Micrometeorites in towns and cities
For decades, scientists believed micrometeorites could never be picked out of city dirt. In 2016 that idea was overturned, when hundreds of genuine space particles were identified in the grime from roof gutters in three European capitals.
Space dust lands on roofs just as it lands on Antarctic ice. Rain washes it into gutters and downpipes, where it collects in the sediment. The trouble is that cities also make huge numbers of tiny magnetic spheres from industry and traffic. A Norwegian amateur spent years learning to tell the two apart, and in 2016 he and a British scientist showed that real micrometeorites can be found in towns.
Key facts
- Confirmed
- Reported in a scientific journal in December 2016
- Cities sampled
- Oslo, Paris and Berlin
- Material searched
- About 300 kg of roof-gutter sediment
- Particles found
- About 500 cosmic spherules, typically around 0.3 mm across
The old view: impossible
Micrometeorites fall everywhere, so in principle they must land on every roof and pavement. But a city is full of look-alikes. Welding, grinding, burning coal, car brakes, railway lines and fireworks all make tiny magnetic balls of melted metal and rock, and in urban dust they outnumber genuine micrometeorites by a huge margin. For most of the 20th century the specialists' view was that searching city dirt was pointless: any space particles would be impossible to find among the man-made ones, and the few reported claims were not convincing.
An amateur's long search
The change came from outside the universities. Jon Larsen, a Norwegian jazz guitarist with a lifelong interest in natural science, became fascinated by the idea of space dust landing all around us, and from about 2009 he began collecting sediment from roofs, gutters and car parks, sieving and sorting it, and examining the magnetic grains one by one under a microscope. Over the following years he built up a catalogue of the industrial particles he kept finding, learning what each kind looked like so that he could exclude them.
After about six years and tens of thousands of particles examined, he found his first convincing candidate in early 2015. He worked with Matthew Genge, a planetary scientist at Imperial College London who specialises in micrometeorites, and they checked the candidates with an electron microscope, which shows a particle's surface in fine detail and measures its chemistry.
The 2016 study
In a paper published in December 2016, Genge, Larsen and colleagues described about 500 cosmic spherules recovered from roughly 300 kg of sediment from roof gutters in Oslo, Paris and Berlin. The particles had the textures of genuine micrometeorites, such as barred olivine and porphyritic olivine crystals and fine magnetite on their surfaces, and chemistry like that of chondritic meteorites, with the nickel and the balance of magnesium, iron and silicon that industrial spheres lack.
Several features stood out. The urban spherules were relatively large, typically around 0.3 mm, because the smaller grains were harder to find and more easily washed away. They were very fresh: because gutters are cleared and roofs replaced, the particles must have landed within the last few years or decades. And compared with Antarctic spherules, which mostly fell over the last million years or so, they had subtly different textures, with fewer of the feather-like magnetite crystals that grow when iron-rich droplets cool. The authors suggested that the urban particles had arrived at somewhat higher speeds, around 12 km/s or more, perhaps reflecting slow changes in the orbits of the dust reaching the Earth.
Why roofs work
A roof acts as a collecting surface. Micrometeorites that land on it are washed by rain into the gutters and downpipes, along with grit, moss, leaf litter and the fine particles from roof tiles and felt. Large, flat roofs that have not been cleaned for many years collect the most, and roofs far from busy roads and industry collect fewer look-alikes. Because the particles have been on the Earth for only a short time, they are among the least weathered micrometeorites known.
A science for everyone
The discovery opened micrometeorite hunting to anyone with a magnet, sieves and a microscope. Since then, amateurs in many countries have found urban spherules of their own, and Larsen's close-up photographs of them, often taken with an electron microscope, have been exhibited and published in books.
It remains hard. The great majority of magnetic spheres in any gutter are not from space, and a convincing identification needs care and experience. See recognising a micrometeorite and search for them yourself, which explains a safe method that never needs anyone to climb onto a roof.
Did you know?
Finding about 500 micrometeorites took roughly 300 kg of sediment: on average, less than two genuine particles per kilogram of gutter dirt.
Jon Larsen is also a founder of the Norwegian gypsy-jazz group Hot Club de Norvège.
A roof's worth of micrometeorites can be younger than the building itself: many urban spherules landed only a few years before they were collected.