When a micrometeoroid enters the Earth’s atmosphere, the heat generated by friction melts it into a liquid droplet. During the subsequent deceleration, its temperature drops and isolated crystal domains start to grow in the melt. Features not unlike small mountain islands begin to form in an ocean of molten glass.
If the temperature curve is not too steep, the crystal domains continue to grow until the entire meteoroid is crystalline. The formation is over in a blink of an eye and the result is a polysomatic crystalline micrometeoroid. From then on, the little particle falls to Earth at terminal velocity and is carried softly to the ground by wind.
This is the story of freshly cryptocrystalline micrometeorite, NMM 2758, found last year on a roof outside Oslo, Norway. The peak temperature during the atmospheric entry must have been close to 1,900°C, because the olivine crystals are fine-grained forming a so-called “turtleback”.
Had the peak temperature been slightly lower, the texture would have been different; barred olivine (BO) with a characteristic stripy surface.