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The Case for Rarity of Chondrules and Calcium-Aluminum-rich Inclusions in the Early Solar System and Some Implications for Astrophysical Models

The high-temperature materials in chondritic meteorites, the chondrules (silicate spherules produced by flash heating) and refractory inclusions (the CAI, or calcium-aluminum-rich inclusions), have been used to place major constraints on astrophysical models for the early solar system. Among the meteorites falling to Earth, the CAI are rare, being only 15% by volume of a class that constitutes 0.84% of observed falls, however chondrules are ubiquitous. Here it is pointed out ~96% of the meteorites entering the atmosphere contain few or no chondrules because atmospheric passage destroys such meteorites 1000 times more effectively than the tougher chondrule-rich material. Furthermore, the mechanisms for transferring meteorites from the asteroid belt to Earth do not sample the belt representatively, so that even the 4% of chondrule-rich material entering the atmosphere may be an overestimate. Most asteroids have surfaces resembling meteorite classes that are chondrule-poor or chondrule-free. Foreign clasts in meteorites, which are an independent sampling of asteroidal material, usually resemble chondrule-poor or chondrule-free carbonaceous chondrites. Cosmic-ray ages and gas-retention ages for meteorites indicate that most stony meteorites are coming from just a few asteroids. A strong case can thus be made that chondrules and CAI were rare in the inner solar system during planet and planetesimal formation and that they do not provide a strong constraint on astrophysical ideas for the evolution of the early solar nebula.