Førhen var partikler fra rummet kun fundet i Antarktis og oceanets dybder, men dette er første gang kosmisk støv optræder i større byer.
Videnskabsfolk undersøgte 300 kilogram snavs fra tagrender i de tre byer og brugte magneter til at filtrere partiklerne, som indeholdte mineraler, der har magnetiske egenskaber. Gennem processen fandt forskerne 500 kosmiske støvkorn.
Partiklerne som er omkring 0,01 mm i størrelsen, faldt til jorden efter deres skabelse. Forskerne håber, at de ved at analysere støvet vil være i stand til at forstå, hvor tidligt solsystemet udviklede sig.
Dr. Matthew Genge fra Department of Earth Science and Engineering ved Imperial College London og amatørvidenskabsmand Jon Larsen fra Norge arbejde sammen på projektet.
Larsen kontaktede sin kollega i 2011, fordi han troede at kosmiske støvpartikler kunne findes i bylandskaber.
Fra de prøver taget i byerne, der blev analyseret, var forskerne i stand til at forstå, at store mængder kosmisk støv for nylig faldt til jorden.
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"When Jon first came to me, I was dubious," Genge said. "Many people had reported finding cosmic dust in urban areas before, but when they were analysed, scientists found that these particles were all industrial in origin."
"We've known since the 1940s that cosmic dust falls continuously through our atmosphere, but until now we've thought that it could not be detected among the millions of terrestrial dust particles, except in the most dust-free environments such as the Antarctic or deep oceans. The obvious advantage to this new approach is that it is much easier to source cosmic dust particles if they are in our backyards," he added.
The particles discovered in urban areas, however, were larger than previous particles found, according to the study. They were around 0.3 millimeters.
Based on the size of the particles, analysis suggests that they were formed by melting as they entered the atmosphere at speeds of around 12km per second, making them the fastest moving dust particles found on the planet, according to Genge.
The study also found that cosmic dust has changed over the last million years. There were fewer feather-like crystals than the particles found in Antarctica, which racked up in ice over the last million years.
It is believed that the difference in the particles comes from changes in the orbits of planets in the solar system. Throughout millions of years, the orbits of planets varied slightly, causing disturbances in the gravity they exerted, influencing the trajectory of the microscopic particles as they traveled through space. All of this, according to the study, results in rising temperatures, which cause the crystals that grew in cosmic dust to form in different shapes and sizes.
"This find is important because if we look at fossil cosmic dust collected from ancient rocks to reconstruct a geological history of our solar system, then we need to understand how this dust is changed by the continuous pull of the planets," Genge said.