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An international team of researchers, including members from the University of Cambridge Department of Earth Sciences, have used an X-ray microscope to investigate meteorite samples from the Sedgwick Museum to learn about the earliest stages of the evolution of the solar system.
The research team took a sample of the Imilac pallasite meteorite (currently on display in the Museum) and ‘illuminated’ it with a beam of intense, coherent X-rays at the BESSY II synchrotron in Berlin. By analysing the response of the sample to the x-ray beam, they were able to image the magnetisation of the meteorite.
This meteorite was originally part of an asteroid, a small planet-like body orbiting our Sun. Here, it could have experienced a magnetic field created by the asteroid (potentially similar to that created today by the Earth). Thanks to the chemistry and structure of these meteorites, they have recorded a reliable record of this field created billions of years ago and have reliably preserved it up to the present day; this concept is similar to how hard drives in computers store data.
By imaging the magnetisation of the sample, the research team were able to visualise this magnetisation and assess if the sample was magnetised. Importantly, this magnetisation reflects the evolution of the properties of the field generated by the asteroid over millions of years.
Back in Cambridge, the team used computer modelling to recreate the observed magnetisation, from which they learnt that the Imilac meteorite recorded an intense field (~2 times as intense as the present day Earth field) over a period of ~4 – 8 million years. Another meteorite from the same asteroid (the Esquel pallasite) recorded a very different trend, with a field that shut off over ~6 – 12 million years.
By comparing these trends to models of the cooling history of the parent asteroid, these two meteorites were found to have recorded this magnetic signal when the core of the asteroid was solidifying (like the Earth’s is at the present day). Core solidification can generate a planetary magnetic field, and through further computer modelling, the researchers found that the field predicted by this mechanism matches the trends drawn from the experiments, implying the recorded field was created by convection driven by core solidification.
Prior to this study, magnetic fields on asteroids were thought to only have been generated through direct cooling of the asteroid. This mechanism is very inefficient at creating magnetic fields, and could only have done so for roughly the first 10 million years of the solar system. On the other hand, solidification driven convection is very efficient, and would have acted for tens to hundreds of millions of years after the solar system formed and likely lasted for a further ten to fifty million years. Also, given its efficiency, magnetic fields created by this mechanism were likely common among asteroids, implying that there was probably a widespread epoch of magnetic activity in the early solar system.
These observations change the way in which researchers think that asteroids (the building blocks of Earth) evolved, and it is becoming increasingly apparent that they appear to be smaller, accelerated versions of our own planet. This opens up the possibility of studying aspects of our own planet by looking at meteorites, as well as using these fascinating rocks as a tool to predict the future behaviour of the Earth.
This research was undertaken by James F. J. Bryson, Claire I. O. Nichols, Julia Herrero-Albillos, Florian Kronast, Takeshi Kasama, Hossein Alimadadi, Gerrit van der Laan, Francis Nimmo & Richard J. Harrison. It is published in nature, the international weekly journal of science.
James Bryson, Department of Earth Sciences