The most primitive meteorites, chondrites, preserve records of the prehistory and early history of our Solar System. The chondrites are composed of three basic components: chondrules and refractory inclusions set in a fine-grained matrix. Chondrules and refractory inclusions formed at high temperatures in the solar nebula. The fine-grained matrix also contains products of high temperature nebular processing, such as crystalline silicates. However, it also contains presolar circumstellar grains that formed around AGB stars and supernovae , as well as amorphous silicates that may include interstellar grains. The non-solar D/H of water accreted as ice by chondrites points to an interstellar contribution , and large D and 15N enrichments in chondritic organic material point to an interstellar or outer Solar System origin . Overall, chondrites seem to have accreted ~5-10% of interstellar material in their matrices .
Thus, one might expect to be able to make a direct comparison between primitive meteorites and astronomical observations. However, chondrites also accreted short-lived radionuclides that internally heated their parent bodies, initially melting ices and promoting clay formation but, in many cases, temperatures subsequently became high enough to dehydrate them. This heating was necessary to lithify these cosmic sediments into ‘rocks’ that could be delivered to Earth. Unfortunately, it also has modified to varying degrees the organic material that they accreted.
The least heated chondrites contain several hundred ppm of complex suites of soluble organic compounds. The best studied of these include amino acids, amines, hydroxyl acids and carboxylic acids. These compounds exhibit almost complete structural diversity, pointing to abiotic synthesis. The dominance of alpha amino acids points to a Strecker-type synthesis involving HCN, NH3 and aldehydes/ketones. This synthesis seems to have occurred quite early as in the meteorites that experienced more extensive alteration of their silicates, amino acid abundances are much lower and carboxylic acids are much higher [5, 6], suggesting that there was progressive destruction of the amino acids. Interestingly, at the same time there appears to have been the development of up to 10-20% L enantiomer excesses in isovaline , one of the few more abundant amino acids that would not racemize in 4.5 billion years. Although H2, CO and CO2 would have been present in the nebula and in the meteorites, there is little evidence for the products of FTT synthesis.
The dominant organic material in chondrite matrices (up to 3-4 wt.%) is a macromolecular organic material (IOM) . In bulk, its composition is similar to comet Halley CHON material and the refractory organic material in comet 67P dust . It is composed of small, highly substituted aromatic moieties that are cross-linked by short, highly branched aliphatic chains and O functionality. At present, there is no consensus on how IOM formed.
It is interesting to speculate why, if many of the building block were present, life did not arise in meteorites. It may have been simply that there was insufficient time. Also, if, as some have suggested, meteorites/comets provided the prebiotic building blocks for life on Earth, it could only have been in objects that were mm-m-sized or less than a few hundred microns across. Otherwise, atmospheric entry and impact vaporization would have destroyed them. Given the low abundances of soluble organics, only if IOM was broken down by weathering on the early Earth could meteorites/comets have been potent sources of O-,N-bearing prebiotic molecules.
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