What is a chondrule?
A chondrule is a small, rounded grain, usually less than a few millimetres across, that formed as a molten droplet floating in space in the early Solar System and then cooled quickly. Chondrules are the defining feature of chondrite meteorites.
If you look closely at the cut face of a chondrite, you may see dozens of tiny round or oval grains, like seeds set in a fine-grained paste. These are chondrules, and the name comes from the Greek word for a grain. Most are well under a millimetre to a few millimetres in diameter, though some meteorites hold larger ones.
Chondrules were once droplets of melted rock. Something in the early Solar System heated clumps of dust to temperatures high enough to melt silicate minerals, then let them cool within minutes to hours. That rapid cooling left distinctive textures that show up beautifully under a microscope: some chondrules contain parallel bars of olivine, others are made of fan-like sprays of pyroxene, and many are packed with small crystals set in glass.
Exactly what melted them is still one of the great puzzles of meteorite science. Leading ideas include shock waves passing through the gas and dust disc around the young Sun, collisions between early bodies, and even lightning-like electrical discharges. Radiometric dating shows that most chondrules formed within the first few million years after the oldest Solar System solids, the calcium-aluminium-rich inclusions.
Chondrules are easiest to see in less heated chondrites. Allende, a carbonaceous chondrite that fell in Mexico in 1969, shows many clearly against its dark matrix. In more strongly heated meteorites, chondrules recrystallise and their edges blur, which is one way scientists judge how much heating a meteorite experienced. A rounded grain in a rock is not proof of a meteorite, though; similar-looking shapes occur in some Earth rocks, so identification needs expert testing.