Iron Age Potters Carefully Recorded Earth's Magnetic Field — By Accident

Rae Ellen Bichell

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16 15797large1 b7423faf3dcfcb43a1d91b08f23b393089c7d01e s800 c85Ancient jar handles like this one, stamped with a royal seal, provide a detailed timeline of the Earth's magnetic field thousands of years ago. Image courtesy of Oded Lipschits



They didn't know it, but in the process, the ancient potters were not just upholding centuries of tax bureaucracy.

As a group of archaeologists and geophysicists wrote Monday in the journal PNAS, they were also creating a long-lasting record of activity some 2,000 miles beneath their feet. And that record is now yielding clues to a big mystery about this planet: how its magnetic field has changed over time.

It's known that iron in the Earth's outer core generates the magnetic field as it moves around the core with the Earth's rotation. But it's still largely unknown how the magnetic field got started and why it changes.

"Albert Einstein defined this problem as one of the five most enigmatic issues in modern physics, and it still is, because the mechanism that creates the magnetic field is not well understood," says Erez Ben-Yosef, an archaeologist at Tel Aviv University in Israel and an author of the paper.

Since the invention about 180 years ago of the magnetometer, a device that can measure the strength of a magnetic field, scientists have noticed something confusing and a little worrisome over the years.

"We are losing the magnetic field," Ben-Yosef says. "We already lost more than 10 percent of its strength, so people are concerned that we might lose the magnetic field entirely."

That's concerning, he says, because the magnetic field protects the Earth from high levels of radiation coming from space.

But fear not, Ben-Yosef says. The jars hold hope.

16 15797large2 wide 3ddccf4f45adae92d4c63e4b278be7afc0fb3906 s800 c85Tiny minerals in the clay of this jar hold information about the strength of the Earth's magnetic field at the time the jar was fired, thousands of years ago. Image courtesy of Oded Lipschits


"It's kind of like a tape recorder," Ben-Yosef says.



So, for example, archaeologists know that the Assyrians destroyed Judah, also known as Judea, in 701 B.C. and subsequently replaced the tax jar seal.

"So, we know what happened to the magnetic field between 730 B.C.E. to 700 B.C.E. by the year," says Ben-Yosef.






16 15797large3 wide b1a3bcc9e8a04d9c0cf2ab9552129c365a527717 s800 c85A jar handle stamped with concentric circles, excavated from a site in Jerusalem called Ramat Rahel. Image courtesy of Oded Lipschits

The jars provide an unprecedented look at the planet's magnetic field over those six centuries, one that's much harder to get from rocks.


"But we didn't have the type of time resolution that the study in PNAS has," he says, because it's a lot harder to pinpoint rocks on a timeline than it is to pinpoint man-made objects. "That's what so cool about what they did. They pulled this out of heated ceramics."

He says the jar record shows the magnetic field is likely a lot more dynamic and nimble than researchers thought it would be.

Scientists knew the Earth could undergo big changes in magnetic intensity — the poles are thought to reverse about every 200,000 to 300,000 years. But in between those times, Forman says, people assumed there wasn't much going on. This study shows magnetic fields going about a quarter of the way toward reversal, and boomeranging back, all within just a few decades.

"When dealing with such large-scale phenomena, we don't usually think it can occur within a few decades. We usually think it would take thousands or tens of thousands of years," Forman says.

The finding, he adds, "opens up a big can of worms" because researchers just don't know how or why that would happen.

So there's something missing about scientists' concept of goings on in the Earth's core.

And scientists can thank political instability, war, relentless tax bureaucracy and all those potters for the hint.

Benjamin Franklin was right: "In this world nothing can be said to be certain, except death and taxes." And maybe also ancient potters.