The magnetic history of ice

The history of our planet has been written, among other things, in the periodic inversion of its magnetic poles. Scientists at the Weizmann Institute of Science propose a new way to read this historical record: on ice. Their findings, which were recently reported in Letters of Earth and Planetary Science, could lead to a refined probing of ice cores and, in the future, could be applied to understand the magnetic history of other bodies in our solar system, including Mars and Jupiter’s moon Europa.

The idea of ​​investigating a possible connection between ice and Earth’s magnetic history arose far from the planet’s source of ice, on the sunny island of Corsica, where Professor Oded Aharonson of the Department of Planetary and Earth Sciences of the Institute, attended a conference on magnetism. . More specifically, the researchers were discussing the field known as paleo-magnetism, which is studied primarily through flakes of magnetic minerals that have become trapped in rocks or cores drilled through ocean sediments. Such particles align with Earth’s magnetic field the moment they get caught in place, and even millions of years later, researchers can test their north-south magnetic alignment and understand the position of Earth’s magnetic poles. at that distant moment. The latter is what gave Aharonson the idea: if small amounts of magnetic materials could be detected in ocean sediments, perhaps they could also be trapped in ice and measured. Some of the ice frozen in glaciers in places like Greenland or Alaska is many millennia old and layered like tree rings. Ice cores drilled through them are investigated for signs of global warming or glaciations. Why not also investments in the magnetic field?

The first question Aharonson and his student Yuval Grossman, who led the project, had to ask was whether it was possible that the process in which ice forms in regions near the poles could contain a detectable record of magnetic pole reversals. These randomly spaced reversals have occurred throughout our planet’s history, fueled by the chaotic movement of the liquid iron dynamo in the planet’s core. In banded rock formations and layered sediments, the researchers measure the magnetic moment, the north-south magnetic orientations, of the magnetic materials in them to reveal the magnetic moment of Earth’s magnetic field at that time. Scientists thought that such magnetic particles could be found in trapped dust, along with water ice, in glaciers and ice sheets.

The research team built an experimental setup to simulate ice formation like that of polar glaciers, where dust particles in the atmosphere can even provide the nuclei around which snowflakes form. The researchers created artificial snowfall by finely grinding ice made from purified water, adding a bit of magnetic powder and dropping it through a very cold column that was exposed to a magnetic field, the latter with an orientation controlled by scientists. By maintaining very cold temperatures, around 30 degrees Celsius below zero, they discovered that they could generate miniature “ice cores” in which snow and dust were solidly frozen in hard ice.

It would be exciting to search for magnetic field inversions in the ice sampled from other bodies in our solar system

“If the dust is not affected by an external magnetic field, it will settle in random directions that cancel each other out,” says Aharonson. “But if a part is oriented in a particular direction just before the particles freeze, the net magnetic moment will be detectable.”

To measure the magnetism of the “ice cores” they had created in the laboratory, Weizmann’s scientists took them to the Hebrew University of Jerusalem, to Prof. Ron Shaar’s laboratory, where a sensitive magnetometer installed there is capable of measuring what least. Magnetic Moment The team found a small, but definitely detectable, magnetic moment that matched the magnetic fields applied to their ice samples.

“The paleo-magnetic history of Earth has been studied from the rocky record; reading it in ice cores could reveal additional dimensions or help assign precise dates to the other findings in those cores, “says Aharonson.” And we know that the surfaces of Mars and large icy moons like Europe have been exposed to magnetic fields. It would be exciting looking for magnetic field reversals in ice sampled from other bodies in our solar system. “

“We have shown that it is possible,” he adds. Aharonson has even proposed a research project for a future space mission that will include sampling of ice cores on Mars, and he hopes that this demonstration of the feasibility of measuring such a core will advance the appeal of this proposal.

Prof. Oded Aharonson is Head of the Helen Kimmel Center for Planetary Science; His research is also supported by the Minerva Center for Life in Extreme Planetary Conditions; the Zuckerman STEM Leadership Program; and the Adolf and Mary Mil Foundation.