Life History of a Dust Grain (IMAGE) University of California - Berkeley Caption All CAIs (calcium-aluminum-rich inclusions) are thought to have originated near the protosun, which enriched the nebular gas with the isotope oxygen-16. In a single CAI, the abundance of oxygen-16 was found to decrease outward from the center of the core, suggesting that the CAI formed in the inner solar system, where oxygen-16 was more abundant, but later moved farther from the sun, where the CAI lost oxygen-16 to the surrounding 16O-poor gas. Initial rim formation could have occurred as CAIs fell back into the midplane of the disk, indicated by the dashed path A; as they migrated outward within the plane of the disk, shown as path B; and/ or as they entered high density waves (i.e., shockwaves). Shockwaves would be a reasonable source for the implied 16O -poor gas, increased dust abundance and thermal heating. The first mineral layer outside the core had more oxygen-16, implying that the grain had subsequently returned to the inner solar system. Outer rim layers had varying isotope compositions, but in general indicate that they also formed closer to the sun, and/or in regions where they had lower exposure to the 16O -poor gas from which the terrestrial planets formed. Credit Justin Simon/NASA Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.