It takes luck, and a sharp eye, to see a shooting star. Watchers of the sky should be in for a treat this November during the annual Leonid meteor shower, as the Earth passes through the tail of Comet Temple-Tuttle. Yet a great deal of space-dust falls unseen: each year, 40,000 tonnes of asteroid and comet débris falls to Earth, most of it as tiny particles called micrometeorites, less than 200 microns (thousandths of a millimetre) across. Where does this enormous quantity of material end up? By diligent searching, researchers have found micrometeorites in deep-sea sediments, in the ice sheets of Greenland and Antarctica, and in sedimentary rocks.
The most abundant micrometeorites are the so-called 'cosmic spherules', in which the original metallic or rocky substance of the micrometeorite has been melted, oxidized and recrystallized during atmospheric entry. Plummeting at high velocity through the upper atmosphere, micrometeorites are flash-heated to more than 1,500 °C, melting to form spherical blobs of liquid rock and metal. For a fraction of a second they become incandescent - this is what we, on the ground far below, see as shooting stars. The molten droplets are rapidly quenched and cooled as they plunge further, finally landing as the particles we see as cosmic spherules. Buried in rocks, in the ocean bed, or under ice, cosmic spherules are the fossils of shooting stars.
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