Ancient rocks show how Earth avoided a Mars-like fate

In an article published in the journal nature communication, the Rochester scientists explain that the strength of the magnetic field decreased to 10% of its current strength about 565 million years ago. Then the field mysteriously revived, regaining strength just before the Cambrian explosion of multicellular life on Earth.

Such rejuvenation occurred within a few tens of millions of years, which is fairly rapid on geologic timescales, and coincided with the formation of Earth’s solid inner core, suggesting that the core is likely a direct cause.

“The inner core is hugely important,” said study co-author John Tarduno in a media statement. “Just before the inner core started growing, the magnetic field was about to collapse, but as soon as the inner core started growing, the field was regenerated.”

Tarduno explained that Earth’s magnetic field is created in its outer core, where swirling liquid iron causes electric currents and powers a phenomenon called geodynamo, which creates the magnetic field.

Because of the relationship of the magnetic field to the Earth’s core, scientists have been trying for decades to determine how the Earth’s magnetic field and core have changed throughout our planet’s history. You cannot directly measure the magnetic field due to the location and extreme temperatures of the materials in the core. Fortunately, minerals that rise to the surface contain tiny magnetic particles that pinpoint the direction and intensity of the magnetic field as the minerals cool from their molten state.

To better constrain the age and growth of the inner core, Tarduno and his team used a CO2 laser and a SQUID magnetometer (superconducting quantum interference device) to analyze feldspar crystals from the anorthosite rock. These crystals contain tiny magnetic needles that are considered perfect magnetic recording devices.

By examining the magnetism trapped in ancient crystals – a field known as paleomagnetism – the researchers determined two new important dates in the history of the inner core.

The first date is 550 million years ago, when the magnetic field began to be rapidly renewed after a near collapse 15 million years earlier. The researchers attribute the rapid renewal of the magnetic field to the formation of a solid inner core, which recharges the molten outer core and restores the strength of the magnetic field.

The second date is 450 million years ago, when the structure of the growing inner core changed, marking the boundary between the innermost and outermost cores. These changes in the inner core coincide with changes in the structure of the overlying mantle at about the same time, due to plate tectonics at the surface.

“Because we were able to narrow down the age of the inner core, we were able to examine the fact that today’s inner core actually consists of two parts,” Tarduno said. “Plate tectonic movements on the Earth’s surface indirectly affected the inner core, and the history of these movements is imprinted deep within the Earth in the structure of the inner core.”

For the scientist, a better understanding of the dynamics and growth of the inner core and magnetic field has important implications, not only in uncovering Earth’s past and predicting its future, but also in unraveling how other planets are magnetic Shields could create and maintain the conditions necessary to house life.

Researchers believe that Mars, for example, once had a magnetic field, but the field dissipated, leaving the planet vulnerable to solar wind and the surface devoid of oceans. While it’s unclear whether the absence of a magnetic field would have inflicted the same fate on Earth, Tarduno said the blue planet would have lost a lot more water if its magnetic field hadn’t been regenerated.

“The planet would be much drier and very different than the planet today,” he pointed out.