The Earth’s magnetic field is thought to have been crucial to the survival of life. If so, knowing its origin is essential not only for understanding our own planet, but establishing the prospects for life elsewhere. Unfortunately, geologists trying to establish when the field appeared may be back to square one, with the discovery of a flaw in what was thought to be evidence for the field’s early formation.
The zircon crystals of the Jack Hills, Western Australia, are the oldest materials to originate on Earth. Much of what we know of the first 10 percent of the Earth’s existence comes from these time capsules. So they represent the logical place for geologists to turn to answer the question of when the planetary magnetic field made its appearance.
Zircons aged 3.3-4.2 billion years old from these hills have been found to be magnetized, leading some geologists to conclude the field dates back to soon after the Earth’s formation 4.5 billion years ago. Since this is long before the solid core responsible for today’s field formed, such a finding would prove liquid cores can also produce strong fields. However, Cambridge University scientists have undermined this conclusion, reporting in the Proceedings of the National Academy of Sciences that the magnetism could be a much later addition.
Zircon crystals are not magnetic themselves. However, their value to geologists lies in their capacity to incorporate other elements. The uranium they capture during formation turns to lead at a known rate, revealing their ages. Jack Hills crystals contain magnetic minerals, and if these, like the uranium, are legacies of the original crystallization, the Earth must have had a strong field at the time.
However, when Dr Richard Taylor and colleagues studied two 4-billion-year-old crystals, both showed nanoscale pores resulting from radiation damage. Magnetite (Fe3O4) had accumulated within these pores, possibly long after their initial formation. As the name suggests, magnetite is easily magnetized in appropriate fields during its crystallization, and will hold this magnetism provided it is not exposed to temperatures above around 550ºC (1,020ºF).
Consequently, there is a high chance the magnetism reflects the Earth’s circumstances at a much later date, leaving the question open as to when the field first appeared.
The Earth’s magnetic field let sailors navigate the oceans. For geologists, its changes proved the existence of tectonic plates. Its greatest significance, however, lies in the way it protects the Earth’s atmosphere from the solar wind. It is thought the lack of a similar field was a major contributor to Mars losing most of its atmosphere, and with it the prospects of hosting much life.