Researchers from the University of Oxford and the Massachusetts Institute of Technology (MIT) have made an important discovery about the history of Earth’s magnetic field. They found the oldest clear evidence that Earth had a magnetic field, dating back 3.7 billion years. This is a significant finding that helps us understand more about the ancient Earth and the conditions that allowed life to develop.

The Earth’s magnetic field is crucial for protecting life on our planet. It shields us from harmful cosmic radiation and solar wind, which can be dangerous for living things. Until now, the exact timing of when Earth’s modern magnetic field was first established has been uncertain. This new study, led by Claire Nichols from Oxford, provides a clearer picture of the ancient magnetic field.

The researchers analyzed ancient rocks containing iron from Isua, Greenland. These iron particles act like tiny magnets, recording both the strength and direction of the magnetic field when they formed. They found that rocks from 3.7 billion years ago showed a magnetic field strength of at least 15 microteslas, comparable to the strength of Earth’s modern magnetic field, which is around 30 microteslas.

An example of the 3.7 billion year old banded iron formation found in the northeastern part of the Isua Supracrustal Belt
An example of the 3.7 billion year old banded iron formation found in the northeastern part of the Isua Supracrustal Belt. Credit: Claire Nichols

This is the oldest reliable measurement of Earth’s magnetic field strength derived from whole rock samples, rather than just individual crystals. This approach gives a more accurate and trustworthy assessment of the ancient magnetic field.

While the strength of Earth’s magnetic field has remained relatively constant over billions of years, the solar wind was much stronger in the past. This means that Earth’s surface protection against solar radiation has improved over time, allowing life to eventually transition from the oceans to land, away from the shielding of water.

The Earth’s magnetic field is generated by the churning of molten iron in the outer core, driven by the solidification of the inner core. In the early days of Earth’s formation, the inner core had not yet fully solidified, raising questions about how the primitive magnetic field was sustained. These new results suggest the early geodynamo mechanism was about as efficient as the current process.

At MIT's Paleomagentism Laboratory, entire rock samples from Isua were measured to extract their ancient record of the Earth's magnetic field
At MIT’s Paleomagentism Laboratory, entire rock samples from Isua were measured to extract their ancient record of the Earth’s magnetic field. Credit: Claire Nichols

Reconstructing Earth’s ancient magnetic field is challenging, as geological events can erase previous magnetic signals. But the Isua Supracrustal Belt, where these rocks were found, has been protected from significant tectonic activity and deformation that allowed the researchers to collect clear evidence of the 3.7 billion-year-old magnetic field.

Understanding variations in Earth’s magnetic field strength over time is important for determining when the solid inner core began to form. This, in turn, provides insights into the rate of heat escaping from Earth’s deep interior, which is key to processes like plate tectonics.

Future research aims to explore Earth’s magnetic field before the appearance of oxygen in the atmosphere, around 2.5 billion years ago. This could help assess whether planetary magnetic fields are critical for the development of life and its role in atmospheric evolution.


Sources

University of Oxford | Nichols, C. I. O., Weiss, B. P., Eyster, A., Martin, C. R., Maloof, A. C., Kelly, N. M., et al. (2024). Possible Eoarchean records of the geomagnetic field preserved in the Isua Supracrustal Belt, southern west Greenland. Journal of Geophysical Research: Solid Earth, 129, e2023JB027706. doi.org/10.1029/2023JB027706


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