Chondrules, such as the one shown in figure 1, are millimeter-scale, previously molten droplets that make up more than half the volume of most meteorites that fall to Earth. Isotopic dating suggests they formed during the roughly 5 million years when the solar nebula was coalescing into solids (see the article by Robin Canup, Physics Today, April 2004, page 56). Unfortunately, there’s no consensus as to how they formed. The conventional and perhaps still leading view holds that chondrules originated as rapidly melted dust balls—flash heated by shock waves, lightning bolts, solar flares, or some other process—that crystallized in hours to days and sometime afterward became part of accreting planetesimals and planetary embryos. But no one has convincingly explained which of those potential heat sources was actually responsible.
Other theories posit collisions between asteroids as the source of chondrule-forming droplets. The idea goes back to the 1950s...
