Cosmic dust found for the first time in urban rooftops

Scientists have found cosmic dust for the first time in urban places, on rooftops in three of the world's major cities, which can help unveil how the early solar system has evolved.

Cosmic dust is made up of left over particles that have been around since the formation of our solar system. These particles are tiny, roughly around 0.01 millimetres in size, and have been falling to Earth since it was formed billions of years ago. Analysing their chemical and mineral content can tell scientists about how the early solar system has evolved.

Most dust is collected by scientists from the frozen wastes of places like Antarctica, as it was thought too difficult to unearth these little time capsules in urban debris. For a new study, Matthew Genge from Imperial College London teamed up with Jon Larsen, an amateur scientist from Norway, to sift through 300 kilograms of sediment that was trapped in gutters on roofs in Paris, Oslo and Berlin.

Cosmic dust particles contain minerals that make them magnetic, so they used magnetism to separate the particles under the microscope, finding 500 of them in the debris. The samples analysed show the comparatively big cosmic dust that has recently, in geological terms, fallen on Earth.

In 2011, Larsen contacted Genge to say he believed it was possible to find to find cosmic dust particles in an urban landscape such as cities. "When Jon first came to me I was dubious. 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," said Genge. "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," he said. "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 said.

Researchers found that the cosmic dust particles recovered from roof tops are larger than other particles previously recovered, at around 0.3 millimetres. The analysis suggests they were formed by melting during atmospheric entry at speeds of around 12 km per second. This would make them the fastest moving dust particles found on Earth, Genge said.

The new study also suggests that the dust arriving on Earth has changed over the last million years. The cosmic dust found in cities contains fewer feather-like crystals than old cosmic dust from Antarctica, which has accumulated in ice over the last million years. However, it is similar to cosmic dust that fell on Earth during medieval times. The study was published in the journal Geology.