Scoop a handful of gunk from your rooftop gutter and amid all those leaves and soggy dirt, you might just hold some of our solar system's oldest secrets. Scientists have studied sediment taken from rooftop gutters across Europe and discovered cosmic dust hidden within, marking the first time these extraterrestrial sprinkles have turned up in urban environments.

Cosmic dust refers to the microscopic particles that float around in outer space, and also make their way through our atmosphere and onto the Earth's surface. These particles are old, so old that many astronomers believe they were on the scene during the formation of our solar system billions of years ago.

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The thinking is that anything that's been around that long has has a few stories to tell, so scientists have gone to great lengths to probe the secrets of these microscopic particles by analyzing their chemical and mineral contents. Spacecraft like NASA's Cassini have special instruments onboard for analyzing cosmic dust out in space, but even the particles that make their way down to Earth aren't so easy to come across.

This is because the dusty nature of urban environments effectively drowns them out and was thought to make them undetectable, leading scientists to more extreme locations to find them, such as the Australian outback, Antarctic ice fields, and the deep ocean. But now it seems that they can be found a lot closer to home.

Five years ago, an amateur scientist from Norway approached a researcher at Imperial College London called Matthew Genge with an audacious idea: that it was indeed possible to find cosmic dust particles in urban landscapes. This idea had been bandied about in amateur science circles for decades, but lacked the conclusive evidence to back it up.

"When Jon first came to me I was dubious," says Genge. "Many people had reported finding cosmic dust in urban areas before, but when they were analyzed scientists found that these particles were all industrial in origin."

So the duo collected 300 kg (661 lb) of material from rooftop gutters in Paris, Oslo and Berlin. Because cosmic dust particles contain minerals that make them magnetic, the researchers were able to use magnetism to pull the particles from the sediment under a microscope, eventually winding up with 500 of the little suckers.

A cosmic particle taken from a rooftop gutter

"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," says Genge. "The obvious advantage to this new approach is that it is much easier to source cosmic dust particles if they are in our backyards."

Further investigation of the cosmic dust revealed a couple of other interesting tidbits. The particles found in the cities happened to be a bit larger than those found in other locations, around 0.3 mm compared to roughly 0.01 mm regular size. The researchers say this indicates that they whizzed through the hot atmosphere faster than normal, at around 12 km per second (7.45 mi), which would make them the fastest moving dust particles found on Earth.

The makeup of the particles, which the researchers say likely fell to Earth within the last six years, also seems different to older particles taken from Antarctica that built up over the last million years, with fewer feather-like crystals within. Genge believes that this is due to the slight changes in orbits of the solar system's planets over millions of years, which disturbs their gravitational pull and in turn the trajectory of cosmic dust racing through space.

"This find is important because if we are to 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," he says.

The research was published in the journal Geology.

Source: Imperial College London

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