Scientists have found traces of cosmic dust lying on the rooftops of buildings in Paris, Oslo, and Berlin – and it's the first time researchers have confirmed the discovery of these tiny particles in urban environments.
Cosmic dust is the name for small amounts of matter in space, including those left over from the formation of our Solar System some 4.6 billion years ago, and new research shows that these micrometeorites are still falling on Earth billions of years later.
"We've known since the 1940s that cosmic dust falls continuously through our atmosphere," says planetary scientist Matthew Genge from Imperial College London in the UK.
"[B]ut 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."
Previous attempts to identify cosmic dust in cities had only uncovered terrestrial grime and particles left over from industrial pollution.
But after sifting through 300 kilograms of sediment collected by roof gutters in the three European cities, Genge and fellow researcher Jon Larsen – who runs micrometeorite enthusiast site Project Stardust – found some 500 of the particles.
Cosmic dust grains contain magnetic minerals, so the team was able to extract them from the rest of the sediment using magnets, and identified them on the basis of their composition.
The grains they found are what's called silicate-dominated (S type) cosmic spherules, which are melted into non-spherical shapes due to the intense heat they encounter during entry into Earth's atmosphere.
Cosmic dust particles are usually extremely tiny – measuring around 0.01 millimetres in size – but the samples the team uncovered from the rooftops are slightly larger, at about 0.03 millimetres.
The newly discovered micrometeorites also contain subtle crystal variations in their structure, which resemble samples dating from medieval times. By contrast, older samples dating back millions of years found in Antarctica feature a different crystal composition.
While the exact reason for these differences is unknown, the researchers speculate that it's to do with minor changes in the orbits of planets in the Solar System.
Over millions of years, the way planets move around the Sun varies slightly due to gravitational variations, and this in turn affects the gravity exerted by each planet on the matter around it.
Based on the size and shape of the particles, Genge thinks they were melted as they plummeted to Earth at speeds of around 12 kilometres per second (7.5 miles per second), which would make them the fastest-moving dust particles found on Earth.
The researchers also suggest these could be the youngest cosmic dust particles ever discovered, estimating that they fell to Earth within the past six years.
This is based on the regularity of gutter cleaning of commercial buildings, and also the low amount of rusting evident in their metallic content, which would have occurred during the particles' short stay on Earth.
According to the researchers, the techniques they used to identify cosmic dust in urban environments means that these space particles could be found anywhere on Earth.
And the more of them that we can detect and analyse, the greater we can understand how the early Solar System evolved, as well as the changes that have taken place in our corner of the Universe since then.
"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," 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."
The findings are published in Geology.