A pulse of micrometeorites reaching the Earth's surface following the breakup of an asteroid has been proposed to explain the abundance of extraterrestrial grains in 470-million-year-old rocks in Sweden. An unusual concentration of extraterrestrial material has also been identified in sedimentary rocks of this age in China, suggesting that the micrometeorite rain was a global event.
Carl Alwmark of Lund University and colleagues analysed chromite grains found in 470-million-year-old rocks near the Puxi River, China. The concentrations and isotopic ratios of neon trapped in these grains indicate that they had been directly exposed to the solar wind, suggesting that the grains were formed in the solar system and then delivered to Earth as micrometeorites. The high number of chromite grains points to an influx of micrometeorites two to three orders of magnitude higher than background levels, consistent with the breakup of an asteroid.
Age estimates of the Chinese rocks suggest that the micrometeorite pulse continued for at least two million years following the meteorite-forming event.
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Goldin, T. Cosmic rain. Nature Geosci 5, 590 (2012). https://doi.org/10.1038/ngeo1579
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DOI: https://doi.org/10.1038/ngeo1579
