 times stronger than they are now.

"The young Sun was very active, and so having a strong magnetic field early on allows you to hang on to your atmosphere," says Nimmo.

"Mars had a dynamo early on, but then that dynamo died," he says.

"So part of the reason that Mars lost its atmosphere is not simply that it has less gravity, but also that it didn't have a magnetic field protecting the atmosphere from being blown away."

Zircon crystals

In their new study Nimmo and colleagues calculated when Earth's geodynamo started by analysing zircon crystals from Jack Hills in Western Australia.

"Jack Hills zircons are famous because they are the oldest minerals we know of on this planet," says Nimmo.

The zircons, which are billions of years old, contain a naturally occurring magnetic iron oxide (magnetite), which records the strength of the magnetic field at the time it solidified from its molten state.

The researchers used a unique magnetometer to measure the magnetic intensity of the magnetite in zircons of different ages, to probe the history of the planet's magnetic field.

To assess whether the magnetic information in the zircons was reliable, the team confirmed that the minerals hadn't been disturbed from the time they were first deposited.

Field strength

Solar winds hitting Earth's atmosphere can create small magnetic fields without a geodynamo, but these would have a maximum magnetic field strength of only about 0.6 micro-Teslas.

In comparison, the values measured by the authors in the ancient zircons were far higher, indicating the presence of a geodynamo some 4.22 billion years ago.

Prior to this research, the best estimate of the age of Earth's magnetic field was between 3.2 billion and 3.45 billion years, based on samples from igneous rocks in the Nondweni and Barberton Greenstone Belts of South Africa.

"These new results represent the first time that we know anything about the Earth's magnetic field prior to the great heavy bombardment 3.9 billion years ago when the planets experienced a huge meteor storm," says Nimmo.

"There was this sort of spike of impacts and that's erased a lot of evidence of what happened before, but these zircons survived that impact and so they're telling us something about the state of the Earth before it was hit by all these asteroids."

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