The oldest indisputable evidence of the Earth's magnetic field was discovered in Greenland

Collaborative study by Oxford university And Massachusetts Institute of Technology NASA has revealed a 3.7 billion-year-old magnetic field record from Greenland, showing that Earth's ancient magnetic field was as strong as it is today, which is crucial to protecting life by shielding from cosmic and solar radiation.

A new study has recovered a 3.7 billion-year-old record of Earth's magnetic field, finding that it looks remarkably similar to the field surrounding Earth today. The results were published today (April 24) in the journal Journal of Geophysical Research.

Without the magnetic field, life on Earth would not be possible as this protects us from harmful cosmic radiation and charged particles emitted by the Sun (“solar wind”). But until now, there is no reliable date for when the modern magnetic field first emerged.

Samples were extracted along the transects to compare the difference between volcanic intrusions dating back 3.5 billion years, and the surrounding rocks that researchers have shown hold a record of the 3.7 billion-year-old magnetic field. Credit: Claire Nicholls

Examination of ancient rocks

In the new study, the researchers examined an ancient sequence of iron-containing rocks from Isua, Greenland. The iron particles effectively act as tiny magnets that can register the strength and direction of the magnetic field as the crystallization process holds them in place. The researchers found that rocks dating back to 3.7 billion years ago had a magnetic field strength of at least 15 microtesla, compared to the modern magnetic field (30 microtesla).

These results provide the oldest estimate of the strength of the Earth's magnetic field derived from whole rock samples, which provides a more accurate and reliable assessment than previous studies that used individual crystals.

Study co-author Athena Easter stands in front of a large area of ​​the Banded Iron Formation, the iron-rich deposit from which ancient magnetic field signals have been extracted. Credit: Claire Nicholls

Insights from the study

Lead researcher Professor Claire Nicholls (Department of Earth Sciences, University of Oxford) said: “Extracting reliable records from rocks of this age is extremely difficult, and it was really exciting to see the initial magnetic signals starting to emerge when we analyzed these samples in the laboratory.” . This is a really important step forward as we try to determine the role of the ancient magnetic field when life first appeared on Earth.

While the strength of the magnetic field appears to have remained relatively constant, the solar wind is known to have been much stronger in the past. This suggests that the protection of the Earth's surface from solar winds increased over time, which may have allowed life to move to the continents and leave the protection of the oceans.

The Earth's magnetic field is created by mixing molten iron in the outer core of liquid, driven by buoyant forces while the inner core solidifies, creating a dynamo. During the early formation of the Earth, the solid inner core had not yet formed, leaving open questions about how the early magnetic field was maintained. These new findings suggest that the mechanism driving Earth's early dynamo was similarly efficient to the solidification process that generates 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 Earth's solid inner core began to form. This will help us understand how quickly heat escapes from the Earth's deep interior, which is key to understanding processes such as plate tectonics.

Geological and meteorological effects

One of the big challenges in reconstructing the Earth's magnetic field so far in time is that any event that causes rocks to heat up can alter the preserved signals. Rocks in the Earth's crust often have a long and complex geological history that erases past magnetic field information. However, the Isoa supracrustal belt has a unique geology, as it lies atop thick continental crust that protects it from widespread tectonic activity and deformation. This has allowed researchers to build a clear body of evidence supporting the existence of a magnetic field 3.7 billion years ago.

The results may also provide new insights into the role of our magnetic field in shaping the evolution of Earth's atmosphere as we know it, especially regarding the leakage of gases from the atmosphere. A currently unexplained phenomenon is the loss of unreacted xenon gas from our atmosphere over 2.5 billion years ago. Xenon is relatively heavy, and therefore unlikely to have simply drifted out of our atmosphere. Recently, scientists have begun to investigate the possibility of removing charged xenon particles from the atmosphere by a magnetic field.

In the future, researchers hope to expand our knowledge of the Earth's magnetic field before the appearance of oxygen in the Earth's atmosphere about 2.5 billion years ago, by examining other ancient rock sequences in Canada, Australia and South Africa. A better understanding of the ancient strength and variability of Earth's magnetic field will help us determine whether planetary magnetic fields are necessary to host life on the planet's surface and their role in the evolution of the atmosphere.

Reference: “Probable Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southwest Greenland” by Clare I. O. Nicholls, Benjamin B. Weiss, Athena Easter, Craig R. Martin, Adam C. Maloof, Nigel M. Kelly, Mike J. Zawaski, Stephen J. Mojzis, E. Bruce Watson, and Daniele J. Czerniak, April 24, 2024, Journal of Geophysical Research: Solid Earth.
doi: 10.1029/2023JB027706