Jun 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011psrd.repte.156t&link_type=abstract
Planetary Science Research Discoveries
Other
Allende, Cai, Cai Formation, Rim, Oxygen Isotopes, Wark-Lovering, Circulation, Solar Nebula
Scientific paper
CAIs are the oldest solids to form in the Solar System. They occur in every type of chondrite, but are particularly abundant and well studied in the Allende carbonaceous chondrite. Justin Simon (University of California, Berkeley, but now at the Johnson Space Center) and colleagues at Berkeley, Lawrence Livermore National Laboratory, and the University of Chicago analyzed the oxygen isotopic composition of a CAI (designated A37) and its rim, using a nanoSIMS to obtain micrometer spatial resolution. They found that the abundances of oxygen isotopes varied: It is high in oxygen-16 in the center of the inclusion, low near the rim, and then high again in the outer rim. Cosmochemists have concluded that CAIs formed close to the Sun (inside the orbit of Mercury, though Mercury was not present at the time), where oxygen-16 was highest. The decrease near the rim of the inclusion A37 indicates that the CAI must have traveled to a part of the dusty early Solar System where oxygen-16 was relatively less abundant, which is further from the Sun, perhaps around the asteroid belt. The increase in oxygen-16 in the rim indicates that the inclusion again moved to a region rich in oxygen-16, probably back to the region near the Sun, and then traveled back to where asteroids formed so it could be incorporated into the parent body of the Allende meteorite. This wandering is consistent with other observations that suggest transport of materials from the inner to the outer solar nebula, but this is the first documentation of small objects migrating back in towards the Sun.
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