A collaborative study by the University of Oxford and the Massachusetts Institute of Technology (MIT) has uncovered 3.7 billion-year-old magnetic field records from Greenland, proving that Earth's ancient magnetic field was as strong as it is today and was essential for protecting life from the universe and solar radiation.
There are 3.7 billion-year-old banded iron formations in the northeastern part of the Izua's super crustal belt. Source: Claire Nichols
A new study has reconstructed a 3.7-billion-year-old record of the Earth's magnetic field and found that it is very similar to the magnetic field surrounding the Earth today. The findings were published today (April 24) in the Journal of Geophysical Research.
Life on Earth would not exist without a magnetic field, which protects us from harmful cosmic radiation and charged particles emitted by the sun (the "solar wind"). However, to date, there is no reliable date as to when the modern magnetic field began to form.
The researchers took samples along the cross-section to compare the differences between the 3.5 billion-year-old igneous intrusion and the surrounding rocks. Source: Claire Nichols
In the new study, the researchers examined an ancient sequence of iron-bearing rocks in Isua, Greenland. When the crystallization process locks the iron particles in place, the iron particles can effectively act as tiny magnets, recording the strength and direction of the magnetic field. The researchers found that the 3.7 billion-year-old rock captured a magnetic field of at least 15 microtesla, comparable to the modern magnetic field (30 microtesla).
These results provide the oldest estimate of the strength of the Earth's magnetic field derived from a sample of entire rocks, and the results provide a more accurate and reliable assessment than previous studies using individual crystals.
Athena Eyster, co-author of the study, stands in front of a large expanse of exposed banded ferrous formations, an iron-rich sediment that was the source of the extraction of the ancient magnetic field signal. Image credit: Claire Nichols
Principal Investigator Professor Claire Nichols (Department of Earth Sciences, University of Oxford) said: "It was extremely challenging to extract a reliable record from such an ancient rock and it was exciting to see the native magnetic field signals start to emerge as we analysed these samples in the lab. This is a very important step forward as we try to determine the role of paleomagnetic fields when life on Earth first appeared. "
While the magnetic field strength seems to have remained relatively stable all along, the solar wind is known to have been much stronger in the past. This suggests that over time, the Earth's surface has become more protected from the solar wind, which may have allowed life to migrate to the continent and leave the protection of the ocean.
The Earth's magnetic field is created by a mixture of molten iron in the outer core of a fluid, which is driven by buoyancy as it solidifies, creating a generator. In the early days of the formation of the Earth, the solid core had not yet formed, so the question of how the early magnetic field was maintained remained unresolved. These new results suggest that the mechanisms that drove Earth's early generators were similar to the solidification processes that generate the Earth's magnetic field today.
Understanding how the strength of the Earth's magnetic field changes over time is also key to determining when the solid core of the Earth's interior began to form. This will help us understand how quickly heat escapes from the depths of the Earth's interior, which is critical to understanding processes such as plate tectonics.
One of the major challenges in reconstructing the Earth's magnetic field over such a long period of time is that any event that heats the rock will alter the preserved signal. Rocks in the earth's crust tend to have a long and complex geological history, erasing previous magnetic field information. However, the Isua Super Crustal Belt is geologically unique in that it sits atop a thick continental crust, making it immune to extensive tectonic activity and deformation. This allowed the researchers to build a clear body of evidence supporting the existence of a magnetic field 3.7 billion years ago.
These results may also shed light on the role of magnetic fields in shaping the development of the Earth's atmosphere as we know it, particularly the escape of gases from the atmosphere. One phenomenon that cannot be explained today is that more than 2.5 billion years ago, the atmosphere lost its inactive gas xenon. Xenon is relatively heavy, so it is impossible to simply drift out of the atmosphere. Recently, scientists have begun to study the possibility that charged xenon particles are removed from the atmosphere by magnetic fields.
In the future, researchers hope to expand our understanding of the Earth's magnetic field before the increase in oxygen in the Earth's atmosphere about 2.5 billion years ago by studying other ancient rock sequences in Canada, Australia, and South Africa. A better understanding of the strength and variability of the ancient Earth's magnetic field will help us determine whether planetary magnetic fields are key to hosting life on planetary surfaces and their role in the evolution of the atmosphere.
编译来源：ScitechDaily