Saturday 17 December 2016

Micrometeorites from urban environments.

Micrometeorites are dust-sized particles of extraterrestrial origin, that are able to survive the fall through our atmosphere without being destroyed by friction-induced heating due to their low mass (the largest micrometeorites do undergo significant heating and melting, and are known as spherules due to their shape). Micrometeorites can be derived from asteroids or comets, and the study of these bodies is considered to provide significant insights into the evolution of small planetary bodies in our Solar System. However distinguishing micrometeorites from similarly sized mineral particles of terrestrial origin is difficult, and planetary scientists go to great lengths to collect such particles from environments where input from terrestrial sedimentary sources is minimal, such as Antarctic ice- and snow-fields and oceanic basins. Many amateur astronomers have suggested that such particles could also be collected from urban environments, particularly rooftops, but this suggestion has, to date, gained little traction with professional scientists.

In a paper published in the journal Geology on 5 December 2016, Matthew Genge of the Department of Earth Sciences and Engineering at Imperial College London and the Department of Earth Science at the Natural History Museum, Jon Larsen of Project Stardust, Matthias Van Ginneken of the Département des Géosciences at the Université Libre de Bruxelles and Martin Suttle, also of the Department of Earth Sciences and Engineering at Imperial College London and the Department of Earth Science at the Natural History Museum, describe the results of a study of micrometeorites collected from sediments accumulated on rooftops and gutters in Oslo and Paris.

Genge et al. were able to isolate 500 particles identified as micrometeorites from 300 kg of sedimentary material collected from a surface area of ~30,000 m² of rooftops in Olso and Paris (499 micrometeorites came from Oslo and one from Paris). All of these micrometeorites were between 300 and 400 μm in diameter, and were identified as micrometeorites on the basis of mineralogical similarities to other meteorites.

Backscattered electron images of urban cosmic spherules. REL—relict grain, SUL— sulphide, MET—metal, MGR—magnetite rim, OLR—olivine rim. A–C: Porphyritic spherules. D–F: Barred olivine spherules. G, H: Cryptocrystalline spherules. Genge et al. (2016).

These particles included porphyritic olivine spherules dominated by phenocrysts of olivine within glassy mesostasis, barred olivine spherules dominated by parallel growth dendrites of olivine with interstitial glass and cryptocrystalline spherules dominated by radiating clusters of fine olivine dendrites within glass. These compositions are consistent with S-type spherules derived from Antarctic and deep sea sediments, supporting the idea that these urban particles are of extra-terrestrial origin.

As well as providing a source of micrometeorites that does not involve research expeditions to remote locations, this study provides as source of very young material that has not been previously available. Most studies of micrometeorites have relied on sources that time-average material over long periods (thousands of years), enabling them to be found in sufficient numbers to justify collection. However the majority of the rooftop material is thought to have fallen in the last six years, as it is collected from gutteiring which people periodically clean, and all of the material to be less than 50 years old, the age of the oldest building from which it was collected.

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