Earth’s Ancient Magnetic Field

Earth’s Ancient Magnetic Field

Not only are magnets fun to play with, they play prominent roles in humanity’s just-right habitat. Magnetic fields affect the way stars and galaxies form, provide navigational landmarks for human and animal travel, and ensure Earth retains its water for more than four billion years. But has Earth’s magnetic field existed for most of the planet’s history? Recent research addresses this question.

The main difficulty of studying Earth’s ancient magnetic field is finding materials that record its strength. Usually, geophysicists use long-solidified crustal rocks. The magnetic field affects the formation of certain kinds of rocks in such a way that allows scientists to determine the field’s strength and direction. While finding these ancient rocks is difficult enough, other factors impact researchers’ ability to extract information from them. For example, any significant heating of the rocks removes the original information recorded about the magnetic field. Also, researchers must keep in mind that a steady stream of charged particles impacts the top of Earth’s atmosphere, inducing a magnetic field. In order to determine the characteristics of the magnetic field generated by the planet core, this atmospheric contribution must be accounted for.

A team of international scientists recently developed a technique to carry out the necessary measurements.1They started with dacite rocks found in South Africa dating from 3.4 to 3.45 billion years ago. These rocks contain magnetic inclusions that record information about Earth’s core-generated magnetic field (along with atmospheric magnetic noise) from this time. By carefully selecting inclusions meeting specific criteria, they isolated a pristine sample of small quartz crystals to analyze for magnetic field information. Using a SQUID magnetometer, the scientists determined that 3.4 to 3.45 billion years ago Earth’s magnetic field was 50 to 70 percent of today’s values.

Three aspects of these results warrant further note. First, this measurement provides the earliest measurement of Earth’s magnetic field—besting the previous value by 200 million years. Second, the field strength measured here indicates that scientists are finding evidence from near the time when Earth’s rotation-driven dynamo first started gaining significance. If so, even more ancient measurements will provide important insights into the process of core formation and dynamo generation.

Third, and perhaps most important, these measurements identify a critical period for maintaining habitability on Earth for the next 3.5 billion years. A smaller magnetic field means that the solar wind interacts more directly with Earth’s atmosphere. The key interaction consists of radiation breaking water into hydrogen and oxygen and the hydrogen escaping into space. This reaction results in loss of water. If our planet had experienced a stronger solar wind and weaker magnetic field for the first billion years of Earth’s history, water would have been rapidly stripped from early Earth. This situation may explain why Earth has the just-right amount of water today (and not as much as models predict).

As crucial as water is to life, it must come in the right amounts. This research of quartz crystals from South Africa adds to a growing body of evidence that multiple, diverse processes (astronomical, geophysical, atmospheric, and biological) must all interact precisely to ensure Earth has the right amount of water. Such finely-tuned interactions point to a divine Designer.

  1. John A. Tarduno et al., “Geodynamo, Solar Wind and Magnetopause 3.4 to 3.45 Billion Years Ago,” Science 327 (March 5, 2010): 1238–40