Compound-specific Carbon Isotope Compositions of Aldehydes and Ketones in the Murchison Meteorite

Compoundspecific carbon isotope analysis (13C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Streckercyanohydrin synthesis in the early solar system. Previous 13C investigations have targeted amino acid and hydroxy acid Strecker products and reactant HCN; however, 13C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O(2,3,4,5,6pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and 13C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. 13C values, ranging from 10.0 to +66.4, were more 13Cdepleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar 12C/13C ratios. These relatively 13Cdepleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between 13C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alterationdriven chemical pathways

New Insights into the Heterogeneity of the Tagish Lake Meteorite: Soluble Organic Compositions of Variously Altered Specimens

The Tagish Lake carbonaceous chondrite exhibits a unique compositional heterogeneity that may be attributed to varying degrees of aqueous alteration within the parent body asteroid. Previous analyses of soluble organic compounds from four Tagish Lake meteorite specimens (TL5b, TL11h, TL11i, TL11v) identified distinct distributions and isotopic compositions that appeared to be linked to their degree of parent body processing (Herd et al. 2011; Glavin et al. 2012; Hilts et al. 2014). In the present study, we build upon these initial observations and evaluate the molecular distribution of amino acids, aldehydes and ketones, monocarboxylic acids, and aliphatic and aromatic hydrocarbons, including compoundspecific 13C compositions, for three additional Tagish Lake specimens: TL1, TL4, and TL10a. TL1 contains relatively high abundances of soluble organics and appears to be a moderately altered specimen, similar to the previously analyzed TL5b and TL11h lithologies. In contrast, specimens TL4 and TL10a both contain relatively low abundances of all of the soluble organic compound classes measured, similar to TL11i and TL11v. The organicdepleted composition of TL4 appears to have resulted from a relatively low degree of parent body aqueous alteration. In the case of TL10a, some unusual properties (e.g., the lack of detection of intrinsic monocarboxylic acids and aliphatic and aromatic hydrocarbons) suggest that it has experienced extensive alteration and/or a distinct organicdepleted alteration history. Collectively, these varying compositions provide valuable new insights into the relationships between asteroidal aqueous alteration and the synthesis and preservation of soluble organic compounds

Origin and Evolution of Prebiotic Organic Matter as Inferred from the Tagish Lake Meteorite

The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites' asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration and at least some molecules of pre-biotic importance formed during the alteration

Compound-Specific Carbon Isotope Compositions of Aldehydes and Ketones in the Murchison Meteorite

Compound-specific carbon isotope analysis (delta(exp 13)C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker-cyanohydrin synthesis in the early solar system. Previous delta(exp 13)C investigations have targeted alpha-amino acid and alpha-hydroxy acid Strecker products and reactant HCN; however, delta(exp 13)C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and delta(exp 13)C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. delta(exp 13)C values, ranging from -10.0(0/00) to +66.4(0/00), were more (exp 13)C-depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar (exp 12)C/(exp 13)C ratios. These relatively (exp 13)C-depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between delta(exp 13)C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration-driven chemical pathways

Extraterrestrial organic compounds and cyanide in the CM2 carbonaceous chondrites Aguas Zarcas and Murchison

Evaluating the water-soluble organic composition of carbonaceous chondrites is key to understanding the inventory of organic matter present at the origins of the solar system and the subsequent processes that took place inside asteroid parent bodies. Here, we present a side-by-side analysis and comparison of the abundance and molecular distribution of aliphatic amines, aldehydes, ketones, mono- and dicarboxylic acids, and free and acid-releasable cyanide species in the CM2 chondrites Aguas Zarcas and Murchison. The Aguas Zarcas meteorite is a recent fall that occurred in central Costa Rica and constitutes the largest recovered mass of a CM-type meteorite after Murchison. The overall content of organic species we investigated was systematically higher in Murchison than in Aguas Zarcas. Similar to previous meteoritic organic studies, carboxylic acids were one to two orders of magnitude more abundant than other soluble organic compound classes investigated in both meteorite samples. We did not identify free cyanide in Aguas Zarcas and Murchison; however, cyanide species analyzed after acid digestion of the water-extracted meteorite mineral matrix were detected and quantified at slightly higher abundances in Aguas Zarcas compared to Murchison. Although there were differences in the total abundances of specific compound classes, these two carbonaceous chondrites showed similar isomeric distributions of aliphatic amines and carboxylic acids, with common traits such as a complete suite of structural isomers that decreases in concentration with increasing molecular weight. These observations agree with their petrologic CM type-2 classification, suggesting that these meteorites experienced similar organic formation processes and/or conditions during parent body aqueous alteration.NASA’s Planetary Science Division Research Program12 month embargo; first published online 22 June 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Variations in microbial carbon sources and cycling in the deep continental subsurface

Deep continental subsurface fracture water systems, ranging from 1.1 to 3.3 km below land surface (kmbls), were investigated to characterize the indigenous microorganisms and elucidate microbial carbon sources and their cycling. Analysis of phospholipid fatty acid (PLFA) abundances and direct cell counts detected varying biomass that was not correlated with depth. Compound-specific carbon isotope analyses (δ13C and Δ14C) of the phospholipid fatty acids (PLFAs) and carbon substrates combined with genomic analyses did identify, however, distinct carbon sources and cycles between the two depth ranges studied. In the shallower boreholes at circa 1 kmbls, isotopic evidence indicated microbial incorporation of biogenic CH4 by the in situ microbial community. At the shallowest site, 1.05 kmbls in Driefontein mine, this process clearly dominated the isotopic signal. At slightly deeper depths, 1.34 kmbls in Beatrix mine, the isotopic data indicated the incorporation of both biogenic CH4 and dissolved inorganic carbon (DIC) derived from CH4 oxidation. In both of these cases, molecular genetic analysis indicated that methanogenic and methanotrophic organisms together comprised a small component (<5%) of the microbial community. Thus, it appears that a relatively minor component of the prokaryotic community is supporting a much larger overall bacterial community in these samples. In the samples collected from >3 kmbls in Tau Tona mine (TT107, TT109 Bh2), the CH4 had an isotopic signature suggesting a predominantly abiogenic origin with minor inputs from microbial methanogenesis. In these samples, the isotopic enrichments (δ13C and Δ14C) of the PLFAs relative to CH4 were consistent with little incorporation of CH4 into the biomass. The most 13C-enriched PLFAs were observed in TT107 where the dominant CO2-fixation pathway was the acetyl-CoA pathway by non-acetogenic bacteria. The differences in the δ13C of the PLFAs and the DIC and DOC for TT109 Bh2 were ∼−24‰ and 0‰, respectively. The dominant CO2-fixation pathways were 3-HP/4-HB cycle > acetyl-CoA pathway > reductive pentose phosphate cycle

The grayness of the origin of life

In the search for life beyond Earth, distinguishing the living from the non-living is paramount. However, this distinction is often elusive, as the origin of life is likely a stepwise evolutionary process, not a singular event. Regardless of the favored origin of life model, an inherent “grayness” blurs the theorized threshold defining life. Here, we explore the ambiguities between the biotic and the abiotic at the origin of life. The role of grayness extends into later transitions as well. By recognizing the limitations posed by grayness, life detection researchers will be better able to develop methods sensitive to prebiotic chemical systems and life with alternative biochemistries