Earth's Magnetic Field 'Preserved Habitability' at Two Key Points in Planet's History

Earth's magnetic field preserved the planet's habitability at two key points in its history, scientists have said. By developing a strong shield around the planet early after it formed, the magnetic field may have helped protect the components necessary for life to emerge—a discovery that could have implications for the search for alien life on planets beyond our solar system.

The magnetic field around our planet helps deflect harmful particles coming from the sun—the solar wind. It is thought Mars once had a magnetic field but at some point it was lost. After this happened, the charged particles stripped the planet of its atmosphere and its oceans were lost to space. By maintaining a magnetic field, Earth was able to become and remain habitable.

In a study published in PNAS, researchers have now looked at Earth's early magnetic field, just after the planet formed 4.5 billion years ago. Until fairly recently, it was thought the magnetic field had formed around 3.5 billion years ago. However, in 2015, researchers led by John Tarduno, Professor of Geophysics at the University of Rochester, found evidence to suggest it was at least 4.2 billion years old.

"Early in Earth history the planet had to contend with intense solar winds streaming from the young sun," Tarduno said. "These winds had the potential to erode the atmosphere, ultimately stripping Earth of its water. Thus, Earth might have looked more like Mars today. Instead, Earth's magnetic field provided a shield against the solar wind, preventing atmospheric erosion and massive water loss."

In their latest research, Tarduno and colleagues have been able to determine that the early magnetic field was stronger than previously believed. To do this they studied zircon crystals, which are some of the oldest materials on Earth.

"Measuring Earth's earliest magnetic field is so difficult because the planet's oldest rocks have been metamorphosed over the billions of years since their formation," Tarduno explained. "However, the magnetization held by select crystals bearing magnetite inclusions—zircons—can preserve records of the earliest field." However, the zircons are about the width of a human hair, so measuring them requires specialized equipment.

Their analysis suggests the Earth had a magnetic field by 4.2 billion years ago, and it was particularly strong between 4.1 and 4 billion years ago. It is thought that at this time, the field was being generated by chemical reactions inside the planet as it cooled down after forming. This is different to the way the magnetic field is generated now, by movement in the liquid iron core.

Eventually, the chemical reactions stopped and the newly formed liquid core took over, the researchers believe. "We believe that the magnetic field preserved planetary habitability at two key times in Earth history: the Hadean and in the latest Precambian (Ediacaran) about 565 million years ago when the field almost completely collapsed just before the start of inner core growth," Tarduno said. "Our working hypothesis is that the field was initially strong, but over billions of years it gradually became weaker as the dynamo became less efficient. Then the inner core started to grow, and the field grew strong again. We are currently collecting data from sites worldwide to trace this history."

Findings also have implications for the search for life on exoplanets—planets found outside of our own solar system. "Exoplanets in the habitable zone—that distance from the star where liquid water can exist—might have their atmospheres and water eroded by early stellar winds if they do not have magnetic fields. Thus magnetic fields can be thought of as a key component for planetary habitability," Tarduno said.

earth magnetic field
Artist impression of Earth's magnetic field protecting the planet from the solar wind. iStock