A Heterogeneous Chemical Origin for the ¹⁶ O-Enriched and ¹⁶ O-Depleted Reservoirs of the Early Solar System

The oldest solids formed in the solar system, calcium–aluminum inclusions, are 16 O-enriched compared to chondrules, asteroids, Earth, and Mars. Based on the preliminary measurements of the solar wind by Genesis , the Sun also appears to be 16 O-enriched. This distribution of oxygen isotopes in the solar system cannot be reconciled via conventional mass-dependent isotopic fractionation processes and instead require the existence and/or production of distinct 16 O-enriched and 16 O-depleted reservoirs in the early solar system. The origin of these distinct reservoirs is unknown, although several mechanisms have been proposed to date including the following: (1) the injection of pure 16 O by a supernova into the protoplanetary disk or parent molecular cloud, (2) self-shielding of CO in the parent molecular cloud or protoplanetary disk, (3) symmetry-dependent chemical fractionation processes in the protoplanetary disk, and (4) Galactic chemical evolution. While some of these proposals have been ruled out, the validity of others is still open. Here I propose that the 16 O-enriched and 16 O-depleted reservoirs present in the early solar system originated in the parent molecular cloud via the heterogeneous chemical processes that form H 2 O, a significant oxygen reservoir, on the surface of interstellar (IS) dust grains in dense molecular clouds, the astrophysical setting where star formation is observed to occur. As a consequence, this model predicts that molecular cloud H 2 O and possibly other IS solids inherited from the molecular cloud were depleted in 16 O compared to the bulk gas-phase O present, thus providing distinct 16 O reservoirs at the earliest stages of planetary formation.