Around 2800 kilometres below where you are standing right now, there is a large amount of molten iron swirling around and generating our planet’s magnetic field. This magnetic field may be invisible but it is vital for life on Earth since it shields the planet from streams of radiation from the sun, known as solar wind.
But around 565 million years ago, our planet’s magnetic field decreased to less than ten per cent of its strength today. Then, almost mysteriously, the field regained its strength just before the Cambrian explosion or the “biological big bang” when various phylae and species of multicellular life emerged on earth.
A new paleomagnetic research study published in Nature Communications says that this rejuvenation of the magnetic field happened within the span of a few tens of million years (which is rapid in a geological contact) and also coincided with the formation of Earth’s solid inner core. This suggests that the core is likely a direct cause of the rejuvenation.
“The inner core is tremendously important. Right before the inner core started to grow, the magnetic field was at the point of collapse, but as soon as the inner core started to grow, the field was regenerated,” said John Tarduno, corresponding author of the paper, in a press statement. Tarduno is the William R. Kenan, Jr. Professor of Geophysics in the Department of Earth and Environmental Sciences and dean of research for Arts, Sciences & Engineering at the University of Rochester.
Our planet’s magnetic field is generated in its outer core, which lies between the Earth’s mantle and the solid inner core. The solid inner core is composed of an outermost inner core and innermost inner core. In the outer core, swirling liquid iron generates electric currents due to a geodynamo process and in turn, these electric currents induce the magnetic field.
For decades, scientists have been trying to figure out how the Earth’s magnetic field and core have changed throughout or history. But they cannot directly measure the magnetic field due to the location and extreme temperatures of materials in the core. Thankfully, minerals that rise to Earth’s surface from the core contain tiny magnetic particles that lock in the direction and intensity of the magnetic field at the time the minerals cool from their molten state.
To better understand the age and growth of the inner core, Tarduno and his team used a carbon dioxide laser and a superconducting quantum interference device (SQUID) magnetometer to analyse particular mineral crystals from the rock anorthosite. These “feldspar” minerals have minute magnetic needles within them that Tarduno calls “perfect magnetic recorders.”
By studying the magnetism “locked” in these ancient crystals, researchers were able to deduce two important events in the history of the Earth’s inner core.
First, the formation of a solid inner core happened about 550 million years ago. Researchers attribute the rapid renewal of the magnetic field at the same time to this formation and deduce that the solid inner core recharged the molten outer core and restored the magnetic field’s strength.
Second, the growing inner core’s structure changed about 450 million years ago. This marked the boundary between the innermost and outermost inner core. The mantle that lies above the core also saw some changes around the same time, due to plate tectonics on the surface.
“Because we constrained the inner core’s age more accurately, we could explore the fact that the present-day inner core is actually composed of two parts. Plate tectonic movements on Earth’s surface indirectly affected the inner core, and the history of these movements is imprinted deep within Earth in the inner core’s structure,” added Tarduno, in the press statement.
Researchers believe that Mars once had a magnetic field that later dissipated, leaving the planet oceanless and vulnerable to solar winds. While it is not easy to conclude that the Earth would have met the same fate without the magnetic field, our planet would have lost a lot more water if the field was not generated.
By understanding how these processes work, scientist get insights into how other planets could also form magnetic shields and sustain the conditions needed to harbour life as we know it.