Protosolar nebula

Dauphas N, Marty B, Reisberg L (2002b) Molybdenum evidence for inherited planetary scale isotope heterogeneity of the protosolar nebula. Astrophys J 565 640-644. 

Hashizume, K. and Chaussidon, M. (2005) A non-terrestrial 160-rich isotopic composition for the protosolar nebula. Nature, 434, 619-622. 

Dauphas N., Cook D. L., Sacarabany, A. et al. (2008) Iron-60 evidence for early injection and efficient mixing of stellar debris in the protosolar nebula. Astrophysical Journal, 686, 560-569. 

Nevertheless, all abnormal isotopic ratios are not necessarily related to an initial heterogeneity of the protosolar nebula. Magnetic isotope effects could have played a role during the accretion and even later, during the long life-time of the chondrites parent bodies. 

What is important in the context of chondrite chemistry is the fact that the probable presence in the protosolar nebula of dust particles and complex organic molecules is evidenced. This does not mean that all the organic matter detected in carbonaceous chondrites is necessarily molecules still present in the protosolar nebula. Readers interested in details of the formation of the solar system and the accretion phenomena will find a lot of information in the papers by Larimer 14) and Cameron 1S). 

The heterogeneities of the protosolar nebula are considered as very significant by some authors I7), and as quantitatively not important by others 9). Nevertheless, it remains a fact that the model of a homogeneous accretion disc where equilibrium reaction takes place has now been replaced by a much more subtle description where a lack of homogeneity and equilibrium are facts that must be taken into account. This problem will be discussed below. 

Carbon in the carbonaceous chondrites does not exist as polymer or organic molecules alone. Carbonates are also present in relatively small amounts 20,23) and the same is true for elemental carbon. Elemental carbon seems to exist as carbynes (triple-bonded allotropes of carbon). At least three types of carbynes have been described in Murchison 341 but these results were questioned in 1982 by Smith and Buseck63). According to these authors, sheet silicates mixed with elemental carbon could be misidentified as carbynes in X-ray diffraction patterns. These particular carbonaceous phases (carbynes or otherwise) and other carbonaceous phases (polymer and amorphous carbon phases called C-oe and C- 3) are carriers of noble gases trapped in the chondritic material. Some of these carbynes seem to be condensates from the protosolar nebula while others are probably of presolar origin34 >. 

As we said in Sect. 4, these now generally accepted ideas were recently introduced to the literature, and are of great importance in the context of this review article. In our opinion, it is impossible to discuss the problem of the origin (or better, the origins) of organic matter in chondrites without taking into account the heterogeneity of the protosolar nebula and its consequences. In the next section, we will illustrate this last remark. 

In 1973, Clayton et al. U) developed the model of an inhomogeneous protosolar nebula containing solid particles, which had been preformed at a previous stage and which had never participated in a general homogenisation process in the gas phase. These solid particles were formed in a particular environment and injected into the protosolar nebula. They are therefore called presolar or, sometimes, interstellar. 

Carbonates Cl, some in C2 2000 + 72 to + 47 Solar values Protosolar nebula... 

The Synthesis of Organic Molecules in the Protosolar Nebula and During the Accretion Process... 

What kind of chemistry can we envisage taking place in such an environment To answer this question it is important, though not obviously necessary, to ask another question What was the major carbon-containing molecule in the protosolar nebula The response carbon monoxide seems trivial because we know that CO is the most abundant carbon-containing molecule in the universe. Nevertheless, at 300 400 K and under the pressures prevailing in the protosolar nebula, the mixture... 

The Stardust mission succeeded in bringing solid particles from the Wild 2 comet to the Earth. Comets are the intriguing celestial bodies that are believed the last witnesses of the formation of the solar system. They are considered to be aggregates of primordial interstellar dust particles, the last reminders of protosolar nebula. Even simple spectroscopic observation indicates that comets are rich in simple organic species, which may undergo further photochemical transformations during their life within the solar system [35, 36], Hundreds of organic molecules... 

Various stages can be identified by the main physical process acting in the evolution of low-mass protosolar nebula models. 

The chemical composition of ices in space is inferred theoretically on the basis of condensation theory, which predicts tlie composition of solids condensed from gas of Uie cosmic abundance of elements. In Table 9.2, chemical compositions of ices and corresponding equilibrium condensation temperatures are shown in protosolar nebula [1] and interstellar molecular clouds [2],... 

In the protosolar nebula, H2O ice condenses at temperatures lower than 150K. Upon lurther lowering of the temperature, part of the H2O ice transforms into NH3 hydrate and CH4 clathrate hydrate. These are the compositions of ice predicted by the equilibrium condensation tlieory. It should be pointed out, however, tliat tlie equilibrium might not actually be realized at low temperatures. Thus, Uie ice composition predicted by the equilibrium condensation theory may not be tlie actual composition, but should be regarded as a model composition. 

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).

---