A "unique" micrometeorite found in Antarctica is challenging ideas about how planets can form. Detailed analysis has shown that the sample, known as MM40, has a chemical composition unlike any other fragment of fallen space rock. This, say experts, raises questions about where it originated in the Solar System and how it was created. It also means that astrochemists must expand their list of the combinations of materials in planetary crusts. The detailed analysis of MM40 was led by Matthieu Gounelle from the Laboratory of Mineralogy and Cosmochemistry at the French Natural History Museum. Published in PNAS, the analysis revealed the "unique" chemical composition of MM04 despite it being only 150 microns across as its widest point - about half the width of a written full stop. Micrometeorites are often seen as the 'poor man's space probe'

Dr Caroline Smith, Natural History Museum Dr Caroline Smith, curator of meteorites at the Natural History Museum, London, UK, said the sample was important because of the role that the study of meteorites played in our understanding of Solar System and planetary formation. MM40 was a basaltic achondritic micrometeorite, said Dr Smith. Achondritic meteorites were formed when the Solar System's planets were coming into being. The substances in such meteorites and the processes they have undergone can give clues about how the larger bodies were formed. By contrast, chondritic meteorites were formed during the the Solar System's early days before material had accreted into planets. They have not been altered by the melting and re-crystalisation that has utterly transformed the nature of, say, Earth rocks. Dr Mahesh Anand, an astrochemist from the department of Earth & Environmental Sciences at the Open University, said: "It is fascinating as to how much information can be retrieved about the processes involved in planetary formation from tiny fragments of extra-terrestrial material that routinely arrive on Earth anonymously." For Dr Smith, the excitement of MM40 lay in the mystery of its origins. "We have basaltic meteorites that are thought to come from an asteroid called 4 Vesta and we also have basaltic meteorites from the Moon and Mars," said Dr Smith. "But [MM40's] chemistry does not match any of those places," she said. "It has to be from somewhere else." While its ultimate origins are a mystery it does have implications for the ways that astrochemists thought planets could be formed. The analysis of MM40 showed that the "inventory" of such processes must be expanded, said Dr Smith. "Micrometeorites are often seen as the 'poor man's space probe'," said Dr Smith "They land on Earth fortuitously and we do not have to spend millions of dollars or euros on a robotic mission to get them."



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