Chondrules and Nebular Shocks

Beneath the fusion-encrusted surfaces of the most primitive stony meteorites lies not homogeneous rock, but a profusion of millimeter-sized igneous spheres. These chondrules, and their centimeter-sized counterparts, the calcium-aluminum-rich inclusions, comprise more than half of the volume fraction of chondritic meteorites. They are the oldest creations of the solar system. Their chemical composition matches that of the solar photosphere in all but the most volatile of elements, reflecting their condensation from the same pristine gas that formed the sun. In this invited editorial, we review the nebular shock wave model of Desch & Connolly (Meteoritics and Planetary Science 2002, 37, 183) that seeks to explain their origin. While the model succeeds in reproducing the unique petrological signatures of chondrules, the origin of the required shock waves in protoplanetary disks remain a mystery. Outstanding questions are summarized, with attention paid briefly to competing models. Subject headings: meteors, meteoroids Beneath the fusion-encrusted surfaces of the most primitive stony meteorites lies not homogeneous rock, but a profusion of millimeter-sized igneous spheres [see Hewins (1996) and other articles in the excellent compendium edited by Hewins, Jones, & Scott]. These chondrules, and their centimeter-sized counterparts, the CAIs (calcium-aluminum-rich inclusions), comprise more than half of the volume fraction of chondritic meteorites. They are the oldest creations of the solar system; the oft-quoted age of the solar system of 4.566 ± 0.002 billion years refers to the crystallization ages of CAIs as determined from radioactive isotope dating. Their chemical composition matches that of the solar photosphere in all but the most volatile of elements, reflecting their condensation from the same pristine gas that formed the sun. Their petrology is consistent with their being heated to super-liquidus temperatures for a period of a few minutes; their roundness suggests that the heating occurred while chondrule precursors were suspended in space, so that surface tension pulled their shapes into spheres. In the two hundred years since the discovery of chondrules, this heating event has shrouded itself in secrecy. Identifying the mechanism would be prize enough in itself; but the stakes are potentially even greater for those who suspect that the primitive character of chondrules and their substantial volume-filling fraction implicate them in the