Terrestrial atmosphere, primitive

It is generally accepted that products of cosmic chemistry were showered on the nascent earth and could likewise fall on many other celestial bodies, brought down by comets, meteorites, and cosmic dust. But there is no agreement on the contribution of such products to the origin of life. Some researchers believe that the bulk of the building blocks of life came from outer space, others that they arose in the primitive terrestrial atmosphere by the kinds of process Miller and others have tried to reproduce in the laboratory. Most likely, both sources contributed, but in proportions that remain to be evaluated. 

The similarities in products and pathways between interstellar molecules and terrestrial laboratory experiments imply a unity of physical and chemical laws in the universe. Given certain conditions and appropriate energy sources, the same chemical pathways will be followed to create certain products from the elements. That is not to say that life, even in primitive form, could be supported in interstellar space. The significant precursor molecules found in interstellar space are at extremely low concentrations, but if they were transported to planetary atmospheres, perhaps by comets, they might then react in the proper environment and evolve into self-replicating systems. 

MIF in oxygen isotopes (O-MIF) is well known in primitive meteorites [1] and in atmospheric O3 [2,3], and MIF in sulfur isotopes (S-MIF) has been discovered in Archean rocks [4] and modern ice core sulfates [5]. In terrestrial environments... 

Fig. 2. Comparison of volatile abundance data in protosolar nebula (PSN), primitive chondrites, terrestrial mantle and atmosphere (atmosphere sensu stricto, crust, sediments, oceans). Data are normalized to Ne and PSN, so that the PSN pattern is flat. The choice of Ne as a normalizing isotope is based on the observation that recycling of atmospheric neon in the mantle is limited as indicated by its isotopic composition. Data sources Mazor et al. (1970), Marty Jambon (1987), Anders Grevesse (1989), Pepin (1991), Moreira et al. (1998), Ozima et al. (1998), Marty Zimmermann (1999).

For example, terrestrial Xe has typically been modeled as starting with a primordial U-Xe composition (very similar to solar wind Xe), and then experiencing (a) isotopic fractionation, (b) addition of radiogenic Xe, and (c) addition of heavy Xe isotopes derived from fission of (Pepin 2000). Martian Xe doesn t quite work that way. Swindle et al. (1986) first pointed out that U-Xe (Pepin 2000) will not work as the primordial Martian Xe. Instead, they found that except for an enrichment of radiogenic Xe, Martian atmospheric Xe could be modeled as isotopically fractionated AVCC Xe (at the time considered a primitive component, now considered a mixture of Q-Xe and Xe from presolar diamonds (Ott 2002, this volume)). In that model there would be no need for any " " Pu fission Xe. In fact, there would be no room for any. This would be strange because " " Pu has a longer half-life than the I would decay away first, yet... 

There are two stable isotopes of helium, He and " He, which vary in their ratio by over three orders of magnitude in terrestrial samples. The Earth s atmosphere is well mixed with respect to helium and contains helium with a uniform isotopic composition of He/" He = 1.39 x 10 . Atmospheric helium is a convenient standard for helium isotope determinations, and terrestrial He/" He ratios are usually normalized to the air ratio and expressed in units of R/Ra, where R = He/ He and Ra = ( He/ He) . In contrast to atmospheric helium R/Ra = 1), the radiogenic helium produced by a-decay of U and Th series isotopes has a much lower ratioof R/Ra0.1, while the volcanic helium that is derived from the Earth s mantle is highly enriched in He R/Ra =5-30). Thus volcanic helium has an isotopic composition distinct from other sources such as atmospheric helium or the helium produced by radioactive decay. This He-rich mantle helium is sometimes called primordial helium, since it is thought to be the remnant of a primitive component... 

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