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The magnetic history of ice

The history of our planet was written, among other things, in the periodic inversion of its magnetic poles. Scientists from the Weizmann Institute of Science propose a new way of reading this historic record: in the ice. Their results, which have recently been reported in Letters of terrestrial and planetary science, could lead to a refinement of the ice cores and, in the future, could be applied to understand the magnetic history of other bodies in our solar system, including the Europe of Mars and the moon of Jupiter.

The idea of ​​investigating a possible connection between ice and the magnetic history of Earth was born far from the ice source of the planet ̵

1; on the sunny island of Corsica, where Prof. Oded Aharonson of the Department of Earth and Planet Sciences of the Institute, attended a conference on magnetism. More specifically, the researchers were discussing the field known as paleo-magnetism, which is primarily studied through magnetic flake minerals that have been trapped in rocks or cores drilled through ocean sediment. These particles align with the Earth’s magnetic field when they are trapped in place, and even millions of years later, researchers can test their north-south magnetic alignment and understand the location of Earth’s magnetic poles in that distant time. The latter is what gave Aharonson the idea: if a small amount of magnetic material could be detected in oceanic sediments, perhaps they could also be found trapped in ice and measured. Part of the ice frozen in glaciers in places like Greenland or Alaska is many millennia old and is layered like tree rings. The ice cores drilled through them are being investigated for signs of things like planetary warming or ice ages. Why not also reverse 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 of ice forming in regions near the poles could contain a detectable record of magnetic pole reversals. These randomly spaced reversals have occurred in the history of our planet, fueled by the chaotic movement of the liquid iron dynamo deep into the core of the planet. In banded rock formations and layered sediments, researchers measure the magnetic moment – the north-south magnetic orientations – of the magnetic materials therein to reveal the magnetic moment of the Earth’s magnetic field at that time. Scientists thought that such magnetic particles could be found in the dust that is trapped, along with water ice, in glaciers and ice caps.

The research team created an experimental setup to simulate ice formation such as that in polar glaciers, where dust particles in the atmosphere can even supply the nuclei around which snowflakes are formed. The researchers created artificial snowfall by finely grinding the ice made with purified water, adding some magnetic dust and dropping it through a very cold column that was exposed to a magnetic field, the latter with a controlled orientation by scientists. By keeping temperatures very cold – around 30 degrees Celsius below zero, they found that they could generate miniature “ice cores” where snow and dust would freeze solidly in hard ice.

It would be exciting to look for magnetic field reversals in the ice sampled by other bodies in our solar system

“If the dust isn’t affected by an external magnetic field, it will settle in random directions that will cancel each other out,” says Aharonson. “But if a part of it 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” that they had created in the laboratory, Weizmann’s scientists took them to the Hebrew University of Jerusalem, to the laboratory of prof. Ron Shaar, where a sensitive magnetometer installed there is able to measure the minimum of magnetic moments. The team found a small but certainly detectable magnetic moment that corresponded to the magnetic fields applied to their ice samples.

“Earth’s paleo-magnetic history has been studied from rock documentation; reading it in ice cores could reveal additional dimensions or help assign precise dates to other results in those cores,” says Aharonson. “And we know that the surfaces of Mars and the large frozen moons like Europe have been exposed to magnetic fields. It would be exciting to look for magnetic field reversals in the ice sampled by 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 involves sampling the ice core on Mars and hopes that this demonstration of the feasibility of measuring that core will advance the appeal of this proposal.

Prof. Oded Aharonson is head of the Planetary Science Center Helen Kimmel; his research is also supported by the Minerva Center for Life Under Extreme Planetary Conditions; Zuckerman’s STEM leadership program; and the Adolf and Mary Mil Foundation.

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