Chondrules

Chondrules are submillimeter spheres that constitute up to 80% of the volume of the most primitive meteorites. That they result from the solidification of a melt in low-gravity in the early solar system has been known for nearly two centuries, but the conditions of their formation and their significance still elude our understanding. It has been variously proposed that they both predate and postdate planet formation and some have suggested that they are the product of the planet-forming process itself. There is mounting evidence that rather than resulting from a trivial event (or series of events) which melted only a small fraction of solids in the disk, chondrule formation significantly transformed the original material present in the early solar system and contributed to the chemical and isotopic compositions of the first planets. The only meteorites that preserved the chemical composition and isotopic signatures of the earliest solar system solids are the CI chondrites that contain no preserved chondrules and probably had very few, if any. All other chondrites have experienced various levels of metal/silicate and refractory/volatile fractionation that may have resulted from chondrule formation, although a number of researchers argue that these fractionations existed before chondrules and probably resulted from nebular-wide condensation. The current most popular mechanisms for forming chondrules in a nebular setting are radiation emitted by the protosun in the X-wind setting or shock waves propagated in the protoplanetary disk. In the latter case, chondrule formation may have contributed to the first stages of accretion, which would have helped preserve the chemical complementarity between chondrules and matrix. It is important that the chemical and isotopic properties, and even the petrology, of chondrules be reassessed in order to allow the development of chondrule formation models that better fit these constraints.