A major question regarding organic matter in carbonaceous chondrites is how primitive this material is: whether it is an interstellar product, a primary condensate from the solar nebula, or a secondary product, introduced into the meteorite during residence in its parent body. We will start this discussion with the study of the evolution of the carbon atom in the solar nebula since one of the few things that we definitely know is that life is based on carbon. Thermodynamics states that carbon monoxide is the stable form of carbon found in the solar nebula at high temperatures, but it becomes less stable on cooling and should transform to methane below 600 K if a pressure of 10–5 atm is assumed. However, methane has a condensation temperature of less than 100 K at this pressure, and its production is kinetically sluggish, so further cooling to 525 K would lead to the CO disproportion reaction into graphite and CO2 . But if these reactions had taken place as described above, there should be no organic molecules anywhere in the solar system. How can we solve this paradox? By taking into account non-equilibrium processes. Two models have been well studied, the Fischer-Tropsch type (FTT) processes and the Miller-Urey (MU) synthesis, both suggested by chemist Harold Urey . In the FTT process, the production of organic compounds takes place through the hydrogenation of carbon monoxide on the surface of active grains, such as iron particles or iron-nickel alloys (kamacite). This model has received support from theoretical work [15,26] as well as laboratory simulations [16,29,30,32,33], although evaluation of the catalytic role of active grains in the formation of organic compounds under nebular conditions is complicated due to the lack of knowledge on real grain-surface properties. The MU synthesis involves the production and recombination of radical and ionic species in a reduced gas atmosphere under the influence of one or more of several possible energy sources (electric discharge, ultraviolet radiation, galactic rays, etc.), followed by secondary reactions in an aqueous phase. This process is usually assumed to have occurred on the surface of asteroids, although the maintenance of an appropriate atmosphere seems difficult to achieve .
The exact nature of the processes responsible for the existence of organic matter in meteorites and the locations where they occurred remain to be clearly established. Gas-grain processes requiring solid surfaces with catalytic properties, electric discharges, and multiple gas-phase reactions could have taken place both in the solar nebula and on asteroids as well as in the interstellar medium. Isotopic studies have revealed that large differences exist in the D/H, 13C/12C, 15N/14N, and 33S/32S ratios associated with different organic compounds in carbonaceous chondrites , adding evidence that more than one source region and production mechanism are necessary to account for their occurrence. Laboratory simulations are essential in order to decipher the role of each type of process. Hydrocarbons have been synthesized under nebular conditions by FTT reactions from CO and H2 gaseous mixtures over kamacite grains [32,33], and amino acids have been produced with excellent results through MU reactions . Also, the immediate precursors of amino acids in the MU synthesis— aldehydes, hydrocyanic acid, ammonia and water— have been detected in space by radioastronomy .
The interstellar heritage
Laboratory FTT and MU reactions can produce many of the organic compounds discussed above. However, none of these processes can produce D enrichments as large as those recognized in the organic acids, amino acids and macromolecular carbon of the carbonaceous chondrites. The existence of macromolecular organic material, similar in many respects to that present in carbonaceous chondrites, in interstellar space, is universally accepted, since the absorption and emission of light by grains in interstellar space are entirely consistent with the presence there of complex carbon compounds. Probably interstellar grains have silicate cores mantled by complex carbon compounds resulting from radiation processing . Carbonaceous chondrites could incorporate interstellar macromolecular organic materials or their precursors, which in turn could be thermally and chemically reprocessed in the solar nebula to varying degrees.
One of the most astonishing findings in cosmochemistry has been the discovery of presolar dust grains in carbonaceous chondrites, which have been extracted and subjected to close scrutiny in the laboratory. They are micrometer-sized mineral grains that existed as part of interstellar dust as a result of nuclear reactions in dying stars and exploding stars, such as supernovae or giant stars , prior to the formation of the Sun and the solar system. Their presolar nature is revealed in the relative abundances of the isotopes of some elements, which differ from those known in all solar system materials . The first interstellar grains were discovered in Murchison in 1987 as minuscule diamonds only a few nanometers across. Since then, several other types of presolar grains have been found, such as silicon carbide, graphite, aluminum oxide, spinel and silicon nitride, in concentrations ranging from 2 parts per billion to 1000 parts per million . The accepted mechanism for the formation of interstellar molecules is based on ion-molecule reactions that, because of the lack of activation energy barriers, can take place rapidly at low temperatures (10 K in interstellar clouds). Ions are originally produced by cosmic rays (cr). The most important process is the ionization of molecular hydrogen: H2 + cr →H2 + + e–. In dark interstellar clouds with abundant H2, this is followed by: H2 + + H2 → H3 + + H. The reason for D enrichment in organic molecules of interstellar origin is that ion-molecule exchange reactions are exothermic, favoring the formation of deuterated molecules: H3 + + HD →H2D+ + H2, H2D+ + CO → DCO+ + H2, etc. The length and complexity of molecules that can be obtained by gas-phase ion-molecule reactions is still unknown but there is certainly a limit. However, condensation of interstellar molecules onto dust grains and subsequent processing and further synthesis on grain surfaces could result in the development of more complex organic species .
Influence of asteroidal environment
It is widely accepted that significant amounts of meteoritic organic matter, or its precursor materials, were synthesized in interstellar and nebular environments. However, the final organic molecular architecture of carbonaceous chondrites was strongly determined by the effects of hydrothermal alteration on their parent bodies. The influence that the asteroidal environment appears to exert on the final constitution of meteoritic organic matter has been well demonstrated by laboratory experiments . In terrestrial systems, clay minerals adsorb organic molecules, and this property has led to a newly proposed mechanism for the formation of high-molecular- weight sedimentary organic matter . Similarly, it is probable that, during hydrothermal processing on the carbonaceous chondrite parent bodies, there was adsorption of organic matter between clay layers, leading to their partial oxidation and condensation into larger organic networks and, eventually, giving rise to the macromolecular material that dominates the organic content of carbonaceous chondrites [25,43].
The discovery that meteoritic organic compounds may be trapped and protected within a clay-mineral matrix has strong implications for our understanding of prebiotic molecular evolution in the early solar system. Clay minerals could trap and concentrate the water-soluble organic compounds present in carbonaceous chondrites, thereby promoting polymerization reactions. Extrapolating forward, the accumulation, protection and consequent increase in organic complexity within the clay-mineral matrix of carbonaceous chondrites could have facilitated the formation and preservation of primitive biopolymers that laid the foundations for early life .
To summarize, four different synthetic routes have been proposed to account for the large number of distinct organic species present in carbonaceous chondrites. They are: (i) interstellar ion-molecule reactions, (ii) Fischer-Tropsch-type catalytic processes, (iii) Miller-Urey-type reactions, and (iv) hydrothermal transformation mediated by clays. These four end-member models are located in different environments and use different starting materials. But none of the synthetic routes alone is consistent with the composition and isotopic data of the organic matter analyzed in carbonaceous chondrites, and one of them, the MU process, is difficult to reconcile with meteorite petrology. It is necessary to combine more than one process in more than one location.
Meteorites contain secreted within them the oldest and most remote known materials as well as an impressive array of organic molecules. A fascinating aspect of this organic matter is that some may be precursors of life. Answering questions such as what might be the product of chemical evolution of such organic materials and when and where they originated awaits the development of yet more precise analytical techniques and procedures than exist today.
Acknowledgements. Support from the Spanish Ministry of Science and Technology (Ramón y Cajal Research Program) and DURSI (Autonomous Government of Catalonia) is acknowledged.