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No urban legend: Our rooftops are collecting stardust

John Timmer
A microscope image of some micrometeorites.
To some extent, scientists are professional downers, the people whose job it is to respond to outrageously improbable stories with "well, actually..." But every now and again they manage to confirm something that lots of people wanted to believe anyway.
This is one of those stories.
For years, amateur astronomers have been suggesting that microscopic, spherical particles collected from their roofs are actually tiny meteorites, the dust that formed our Solar System fallen to Earth. Scientists took the claim at face value but ended up being the downers again, at least initially. As a recent paper on this topic describes it:
A popular belief among amateur astronomers is that modern-day extraterrestrial dust can be collected on roofs in urban environments. Studies by Nininger (1941) reported large numbers of magnetic spherules collected in urban areas; however, later studies showed that the abundance of magnetic particles decreases away from urban areas, and that urban spherules are largely artificial in origin. Despite these studies, amateur collection projects in built-up areas have been common, even though most researchers in micrometeorites consider this occurrence an urban myth.
(The pun on the word "urban" there is their fault, not mine.)
One of those amateurs is a Norwegian artist and jazz guitarist named Jon Larsen, who created a group called Project Stardust. Larsen managed two impressive feats to get the issue revisited. One, he convinced people in Oslo to gather materials from their roof gutters (although, oddly, one sample also came from Paris). And not just a few—material came in from buildings that collectively possessed 30,000 square meters of roof.
The second feat was that Larsen got a small international team of scientists (Belgian and UK) to take this seriously.
Faced with about 300kg of roof debris, the authors separated the material using a combination of magnets and physical shape—micrometeorites are spherical because they melt during atmospheric entry and are shaped by the air. From that 300kg, the researchers isolated 500 particles, all just a few hundred micrometers across, that looked like they were micrometeorites. Forty-eight of them were chosen for detailed analysis.
All 48 of them appear to be genuine micrometeorites. They have levels of seven different elements that are similar to those in chondrites, a common class of interplanetary debris. The most common mineral is olivine, which frequently occurs in micrometeorites because it's easy to form during the rapid heating/cooling of atmospheric entry. Some of them have chemicals that are rare in Earth rocks, and most lack elements like sodium, which tend to boil off as they're heated. Three of the four internal textures found in known micrometeorites appear in this collection.
And in contrast to most of the micrometeorite samples we've collected in the past, these materials have only recently fallen to Earth. Commercial buildings in Oslo apparently clean their gutters every six years on average, and the oldest building that these came from is only 50 years old. The relative precision of these dates, combined with the also precise measure of roof area, allowed the authors to estimate that the Earth is struck by about six tons of micrometeorites every day. Put differently, every square meter of the Earth gets hit about twice a year.
Before you look at those numbers, get excited, and head up to the roof to grab your own little bit of stardust, it's important to keep this all in perspective. The research team had to sort through an average of nearly a kilogram of roof gunk just to find one object that was, typically, 300 micrometers across. But if that's worth it for you, by all means, head for the gutters.
Geology, 2017. DOI: 10.1130/G38352.1  (About DOIs).
No urban legend: Our rooftops are collecting stardust Reviewed by Chidinma C Amadi on 7:16 PM Rating: 5

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