Early Earth's Magnetic Field Was a Weakling
The larger auroral oval relative to the modern is the result of a weaker magnetic field and stronger solar wind. The auroral intensity is brighter due to solar wind densities many times greater than those today.
Credit: J. Tarduno and R. Cottrell. University of Rochester

The protective magnetic field shrouding the early Earth was likely only half as strong as it is today, a new study suggests.

The research also found that the Earth's magnetic field is 200 million years older than previously thought, which has implications for the amount of water that was originally present on the early Earth, and perhaps even on the development of life. Such a weak field in the Earth?s early days may have also made for some spectacular auroras, or Northern Lights, at latitudes as low as what is now New York City, researchers said.

Earth's magnetic field is generated by the turbulent, convective motions of the planet's molten core. The field extends around the Earth for quite some distance into space until it meets the sun's incoming solar wind (the stream of charged solar particles constantly flowing away from the sun). The boundary where the two meet is called the magnetopause.

It is the magnetic field that protects the Earth's surface, and all of its inhabitants, from this energetic solar radiation, which would harm living organisms and strip away much of Earth's atmosphere (Mars has no significant magnetic field, which is thought to be the reason it has such a miniscule atmosphere).

But little is known about the magnetic field as it existed just after the Earth formed, around 4.5 billion years ago. To learn more about this early magnetic field, John Tarduno of the University of Rochester and his colleagues from the University of KwaZulu-Natal in South Africa, turned to the crystals in ancient rocks that preserve magnetic signatures.

Magnetic record

Certain igneous rocks called dacites contain small millimeter-sized quartz crystals, which in turn have tiny nanometer-sized magnetic inclusions that act as mini compasses, locking in a record of the Earth's magnetic field at the time that the dacites cooled from molten magma into hard rock.

To look for preserved records of the early magnetic field, Tarduno and his colleagues used the best preserved grains from 3.5 billion-year-old dacite outcroppins in South Africa, some of the oldest rocks known to still exist on the Earth's surface.

Using a specialized magnetic detector, the team found that the 3.5 billion-year-old crystals in the rocks recorded a field that is about 30 to 50 percent weaker than the field that exists today. The finding is detailed in the March 5 issue of the journal Science.

Some scientists have suggested that there was no magnetic field on the early Earth, so this result "demonstrates that there was a field at that time," Tarduno said.

This weaker magnetic field also has implications for conditions on the early Earth.

Protecting Earth

Because "the magnetic field stands off the solar wind," Tarduno says, it keeps solar particles from eating away at the molecules in Earth's atmosphere.

But in the past, not only was this field weaker, the sun was likely rotating more rapidly and therefore spinning off a stronger solar wind and a magnetopause that was likely much closer to Earth ? today it is at a distance of about 10.7 Earth radii, but then it would likely have been around 5 Earth radii out (Earth's average radius is about 4,960 miles, or 6,370 km).

The solar wind situation on the Earth at the time may have been something like the Halloween solar storm of 2003, which affected satellites, communication, air traffic and power generating systems.

"That means that the particles streaming out of the sun were much more likely to reach Earth," Tarduno said. The implication of that situation is that "it's very likely the solar wind was removing volatile molecules, like hydrogen, from the atmosphere at a much greater rate than we're losing them today," he said. And the loss of hydrogen implies a loss of water as well.

In turn, if a lot of water was stripped away early in Earth's history, to get the amount of water that we have now, the planet must have started "with a fairly robust inventory of water," Tarduno told SPACE.com.

Life as we know it

Both water and a protective magnetic field are essential to the development of life as we know it, so the finding also has implications for understanding how life arose on our own planet, as well as potential life beyond on our solar system.

Tarduno and his team's study "suggests that the magnetic field may predate the establishment of life" on Earth, wrote Moira Jardine, an astronomer at the University of St. Andrews in Scotland who was not involved with the study, in an essay accompanying the new study in Science.

The development of magnetic fields around other planets outside the solar system and how well they guard any potential life is also something to consider when looking for planets around young stars, which seem to have stronger solar winds and more frequent solar storms than the sun does now, Jardine wrote.

It also means that to evolve an extrasolar planet that is Earth-like, "we need to start with a fairly healthy inventory of water," Tarduno said.

The weak magnetic field on the early Earth may have also led to much more spectacular, possibly extending over an area three times the size as they currently do and extending to lower latitudes, possibly as far as the current position of New York City. Auroras are the light shows generated when solar particles are funneled down the polar axes of the magnetic field and interact with atoms in the Earth's atmosphere, exciting them and causing them to give off photons of light.