The mystery of Earth’s magnetic field
Microscopic minerals excavated from an ancient outcrop of Jack Hills, in Western Australia, have been the subject of intense geological study. This evidence seemed to trace the Earth’s magnetic field to as far back as 4.2 billion years ago.
The rocks were understood to contain zircon, the oldest known material on Earth.
These conclusions placed the magnetic field formation almost 1 billion years earlier than it was previously thought to have originated. This almost aligns its creation with the time the Earth itself was formed.
Earth’s magnetic field acts as a shield that deflects solar winds and makes life on Earth possible.
The magnetic field origin remains a contested topic as more new research from Massachusetts Institute of Technology (MIT) appears to contradict the previous evidence after an analysis of the same tiny crystals found located in Jack Hills.
The findings of this most recent research, published in the journal Science Advances, concluded that that the zircon found at the Australian site cannot be used as reliable proof of the presence of a magnetic field dating back more than 3.5 billion years.
“There is no robust evidence of a magnetic field prior to 3.5 billion years ago, and even if there was a field, it will be very difficult to find evidence for it in Jack Hills zircons,” says Caue Borlina, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS).
“It’s an important result in the sense that we know what not to look for anymore,” he continued.
Earth’s magnetic field explained
Earth’s magnetic field is thought to play an important role in making the planet habitable. Not only does a magnetic field set the direction of our compass needles, it also acts as a shield of sorts, deflecting away solar wind that might otherwise eat away at the atmosphere.
Significant amounts of evidence demonstrates that the Earth’s magnetic field existed at least 3.5 billion years ago. The planet’s core, however, is thought to have started solidifying around one billion years ago, meaning that the magnetic field must have been driven by some other mechanism prior to one billion years ago.
Establishing when exactly the magnetic field was formed could help scientists to work out what actually generated it.
Scientists have traditionally used minerals in ancient rocks to determine the orientation and intensity of Earth’s magnetic field back through time. To study the strength and orientation of the Earth’s magnetic field at a given date, scientists rely on a phenomenon known as rock magnetisation - as scientists now know that the Earth’s magnetic field is powered by the solidification of the planet’s liquid iron core.
As a rock forms and cools, the electrons in its atoms change their orientation in the direction of the magnetic field, but cooling below a certain temperature stops them moving and from there they remain fixed.
By knowing the age of a rock, it is possible to establish what the Earth's magnetic field was like at the time that rock was formed.
MIT’s new research states that although the zircon mineral found in the Jack Hills dates back 4.2 million years, this does not mean that its magnetic material was formed at the same time.
To prove their case, scientists extracted more than 3,000 samples of zircon and looked for signs of cracks or deposits.
Around 250 crystals were found to be older than 3.5 billion years. The team isolated and imaged those samples, looking for signs of cracks or secondary materials, such as minerals that may have been deposited on or within the crystal after it had fully formed, and searched for evidence that they were significantly heated over the last few billion years since they formed.
Out of 250 samples, they identified just three zircons that were relatively free of the described “impurities” - meaning they could contain suitable magnetic records.
The team then carried out detailed experiments on the three samples to determine what kinds of magnetic materials they might contain. They eventually determined that a magnetic mineral called magnetite was present in two of the three zircons. Using a high-resolution quantum diamond magnetometer, the team looked at cross-sections of each of the two zircons to map the location of the magnetite in each crystal.
They discovered magnetite lying along cracks or “damaged zones” within the zircons.
Cracks provide pathways that allow water and other elements inside the rock. Such cracks could have let in secondary magnetite that settled into the crystal much later than when the zircon originally formed, explained Caue Borlina, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS).
From this analysis, the team concluded that these zircons could not be used as a reliable recorder for Earth’s magnetic field.
The magnetic field debate has been raging since 2015, and seemingly - it continues on.
