CO Self-Shielding as a Mechanism to Make 16O-Enriched Solids in the Solar NebulaReport as inadecuate




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1

Solar System Exploration Division, NASAs Goddard Space Flight Center, Greenbelt, MD 20771, USA

2

Astrochemistry Laboratory, NASAs Goddard Space Flight Center, Greenbelt, MD 20771, USA

3

International Space University, Strasbourg Central Campus, 1 Rue Jean-Dominique Cassini, 67400 Illkirch-Graffenstaden, France





*

Author to whom correspondence should be addressed.



Abstract Photochemical self-shielding of CO has been proposed as a mechanism to produce solids observed in the modern, 16O-depleted solar system. This is distinct from the relatively 16O-enriched composition of the solar nebula, as demonstrated by the oxygen isotopic composition of the contemporary sun. While supporting the idea that self-shielding can produce local enhancements in 16O-depleted solids, we argue that complementary enhancements of 16O-enriched solids can also be produced via C16O-based, Fischer-Tropsch type FTT catalytic processes that could produce much of the carbonaceous feedstock incorporated into accreting planetesimals. Local enhancements could explain observed 16O enrichment in calcium-aluminum-rich inclusions CAIs, such as those from the meteorite, Isheyevo CH-CHb, as well as in chondrules from the meteorite, Acfer 214 CH3. CO self-shielding results in an overall increase in the 17O and 18O content of nebular solids only to the extent that there is a net loss of C16O from the solar nebula. In contrast, if C16O reacts in the nebula to produce organics and water then the net effect of the self-shielding process will be negligible for the average oxygen isotopic content of nebular solids and other mechanisms must be sought to produce the observed dichotomy between oxygen in the Sun and that in meteorites and the terrestrial planets. This illustrates that the formation and metamorphism of rocks and organics need to be considered in tandem rather than as isolated reaction networks. View Full-Text

Keywords: Fischer-Tropsch reaction; oxygen isotopic fractionation; nebular chemistry; protostellar nebulae; primitive solar nebula Fischer-Tropsch reaction; oxygen isotopic fractionation; nebular chemistry; protostellar nebulae; primitive solar nebula





Author: Joseph A. Nuth, III 1,* , Natasha M. Johnson 2 and Hugh G. M. Hill 3

Source: http://mdpi.com/



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