No pollution reaches Concordia station, 3,200m up in the Antarctic. Neither does much snow — just a few centimetres a year. But there is something else, strewn among the ice. Tiny particles of smooth, oddly glassy dust.

They don’t come from the clouds and, in the middle of an ice cap, they can’t have come from the land, so there is only one other source: space. Scientists have counted them, to estimate that across the Earth this dusting of “micrometeorites” accounts for 5,000 tonnes of rock a year — far more than arrives in visible meteorites. The Earth is constantly bombarded by meteorites. Most are smaller than this full stop. That makes them almost impossible to find. They are, though, important for our understanding of

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