The Earth’s magnetic field is vital for life – without it, the Sun’s radiation would sterilize the planet. But a new study suggests we wouldn’t be here at all if that magnetic field hadn’t almost completely collapsed half a billion years ago.
A lot of factors had to come together just right for Earth to be habitable. Not only did the planet need to be exactly the right distance from the Sun, but it needed to be rocky, watery, contain the right ingredients for life, and of course, have a strong magnetic field that prevents dangerous radiation from the Sun and interstellar space from reaching the surface.
While it’s mostly stable, that magnetic field does fluctuate over time. In a new study, scientists from the University of Rochester identified its weakest point in Earth’s history – but surprisingly, it seems to have happened just before complex life exploded on the scene, rather than coincide with a mass extinction as you might expect.
Ancient minerals can preserve a record of the magnetic field’s strength at the time, thanks to magnetic particles in them. The Rochester researchers measured this magnetization in feldspar and pyroxene crystals, comparing samples from over 2 billion years ago to some from 591 million years ago. They found that the older samples recorded a magnetic field of similar strength to today’s, but the younger samples suggested that the magnetic field at that time was just 3% of its current strength – its weakest point we know about.
It seems that this weaker magnetic field lasted for at least 26 million years, before it started to regain strength. That’s right around the time the Earth’s inner core solidified and stabilized the magnetic field, according to the team’s previous work.
A weaker magnetic field means more cosmic radiation reaches deeper into Earth’s atmosphere, and if that happened today it would likely trigger a mass extinction event. But intriguingly, this historical low point may have helped the ancestor of all animals evolve.
The Ediacaran Period, which lasted from 635 to 539 million years ago, was a key stage in the evolution of life on Earth. This was when complex multicellular lifeforms first emerged, but they barely resemble life as we know it now – they looked like discs, tubes, fans, donuts, or just squishy “mud-filled bags.” In fact, scientists can’t really even figure out if these organisms were algae, fungi, or early versions of plants or animals.
Most of these weirdos seemed to die out by the following period, the Cambrian. But around the same time, there was a massive explosion in the diversity of lifeforms, with almost every modern branch on the evolutionary tree appearing in a relatively short space of time. One of the main triggers for that event is believed to be an increase in oxygen levels in the atmosphere – and according to the new study, we might have that weakened magnetic field to thank.
As mentioned, that dip let more radiation enter Earth’s atmosphere, which allowed more charged particles to strip away atoms like hydrogen. If enough of those are lost to space, then the oxygen atoms in the atmosphere build up, rather than reacting with the hydrogen to form water vapor. Over time, that increases levels of oxygen in the air, giving the burgeoning life a boost.
If the story is true, it goes to show the remarkable luck that advanced life was able to evolve at all. If the magnetic field hadn’t bounced back, Earth might have instead gone the way of Mars.
“If the extraordinarily weak field had remained after the Ediacaran, Earth might look very different from the water-rich planet it is today: water loss might have gradually dried Earth,” said John Tarduno, an author of the study.
The research was published in the journal Nature Communications Earth & Environment.
Source: University of Rochester
