Evidence of Earth’s magnetic field dates back 3.7 billion years

By Stephen Beech via SWNS

The oldest undisputed evidence of Earth’s magnetic field - dating back 3.7 BILLION years - has been discovered.

And it appears "remarkably similar" to the field surrounding our planet today, say scientists.

They explained that life on Earth would not be possible without its magnetic field as it shields us from harmful cosmic radiation and charged particles emitted by the Sun.

But, until now, there has been no reliable date for when the modern magnetic field was first established.

Scientists say that a better understanding of the ancient strength and variability of Earth’s magnetic field will help to determine whether magnetic fields are critical for hosting life on a planet's surface.

The research team, led by scientists at Oxford University and Massachusetts Institute of Technology (MIT) in the United States, examined an ancient sequence of iron-containing rocks from Isua, Greenland.

They explained that iron particles effectively act as tiny 'magnets' that can record both magnetic field strength and direction when the process of crystallization locks them in place.

The team found that rocks dating from 3.7 billion years ago captured a magnetic field strength of at least 15 microtesla comparable to the modern magnetic field (30 microtesla).

The findings, published in the Journal of Geophysical Research, provide the oldest estimate of the strength of Earth’s magnetic field derived from whole rock samples, which offer a more accurate and reliable assessment than previous studies that used individual crystals.

Lead researcher Oxford University Professor Claire Nichols said: "Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analyzed these samples in the lab.

"This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging."

She said that while the magnetic field strength appears to have remained relatively constant, the solar wind is known to have been "significantly" stronger in the past.

Nichols said: "This suggests that the protection of Earth’s surface from the solar wind has increased over time, which may have allowed life to move onto the continents and leave the protection of the oceans."

She explained that Earth's magnetic field is generated by the mixing of the molten iron in the fluid outer core, driven by "buoyancy forces" as the inner core solidifies, which creates a dynamo.

During Earth’s early formation, the solid inner core had not yet formed, leaving questions about how the early magnetic field was sustained.

The new results suggest the mechanism driving Earth’s early dynamo was similarly efficient to the solidification process that generates Earth’s magnetic field today.

Prof Nichols says that understanding how Earth’s magnetic field strength has varied over time is also key to determining when Earth’s inner, solid core began to form.

She said that will help establish how rapidly heat is escaping from Earth’s deep interior, which is key to understanding processes such as plate tectonics.

The research team says the Isua Supracrustal Belt has a unique geology, sitting on top of a thick continental crust which protects it from extensive tectonic activity and deformation.

That allowed them to build a clear body of evidence supporting the existence of the magnetic field 3.7 billion years ago.

Now the research team hopes to further expand its understanding of Earth’s magnetic field before the rise of oxygen in the planet's atmosphere around 2.5 billion years ago by examining other ancient rock sequences in Canada, Australia, and South Africa.