According to a new study, Earth's inner core became solid much later than expected, a fact that had profound consequences on our planet. The nucleation, as it is technically called, led to a dramatic change in our planet's magnetic field, which went from weak and erratic to the more stable configuration we have today.
Researchers, led by a team from the University of Rochester, analyzed the signatures that the ancient magnetic field left on 565-million-year-old rock samples from Quebec. They indicate Earth's magnetic field back then was 10 times weaker than its current value. Their work was published in Nature Geoscience.
Earth's magnetic field is vital in protecting our planet against the solar winds, and without it, our atmosphere would be stripped away and we would be left a barren rock, like Mars. It's thought to be generated by Earth's core. The core is essentially a solid ball of iron surrounded by a layer of liquid metal. As the Earth spins, the molten metal flows, forming electric currents that produce magnetic fields.
The team believes that their findings, plus other evidence of an anomalous magnetic field from the same geological period, suggests that Earth’s magnetic field was once on the verge of collapse, so the nucleation of the core that happened around 565 million years ago, happened at the right time and not a minute too late. As the inner core was first solidifying, lighter elements such as silicon and oxygen moved outwards. This generated convection currents in the core, which led to the strengthening of the magnetic field.
This has been compared to the more extreme cases of magnetic pole flips (the geomagnetic reversal) but in those cases, the magnetic field has not become that weak and certainly not for that long. Reversal episodes are expected to last from a few centuries to a few thousand years, not millions of years.
The findings in this study have theoretical backing too. Simulations of the history of Earth’s magnetic field source, the so-called geomagnetic dynamo, also suggests a nucleation date around 600 million years ago. They also found that before nucleation the magnetic field should lose strength and become a lot more scattered. This would happen because as the core cools, less convection would be happening, and since that drives the formation of the magnetic field, the dynamo system will weaken, which is consistent with the picture painted by these new findings.
Although that picture is very intriguing, the “young core hypothesis” has some complex issues that need addressing. The main one is that the core should be hotter than it is believed to be. A possible solution is that more radioactive elements than we thought ended up in the core. Alternatively, we might be missing some important information regarding the chemistry within the core.