Earth’s protective sky is at least 3.7 billion years old

In southwest Greenland, surrounded by ancient ice, lies the Isua Greenstone Belt. It is the best-preserved rock formation of the Eoarchean era. In these rocks, according to some questioned studies, the first signs of life were recorded 3.7 billion years ago. Now, a group of scientists claims to have found in the same place, and dating back to the same era, the most primitive signal of the Earth’s magnetic field, a kind of dome that protects the Earth and all the life it houses from outside radiation.

Although science is somewhat hesitant about this, it has been posited that the dynamics of the outer part of the Earth’s core, composed essentially of molten iron and nickel rotating around an inner ferric ball, generates electric fields that in their rotation sustain a magnetic field, a bit as if the planet were the dynamo of a bicycle. Its range extends hundreds of kilometers beyond the atmosphere. This magnetosphere encounters cosmic radiation and, in particular, the solar wind, a shower of particles that, if it reached the Earth’s surface, could break the DNA chains that support all living beings. But this protective sky was not always there, and dating its appearance is relevant to finishing writing the first chapters of life on Earth. Also to understand its absence on other planets without magnetism, such as Venus.

That is why the discovery, recently announced by a dozen scientists from as many universities, is so significant. After years of searching, they found what they were looking for in a belt of green rocks in Greenland: an iron ore, magnetite, which preserves the signal of an event that occurred about 3.7 billion years ago that allows them to detect the magnetic field that existed back then. If confirmed by new analyses, it would be the first trace of terrestrial magnetism.

At that time, a geological process — probably tectonic — with a temperature above 580ºC (1,076ºF) modified the shape and composition of the rocks. In one such modification, the iron particles of magnetite, the most magnetic mineral known, were reoriented and captured the intensity of the magnetic field. The rocks became magnetized during an early high-temperature metamorphic event that caused magnetite to form, acquiring a record of the magnetic field 3.7 billion years ago, says Claire Nichols, a professor of Earth Sciences at the University of Oxford and lead author of the work. This dating means advancing the presence of this field by several hundred years. Until now, the oldest paleomagnetism signatures have been found in rock formations in South Africa and Australia.

According to the results of this research, published in the Journal of Geophysical Research, the intensity of the magnetic field at that time was 15 microteslas. Currently, although variable, it has an approximate average value of 30 microteslas. The solar wind has been significantly stronger in the past, suggesting that the protection of the Earth’s surface from outside radiation has increased over time. This invites us to fantasize about the connection between the protection of the field and the evolution of life on the planet, first allowing it and then facilitating the passage from the marine to the terrestrial environment. But Nichols reminds us that their work does not offer evidence either for or against the presence of life 3.7 billion years ago, or earlier, only the conditions that any present life would experience.

The dates do not match: even before the formation of these Greenland rocks, marine bacterial life already existed. It was several hundred million years later that the so-called Great Oxidation Event, which raised oxygen levels, occurred. And it would take many more years for life to emerge from the water and conquer dry land. But none of this could have happened without the Earth’s magnetic field and magnetosphere.

Earth's magnetic field is generated by mixing molten iron in the fluid outer core, driven by convection forces as the inner core solidifies. During the initial phase of planet formation, the solid part had not yet formed, leaving open questions about how the magnetic field was then sustained. The British researcher believes it is very likely that the Earth “has always generated a magnetic field, particularly in its earliest history, when the planet was very hot and thermal convection in the core would have been vigorous.”

For the authors, understanding how the intensity of Earth’s magnetic field has varied over time is also key to determining when the planet’s solid inner core began to form. This would help understand how quickly heat escapes from the Earth’s deep interior, essential for understanding processes such as plate tectonics. And it could be a key piece of information for the future. There is still a long way to go for the Earth’s core to cool and completely solidify, but this process must have happened (or is happening) on other planets that had and no longer have a magnetic field and that had and no longer have an atmosphere.

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