Solar nebula carbon isotopes

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

There are three main classes of chondrites (Table 10.1) that are most readily distinguished by whole-rock chemical and O isotopic compositions [14, 17]. The ordinary chondrites are the most abundant type of meteorite, as they account for about 80% of all meteorite falls. Enstatite chondrites have very reduced compositions and they are rich in reduced Fe and Fe-poor silicate phases, such as enstatite (MgSiOa). Most carbonaceous chondrites have high contents of carbon and other volatile elements that only condensed into solids at very low temperatures in the solar nebula. 

Comet accretion models. Noble gases, as well as water, carbon, and nitrogen, could have been supplied to the inner planets by accretion of volatile-rich icy comets scattered inward from the outer solar system. Although noble gas isotopic distributions in comets are unknown, solar isotopic compositions would be expected in cometary gases acquired from the nebula. There is experimental evidence that the relative elemental abundances of heavier species (Xe, Kr, and Ar) trapped in water ice at plausible comet formation temperatures ( 30 K) approximately reflect those of the ambient gas phase, and trapped noble gas abundances per gram of water are substantial (Bar-Nun et al. 1985 Owen et al. 

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