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Meteorite as cosmic tape

Electron microscope identifies oldest magnetic record in our solar system

The meteorite Bishunpur travelled more than 4.5 billion years through space before it crashed to Earth in northern India over 100 years ago. It contained the oldest magnetic record known today. It dates back to the early phase of our solar system, as researchers from Great Britain, Germany and Norway have discovered. For their investigations they used the cutting-edge electron microscopes of the Ernst Ruska-Centre at Forschungszentrum Jülich, which are normally used for research into semiconductors, superconductors, catalyst and nanomaterials.

The Bishunpur meteorite is a chondrite. This most common and oldest class of meteorites is the source of the oldest material we have on earth today. Its fine-grained rock contains millimeter-sized melting beads, so-called chondrules, which have not been significantly heated or altered since the formation of the solar system. Some of these chondrules include iron grains only a few nanometers in size, from which researchers would like to gain insights about the conditions at the beginning of our solar system.

Ein leuchtender Ball von braun aussehenden Wolken umgebenProtoplanetary Disk Artist Concept.
Copyright: NASA/JPL-Caltech

"Given that these iron grains formed near the beginning of the solar system, 4.6 billion years ago, they have potentially recorded magnetic fields that were present at that time" says Jay Shah, who recently moved from the Imperial College in London to the Massachussetts Institute of Technology in the USA.

The magnetic field, whose traces the Bishunpur may still carry, are of great interest to scientists all over the world. They could have been among the driving forces in the formation of the early solar system, which came into existence out of a disk of gas and dust in - astronomically speaking - a breathtakingly short time span of a few million years. Researchers are still investigating the mechanisms of this process.

The iron granules that the researchers at the Ernst Ruska-Centre in Jülich measured are made of kamacite. This alloy of iron and nickel only occurs in meteorites on Earth. It was formed, as is assumed today, as a result of collisions in the early solar system during which the heated material melted and then solidified again under the influence of the prevailing magnetic field. Similar to a tape recording, the magnetic field of the early solar system was then recorded in the structures of the magnetic material.

The researchers couldn’t say for sure, at first, whether the magnetic signature from today still originates from the primeval times of the solar system. Unlike parts of the Semarkona meteorite that was analyzed a few years ago, the current samples are not uniformly magnetized. "However, the magnetizations that these grains carry are in complex non-uniform vortex structures, and their stability cannot be estimated using previous theories" explains Shah.

As with most meteoric samples from the early days of the solar system, it was unclear whether today's magnetic structures remained unchanged over 4.5 billion years. The researchers finally succeeded in demonstrating the stability of the magnetic vortex structures by using a special method, so-called off-axis electron holography, at Forschungszentrum Jülich. This spatial imaging method has been developed and extended by Rafal Dunin-Borkowski and his colleagues at the Ernst Ruska-Centre in Jülich. It enabled the researchers, together with Jay Shah, to observe the stability of the magnetic vortices while heating the meteoric iron grains to several hundred degrees Celsius. By comparing calculations with the experimental results, the scientists were subsequently able to show that the magnetic structures are remarkably stable and are unlikely to have altered significantly over the life span of our solar system. Their work indicates that iron grains found within unaltered chondritic meteorites are able to carry the oldest magnetic record in our solar system.

Further reading:

Jay Shah, Wyn Williams, Trevor P Almeida, Lesleis Nagy, Adrian R Muxworthy, András Kovács, Miguel A Valdez-Grijalva, Karl Fabian, Sara S Russell, Matthew J Genge and Rafal E Dunin-Borkowski: The oldest magnetic record in our solar system identified using nanometric imaging and numerical modelling, Nature Communications 9 (2018) 1173

Tobias Schlößer