There are the micrometeorites of the fields - long collected in the ice of Antarctica or at the bottom of the oceans is places free of terrestrial dust - and those of the cities, which the professional meteorite hunters had hitherto neglected. This oversight is now repaired: a team of British, Norwegian and Belgian researchers collected mud from the gutters that carry, among other things, the dust that fell on the roofs of Paris and Oslo ... The analysis of 300 kg of this mud has revealed 500 grains from space, larger than 100 micrometers, or one tenth of a millimeter, which are micrometeorites that have failed in cities, after a long journey in the interplanetary environment. They are recognizable by their spherical shape-called spherules. Moreover, their composition rich in metals makes that they can be recovered thanks to a magnet ...

Our distant history

Coming from the collision between meteorites and planetary surfaces like asteroids, they date from the time of the formation of the young solar system, ie 4.6 billion years ago and contain valuable information about this very distant time of our history. The presence of micrometeorites on our roofs is not surprising, the fragments of celestial rocks do not spare some terrestrial areas, but researchers did not suspect of such a harvest. Now it will be easier to scratch the roofs of cities than to organize an expedition to Antarctica.

Their analysis also revealed an enigma: these grains that have been falling for only a few years have been compared to those harvested in Antarctica under ancient layers of ice and thus fallen 800, 000 years ago. Recent micrometeorites in cities are a few dozen times larger than those of 800, 000 years ago. Explaining this difference is not easy, but the team makes some assumptions: the grains of the past would have taken a more tortuous path, and would have burned longer in the atmosphere they would have penetrated with a different speed ... would suffice a small variation of orbit of the Earth or Mars.

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