During early planetary formation, rocks and dust accumulate, gradually increasing in size by drawing in matter from nearby. Hypervelocity asteroidal impacts can shatter these planetoids, with some fragments losing gravitational pull and travelling out into space. Some fragments may be captured by the Earth's gravity and cross our atmosphere as meteors, with some falling to the surface as meteorites.
Iron meteorites are thought to be from asteroid cores and therefore were created early in our solar system's history. Although crossing the Earth's atmosphere has a heating effect on the meteors, large meteors (more than 10cm3) only experience the effect on their external parts, with the interior being unaffected. The interior crystalline structure of the iron meteorites can therefore be studied to give insight into the geochemical and geophysical conditions present during the early formation of the solar system.
Iron meteorites contain a combination of several iron nickel alloys in the crystallite formations, kamacite and taenite. These types of meteorites have been extensively studied in the past using destructive techniques requiring slicing and sectioning much of the sample. At ISIS, researchers from Universita degli Studi di Firenze and Istituto dei Sistemi Complessi have been investigating the possibility of using neutron diffraction to study iron meteorites non-destructively for the first time. Time of flight neutron diffraction was used on two instruments at ISIS to analyse several iron meteorite samples.
Engin-X was used to look at the stresses within several samples from different meteorites. This experiment allowed high resolution diffraction patterns to be generated and analysed, showing that one of the meteorites contained both type I (across large crystal domains) and type II (across small crystal domains) stresses while the other two meteorites only contained type II stresses. The technique used showed to be a robust and viable way of evaluating the residual stress in iron meteorites and showed clear definition between type I and II stresses giving an indication of possible impacts and cooling experienced by the rock in space.
Similarly INES was used to study a collection of nine iron meteorite samples of different chemical groups for insight into their crystallite size, texture and internal strain. The samples showed varying texture which is thought to relate to the conditions during crystallization. A rapid cooling rate or reheating followed by rapid cooling is shown to result in a highly texturized material and rapid post accretion annealing relates to crystallite size and internal strain. The experiment also allowed the size of kamacite cell parameters to be related to pressure experienced during the accretion stage.
Further work needs to be carried out to refine these techniques, but these initial studies show the huge potential of using neutron diffraction to non-destructively investigate iron meteorites. In the future a wider analysis of more samples using this technique could provide a better understanding of planet formation during the early stages of our solar system.
Further information
Type I and type II residual stress in iron meteorites determined by neutron diffraction measurements, published in the journal Planetary Space and Science: https://www.sciencedirect.com/science/article/pii/S0032063317303148
Different Conditions of Formation Experienced by Iron Meteorites as Suggested by Neutron Diffraction Investigation, published in the journal Minerals: http://www.mdpi.com/2075-163X/8/1/19/htm
Learn more on our instruments Engin-X and INES
