What are cosmic spherules?
Cosmic spherules are tiny melted droplets of space dust, usually less than a millimetre across, that solidified as they fell through Earth's atmosphere.
Cosmic spherules are the most common kind of micrometeorite found on Earth's surface. They are small round particles, typically between about 50 micrometres and 1 millimetre across, formed when space dust grains melt completely as they enter the atmosphere and then solidify into droplets.
They come in several types, according to their composition: - Stony, or S-type, spherules are the most common. They are made of silicate minerals and glass, often with a pattern of barred olivine crystals or cryptocrystalline textures, sometimes with metal beads. - Iron, or I-type, spherules are made mostly of iron oxides, such as magnetite and wüstite, sometimes with a metal core. - Glassy spherules are made almost entirely of glass.
Cosmic spherules are found in Antarctic ice and snow, in deep-sea sediments, in desert sands and, as research has shown, even in rooftop gutters in cities. They have been collected using magnets dragged across the ocean floor and by melting large volumes of Antarctic ice.
Scientists study them to learn about the dust in the Solar System, much of which comes from comets and asteroids. Their composition, and the presence or absence of features such as oxidised rims, reveal the speed and angle of entry. Spherules found in ancient rocks can also record changes in the atmosphere or the amount of dust reaching Earth over time.
It is easy to mistake industrial spherules, from welding and burning, for cosmic ones, so careful chemical analysis is needed.