Researchers using the BESSY II synchrotron in Berlin, Germany have captured information stored in ancient meteorites, formed in the early Solar System over 4.5 billion years ago.
Imilac, a pallasite meteorite found in the Atacama Desert, Chile, in 1822. Image credit: Juan Manuel Fluxa / CC BY 2.0.
The team, headed by Dr Richard Harrison of the University of Cambridge, found that the magnetic fields generated by the meteorites’ parent asteroids were much longer-lived than previously thought, lasting for as long as several hundred million years after the asteroids formed, and were created by a similar mechanism to the one that generates the Earth’s own magnetic field.
“Observing magnetic fields is one of the few ways we can peek inside a planet,” said Dr Harrison, who is the senior author on the study published in the journal Nature.
“It’s long been assumed that metal-rich meteorites have poor magnetic memories, since they are primarily composed of iron, which has a terrible memory – you wouldn’t ever make a hard drive out of iron, for instance. It was thought that the magnetic signals carried by metal-rich meteorites would have been written and rewritten many times during their lifetime, so no-one has ever bothered to study their magnetic properties in any detail.”
The particular meteorites used for this study are known as pallasites, which are primarily composed of iron and nickel, studded with gem-quality silicate crystals.
Contained within these unassuming chunks of iron however, are tiny particles just 100 nm across – about one thousandth the width of a human hair – of a unique magnetic mineral called tetrataenite, which is magnetically much more stable than the rest of the meteorite, and holds within it a magnetic memory going back billions of years.
“These tiny particles, just 50 to 100 nm in diameter, hold on to their magnetic signal and don’t change. So it is only these very small regions of chaotic looking magnetization that contain the information we want,” said study first author James Bryson, a PhD student at the University of Cambridge.
Representative image of the kamacite, tetrataenite rim and cloudy zones in the Imilac pallasite: blue and red signals correspond to positive and negative projections of the magnetization along the X-ray beam direction in the meteorite. Image credit: James F.J. Bryson et al.
The measurements the scientists performed at the BESSY II synchrotron demonstrate that the magnetic fields of the asteroids were created by compositional, rather than thermal, convection – meaning that the field was long-lasting, intense and widespread.
“The new technique we have developed is a way of analyzing the images to extract real information. So we can do for the first time paleomagnetic measurements of very small regions of these rocks, regions which are less than one micrometer in size. These are the highest resolution paleomagnetic measurements ever made,” Dr Harrison added.
“The results change our perspective on the way magnetic fields were generated during the early life of the Solar System,” the scientists said.
These meteorites came from asteroids formed in the first few million years after the formation of the Solar System. At that time, planetary bodies were heated by radioactive decay to temperatures hot enough to cause them to melt and segregate into a liquid metal core surrounded by a rocky mantle.
As their cores cooled and began to freeze, the swirling motions of liquid metal, driven by the expulsion of sulfur from the growing inner core, generated a magnetic field, just as the Earth does today.
Scientists now think that the Earth’s core only began to freeze relatively recently in geological terms, maybe less than a billion years ago. How this freezing has affected the Earth’s magnetic field is not known.
“In our meteorites we’ve been able to capture both the beginning and the end of core freezing, which will help us understand how these processes affected the Earth in the past and provide a possible glimpse of what might happen in the future,” Dr Harrison said.
However, the Earth’s core is freezing rather slowly. The solid inner core is getting bigger, and eventually the liquid outer core will disappear, killing the Earth’s magnetic field, which protects us from the Sun’s radiation.
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James F.J. Bryson et al. 2015. Long-lived magnetism from solidification-driven convection on the pallasite parent body. Nature 517, 472–475; doi: 10.1038/nature14114
