Ancient rocks reveal that Earth’s magnetic field existed 3.7 billion years ago

It was a time when the earth can have still wrapped in waterand it was early in life has just started to appear – and the newly discovered traces of the planetary magnetosphere reveal that it was surprisingly similar to the invisible structure that still protects Earth today.

This discovery could help scientists understand the early history of Earth and the factors that contributed to the emergence of life. In addition, the detailed analysis could help measure early traces of Earth’s magnetic field at other locations around the world, helping to recreate its global shape and evolution over the planet’s 4.5 billion-year lifespan.

The Earth’s magnetic field is a huge shell that envelops and encloses our planet. It is believed to be generated deep toward the core, where the rotation of convecting, conducting fluid known as the geodynamo converts kinetic energy into electric and magnetic fields that are ejected into space.

Scientists also believe that we have this magnetic field to thank for Earth’s habitability: it protects the atmosphere from being rapidly stripped by the solar wind and deflects harmful radiation. What we have a less clear idea of ​​is how the magnetic field developed during the earth’s history. What was it like when our planet was young, and how has it changed over the eons since then?

This is where rocks can be really useful. While still soft when they form, any ferromagnetic material in them aligns with external magnetic influences – like the huge magnetic field that wraps around and goes through the Earth.

These geological records can reveal not only the direction of the magnetic field, but its strength. The trick is to find the right kind of stone and to be able to interpret what it has to say.

First, any event or force that heats the rock can alter it, erasing and overwriting previous magnetic information. This is why it is so difficult to find reliable data about the early magnetic field, even though the rock itself is confirmed to be very old.

A team led by geologist Claire Nichols from the University of Oxford in the UK has now found just such a record, in the lonely wilderness Isua in Greenland.

There, a region called the Supracrustal Belt has a special geology: it sits on a thick layer of Earth’s crust that has protected it from tectonic activity and deformation for billions of years.

In that formation there are rocks that are rich in iron. Lead-uranium dating of zircon crystals in this formation has revealed that some of the rocks formed about 3.7 billion years ago.

Nichols and her team studied the iron in these ancient rocks to determine the characteristics of Earth’s magnetic field at the time. Their results revealed that the strength of the magnetic field 3.7 billion years ago was at least 15 microtesla. That is remarkably comparable to the current strength of about 30 microtesla.

It suggests that the early geodynamo was as effective and efficient as it is today. But the sun has not always been as it is now. During the Archaean it was also quite young and much more active, and the solar wind was much stronger.

This suggests that the level of protection from the solar wind provided by the magnetic field has increased over time. This could have implications for the origin of life – that it could not arise until the protection provided was at a certain level.

The findings may also help to understand the evolution of the Earth’s atmosphere. There is evidence that a great deal of ionized xenon and hydrogen was lost during the Archaean; since xenon is relatively heavy, it is unlikely to be removed from a highly shielded atmosphere in any significant amounts.

One possibility is a slightly weaker magnetic field which enables a broadening of the exposure to solar winds at the poles. The team’s research shows that this is possible if the magnetic field strength was at the lower end of the scale they determined from the Greenland rocks.

More research will be needed to determine how the magnetic field behaved and changed, from the Archaean onwards. But one thing is now certain.

“Regardless of its exact strength and stability,” write the researchers“our results suggest that Earth has maintained an 950 intrinsic magnetic field since at least 3.7 billion years ago.”

Go for it.

The team’s research has been published in Journal of Geophysical Research: Solid Earth.