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

In the southwest of Greenland, surrounded by ancient ice, lies the belt of green cliffs of Isua. It is the oldest and best preserved lithological formation on the planet. In these stones, according to some dubious 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 at the same time the most primitive signal of the Earth’s magnetic field, a kind of dome that protects the Earth and all life on it from external radiation.

Although science has made some progress on the matter, it has been suggested that the dynamics of the Earth’s outer core, consisting mostly of molten iron and nickel, rotating around an inner iron ball generates electric fields that, as they rotate, maintain a magnetic field as if the planet were bicycle dynamo. Its range extends hundreds of kilometers beyond the atmosphere. This magnetosphere encounters cosmic radiation and, in particular, the solar wind, a stream of particles that, if it reaches the Earth’s surface, can break the DNA strands that support, for example, all living things. But this protective sky did not always exist, and the dating of its appearance is relevant for completing the writing of the first chapters of life on Earth. Also understand its absence on other planets without magnetism, such as Venus.

That’s why the discovery, recently announced by a dozen scientists from as many universities, is so significant. After years of searching, they discovered the iron mineral magnetite in this Isua green rock belt, which preserves a signal from an event that occurred about 3.7 billion years ago, allowing them to detect the magnetic field that existed then. If this is confirmed by new analyses, this will be the first trace of terrestrial magnetism.

At that time, a geological process – probably tectonic – with temperatures above 580° changed the shape and composition of the rocks. In one such modification, 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 led to the formation of magnetite, which acquired a magnetic field record 3.7 billion years ago,” says Professor of Earth Sciences at the University of Oxford (UK) and. The first author of the work is Claire Nichols. This dating extends the presence of this deposit by several hundred years. So far, the oldest traces of paleomagnetism have been found in rocks from South Africa and Australia.

According to the results of a study published in a scientific journal Journal of Geophysical Research, the magnetic field strength was then 15 microtesla. Currently, although variable, its average value is approximately 30 microtesla. In the past, the solar wind was significantly stronger, suggesting that the protection of the Earth’s surface from external radiation has increased over time. This leads us to imagine the connection between field protection and the evolution of life on the planet, first allowing it and then facilitating its transition from marine to terrestrial environments. But Nichols reminds us that his work “offers no evidence for or against the existence of life or earlier, 3.7 billion years ago, only the conditions that any current life would have experienced.”

Rock samples mined in Greenland preserve information about what the Earth’s magnetic field was like 3.7 billion years ago.Claire Nichols

The dates do not match: even before the formation of these Greenland rocks, marine bacterial life already existed. We will have to wait several hundred million years for the so-called Great Oxidation to occur. And it would take many more years for life to emerge from the water and conquer land. But none of this could happen without the Earth’s magnetic field and magnetosphere.

The Earth’s magnetic field is created by mixing molten iron with a liquid outer core, which is driven by convection forces as the inner core solidifies. At the initial stage of the planet’s formation, the solid part had not yet formed, so questions remain open about how the magnetic field was maintained then. The British researcher believes it is highly likely that the Earth “has always generated a magnetic field, especially in its early history when the planet was very hot and thermal convection in the core was strong.”

For the authors, understanding how the intensity of Earth’s magnetic field has changed over time is also key to determining when the planet’s solid inner core began to form. This will help understand how quickly heat escapes from the Earth’s interior, which is important for understanding processes such as plate tectonics. And the key to the future. The cooling and complete solidification of the Earth’s core is still a long way off, but this process should 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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