The history of our planet has been written, among other things, in the periodic reversal of its magnetic poles. Researchers at the Weizmann Institute of Science propose a new means of reading this historic record: in ice. Their findings, which were recently reported in Earth and Planetary Science Letters, could lead to a refined method of probing ice cores and, in the future, might be applied to understanding the magnetic history of other bodies in our solar system, including Mars and Jupiter’s moon Europa.
The idea for investigating a possible connection between ice and Earth’s magnetic history arose far from the source of that ice: on the sunny isle of Corsica, where Prof. Oded Aharonson of the Institute’s Department of Earth and Planetary Sciences was attending a conference on magnetism. More specifically, the researchers there were discussing paleo-magnetism, which is mostly studied through flakes of magnetic minerals that have been trapped in rocks or found in cores drilled through ocean sediments. Such particles get aligned with the Earth’s magnetic field at the time they are trapped in place; even millions of years later, researchers can test their magnetic north-south alignment and understand the position of the Earth’s magnetic poles at that distant time.
The latter is what gave Prof. Aharonson the idea: If small amounts of magnetic materials could be sensed in ocean sediments, maybe they could also be found trapped in ice and measured. Some of the glacial ice in places like Greenland and Alaska is many millennia old and is layered like tree rings. Ice cores drilled through these spots are investigated for signs of such things as planetary warming or ice ages, so why not reversals in the magnetic field as well?
The first question that Prof. Aharonson and his student Yuval Grossman, who led the project, needed to ask was whether it was possible that the process by which ice forms in regions near the poles could result in a detectable record of magnetic pole reversals. These randomly spaced reversals have occurred throughout our planet’s history, fueled by the chaotic motion of the liquid iron dynamo deep in Earth’s core. Researchers measure the magnetic moment – the magnetic north-south orientations – of the magnetic materials in banded rock formations and layered sediments to reveal the magnetic moment of the Earth’s magnetic field at that time. The scientists thought that such magnetic particles could also be found in the dust that gets trapped, along with water ice, in glaciers and ice sheets.
The research team built an experimental setup to simulate ice formation such as that found in polar glaciers, where dust particles in the atmosphere may even provide the nuclei around which snowflakes form. The group created artificial snowfall by finely grinding ice made from purified water, adding a bit of magnetic dust, and letting it fall though a very cold column that was exposed to a magnetic field, the latter having an orientation controlled by the scientists. By maintaining very cold temperatures – around 30º Celsius (86º Fahrenheit) below zero – they found they could generate miniature “ice cores” in which the snow and dust froze solidly into hard ice.
“If the dust is not affected by an external magnetic field, it will settle in random directions which will cancel each other out,” says Prof. Aharonson. “But if a portion of it gets oriented in a particular direction right before the particles freeze in place, the net magnetic moment will be detectible.”
To measure the magnetism of their tiny ice cores, the Weizmann scientists took them to Prof. Ron Shaar’s lab at the Hebrew University of Jerusalem, which has a sensitive magnetometer that is able to measure the slightest of magnetic moments. The team found a small, but definitely detectible, magnetic moment that matched the magnetic fields applied to their ice samples.
“The Earth’s paleo-magnetic history has been studied from the rocky record; reading it in ice cores could reveal additional dimensions, or help assign accurate dates to the other findings in those cores,” says Prof. Aharonson. “And we know that the surfaces of Mars and large icy moons like Europa have been exposed to magnetic fields. It would be exciting to look for magnetic field reversals in ice sampled from other bodies in our solar system.”
“We’ve proved it is possible,” he adds. Prof. Aharonson has even proposed a research project for a future space mission involving ice core sampling on Mars, and hopes that this demonstration of the feasibility of measuring such a core will advance the project’s appeal.
Prof. Oded Aharonson’s research is supported by the Helen Kimmel Center for Planetary Science, which he heads; the Minerva Center for Life Under Extreme Planetary Conditions; the Zuckerman STEM Leadership Program; and the Adolf and Mary Mil Foundation.
The Weizmann Institute of Science in Rehovot, Israel, is one of the world’s top-ranking multidisciplinary research institutions. The Institute’s 3,800-strong scientific community engages in research addressing crucial problems in medicine and health, energy, technology, agriculture, and the environment. Outstanding young scientists from around the world pursue advanced degrees at the Weizmann Institute’s Feinberg Graduate School. The discoveries and theories of Weizmann Institute scientists have had a major impact on the wider scientific community, as well as on the quality of life of millions of people worldwide.
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