How did Earth avoid a Mars-like fate? Ancient rocks hold clues

A representation of the Earth, at first without an inner core; second, with an inner core beginning to develop about 550 million years ago; third, with an outermost and innermost inner core, about 450 million years ago. Researchers from the University of Rochester used paleomagnetism to determine these two key dates in the history of the inner core, which they believe restored the planet’s magnetic field just before life exploded on Earth. Credit: University of Rochester / Michael Osadciw

About 1,800 miles below our feet, swirling liquid iron in Earth’s outer core generates our planet’s protective magnetic field. This magnetic field is invisible but vital for life on the surface of the Earth because it protects the planet from the solar wind, from the fluxes of radiation from the sun.

However, around 565 million years ago, the strength of the magnetic field decreased to 10% of its current strength. Then, mysteriously, the field rebounded, regaining its strength just before the Cambrian explosion of multicellular life on Earth.

What caused the magnetic field to bounce?

According to new research by scientists at the University of Rochester, this rejuvenation occurred within tens of millions of years – rapidly on geologic time scales – and coincided with the formation of the solid inner core of the Earth, suggesting that the core is likely a direct cause.

“The inner core is extremely important,” says John Tarduno, William R. Kenan, Jr., professor of geophysics in the Department of Earth and Environmental Sciences and dean of research for the arts, sciences, and engineering in Rochester. “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 regenerated.”

In the article published in Nature Communication, the researchers determined several key dates in the history of the inner core, including a more accurate estimate of its age. The research provides clues to the history and future evolution of Earth and how it became a habitable planet, as well as the evolution of other planets in the solar system.

Unlock information in ancient rocks

The Earth is made up of layers: the crust, where life is found; the mantle, the thickest layer of the Earth; the molten outer core; and the solid inner core, which is in turn composed of an outermost inner core and an innermost inner core.

Earth’s magnetic field is generated in its outer core, where swirling liquid iron causes electric currents, resulting in a phenomenon called geodynamo which produces the magnetic field.

Because of the relationship between the magnetic field and the Earth’s core, scientists have been trying for decades to determine how the magnetic field and the Earth’s core have changed throughout our planet’s history. They 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 Earth’s surface contain tiny magnetic particles that lock into the direction and strength of the magnetic field as the minerals cool from their molten state.

To better limit the age and growth of the inner core, Tarduno and his team used a CO2 laser and the lab’s superconducting quantum interference device (SQUID) magnetometer to analyze feldspar crystals from rock anorthosite. These crystals contain tiny magnetic needles that are “perfect magnetic recorders,” says Tarduno.

By studying the magnetism locked in ancient crystals – a field known as paleomagnetism – researchers have determined two new important dates in the history of the inner core:

  • 550 million years ago: the moment when the magnetic field began to rapidly renew itself after a near collapse 15 million years ago. The researchers attribute the rapid renewal of the magnetic field to the formation of a strong inner core which recharged the molten outer core and restored the strength of the magnetic field.
  • 450 million years ago: the time at which the structure of the growing inner core has changed, marking the boundary between the innermost and outermost inner core. These changes in the inner core coincide with changes at about the same time in the structure of the overlying mantle, due to plate tectonics on the surface.

“Because we limited the age of the inner core more precisely, we were able to explore the fact that the current inner core is actually made up of two parts,” says Tarduno. “Plate tectonic movements on the Earth’s surface have indirectly affected the inner core, and the history of these movements is imprinted deep within the Earth in the structure of the inner core.”

Avoid a Mars-like fate

Better understanding the dynamics and growth of the inner core and magnetic field has important implications, not only for uncovering Earth’s past and predicting its future, but also for unraveling the ways in which other planets might form magnetic shields and maintain the conditions necessary to support life. .

Researchers believe that Mars, for example, once had a magnetic field, but the field dissipated, leaving the planet vulnerable to the solar wind and the surface without oceans. While it’s unclear whether the lack of a magnetic field would have caused the same fate for Earth, “Earth would certainly have lost a lot more water if Earth’s magnetic field hadn’t been regenerated,” said Tarduno. “The planet would be much drier and very different from today’s planet.”

In terms of planetary evolution, the research therefore points to the importance of a magnetic shield and a mechanism to maintain it, he says.

“This research really highlights the need for something like a growing inner core that maintains a magnetic field for the lifetime – several billion years – of a planet.”


New research provides evidence of an early strong magnetic field around Earth


More information:
Tinghong Zhou et al, Early Cambrian Geodynamo Renewal and Origin of Inner Core Structure, Nature Communication (2022). DOI: 10.1038/s41467-022-31677-7

Provided by the University of Rochester

Quote: How did Earth avoid a Mars-like fate? Ancient Rocks Hold Clues (2022, July 25) Retrieved July 26, 2022 from https://phys.org/news/2022-07-earth-mars-like-fate-ancient-clues.html

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