Large enantiomeric excesses in primitive meteorites and the diverse effects of water in cosmochemical evolution

Abstract

Carbonaceous chondrites are meteoritic fragments of asteroids that avoided the geological reprocessing of larger planets and allow laboratory probing of early solar-nebula materials. Among these, Renazzo-type (CR) chondrites found in Antarctica appear remarkably pristine and are distinguished by abundant organic materials and water-soluble molecules such as amino acids and ammonia. We present a comprehensive analysis of the organic composition of selected CR meteorites of different petrographic classification and compare compounds' abundance and distribution as they may relate to asteroidal aqueous processing and concomitant evolution of the mineral phases. We found that several CR compounds such as amino acids and sugar alcohols are fully represented in stones with no or minimal water exposure indicating a formation that, if solar, preceded parent body processes. The most pristine CRs also revealed natal enantiomeric excesses (ee) of up to 60%, much larger than ever recorded. However, aqueous alteration appears to affect CR soluble organic composition and abundances, in particular some diastereomeric amino acids may gauge its extent by the consequent racemization of their ee.primitive asteroids | abiotic molecular evolution | solar ices | clays C R meteorites are characterized by several petrologic, geochemical, and isotopic features, such as containing large chondrules, high Fe-Ni metal abundances, and distinct wholerock O-isotopic distributions (1). However, only recent comprehensive analyses of the soluble and insoluble organic materials in two pristine* CR2 † meteorites, GRA 95229 (GRA1) and LAP 02342 (LAP) (2, 3) revealed what is possibly their most distinguishing characteristic vis-à-vis other carbonaceous chondrites (CC) types (i.e., soluble organic suites that are overall unique for having a preponderance of water-over solvent-soluble compounds). Ammonia is the most abundant single molecule detected in CR2 water extracts, and amino acids are their largest component, unlike, for example, in the Murchison meteorite where hydrocarbons are the dominant molecular species.Amino acids have been the most captivating findings in CC meteorites since the fall of Murchison (4) because several of the compounds have exact molecular correspondence with biomolecules and possible pre-biotic significance. In intervening studies, however, the detection of a diverse and apparently stochastic suite of compounds in many meteorites made it difficult to propose how this random mix could have found effective chemical pathways toward the origin of life (5). CR2s have significantly changed that perspective with their main compositional abundances of water-soluble and N-containing molecular species.CC materials relied on a long cosmic history for their formation, which spanned solar as well as presolar environments. In particular, the high D and 13 C isotopic enrichments of some CC organic compounds have revealed their lineage from cold cosmic regimes (6). Among CRs, GRA1 amino acids display δD values up to þ7;200‰ (2), the larges...

Enantiomer excesses of rare and common sugar derivatives in carbonaceous meteorites

“…Racemic mixtures are equated with pristine/uncontaminated abiotic samples when discussing meteoritic compounds. However, some meteoritic amino acids that are rare on Earth, i.e., they are not constituents of proteins and therefore less likely to be contaminants, have been confirmed to contain L enantiomer excesses (EE) (3,12,13). The origins of such excesses are unknown.…”

E

“…The discovery of a large number of meteorites since 1969 has provided new opportunities to search for organic compounds in CM-type carbonaceous chondrites (Pizzarello et al 2001;Glavin et al 2006;Pizzarello and Shock 2010) as well as in Renazzo-type (CR) chondrites found in Antarctica with natal enantiomeric excesses of up to 60 % (Pizzarello et al 2012). …”

Reply to Elsila et al.: Large enantiomeric excesses in primitive meteorites, an analytical and computational supplement

“…We address here Elsila et al's criticism (1) regarding our recent article (2) and, in particular, their claim that our chiral analyses of isoleucine (ile)/alloisoleucine (allo) diastereomers were possibly flawed. We first remind the authors that these compounds' enantiomeric excesses (ee) were already studied recently, validated by compound-specific 13 C analyses (3), and their unambiguous meteoritic indigeneity remains the foundation for our new, expanded analyses (2).…”

“…. is not acidic and subject to racemization" (2). Indeed, α-carbon epimerization cannot remove ile/allo natal ee, but Elsila et al (1) fail to mention that it can decrease larger ee and increase lesser ee if L-ile ee ≠ D-allo ee.…”

“…We first remind the authors that these compounds' enantiomeric excesses (ee) were already studied recently, validated by compound-specific 13 C analyses (3), and their unambiguous meteoritic indigeneity remains the foundation for our new, expanded analyses (2).…”