The Earth had its own magnetic field already 4, 2 billion years ago. Geologists came to this conclusion after analyzing materials from excavations in Australia. The results of the scientists' measurements were published in the scientific journal Proceedings of the National Academy of Sciences.
"Our new data confirm past results, and also indicate the existence of a period of unusually high strength of the Earth's magnetic field, which began and ended about 4 billion years ago. It was probably associated with the deposition of various compounds in the liquid core of the young planet," the scientists write. …
Five years ago, they found out that the Earth's magnetic field and the circulation of rocks in its bowels originated almost immediately after our world collided with the progenitor of the Moon, analyzing the isotopic and chemical composition of the so-called zircons. They were extracted from the deposits of the oldest rocks on the planet, which are located in the town of Jack Hills in Western Australia.
Under the Jack Hills are rocks of the so-called Pilabar Shield. On its territory, as well as in similar deposits found on the territory of South Africa, the pristine ancient crust of the Earth, whose age is at least 3.6 billion years, has been preserved.
In turn, zircons are microscopic crystals of highly refractory minerals that are present inside the igneous rocks that form the basis of the crust on the continents. They arise and break down only at very high temperatures, which makes it possible to use zircons to estimate the age of the most ancient rocks of the Earth and their other features, including the strength of the magnetic field in the past.
John Tarduno, professor of geology at the University of Rochester (USA), and his colleagues found fairly clear traces of the Earth's ancient magnetic field in zircons, which formed about 4.2 billion years ago. Not all geologists believed this, which forced the authors of the article to collect new evidence for their hypothesis.
In particular, many skeptics have suggested that the dating of the zircons may have been distorted by cracks in some of them. Water can penetrate through these cracks into such crystals. The authors of the article took into account all these claims and collected a large number of very high-quality crystals, inside which such a problem could not arise.
Using the same techniques and quantum magnetic field sensors, Tarduno and his colleagues repeated the measurements and obtained the same date as in the first case. Scientists also found hints that the strength of the geomagnetic field in subsequent eras was quite high and at the same time changed noticeably over time.
The large force of the Earth's magnetic "shield" in the early epochs of its existence, which was approximately twice as high as in later ones, came as a surprise to geologists. Previously, scientists believed that the Earth's magnetic field became more powerful only after a solid iron "core" was formed in its core, and an intense circulation of molten iron appeared around it.
So far geologists cannot explain this unambiguously. Nevertheless, they suggest that the powerful magnetic field of the young Earth arose due to the fact that at that time its core was actively mixed under the influence of peculiar "rivers" of iron, which moved towards the center of the planet, and similar flows from magnesium and silicon oxides, which "floated" to the surface of the nucleus.
When the core cooled to a certain point, this process stopped, as a result of which the strength of the magnetic field decreased sharply. It, Tarduno and his colleagues conclude, remained low until the inner part of the core solidified and the modern cycle of metals in its liquid part appeared, which protects us from cosmic rays and the solar wind.