USING MOLECULAR-AVERAGE ISOTOPE VALUES TO MODEL ISOTOPIC STRUCTURES AND METEORITE ORGANIC SYNTHESIS
5.1. Introduction
Carbonaceous chondrites (CCs)—a class of undifferentiated meteorites that have 2-5 wt % C (Glavin et al., 2018)—are amongst the most primitive samples of solar system material. Insights into their chemical evolution elucidate the organic chemistry in the early solar system and that which could occur in others. The organic inventory of the
CC’s includes soluble and insoluble organic matter (SOM and IOM, respectively), includes over 80 amino acids and 100 polyaromatic hydrocarbons (PAHs), and is dominated by its IOM component, which comprises ~70% of the organic matter (OM) (Glavin et al., 2018).
Concentrations of OM vary both between meteorite classes—groups of meteorites thought to be from the same or similar parent bodies, as evinced by their isotopic and mineralogic compositions—and between petrologic types—categories that reflect a meteorite’s degree of aqueous or thermal alteration from parent-body processes (Elsila et al., 2016; Glavin et al., 2018). Typically, CR, CM, and CI chondrites—the less thermally altered CC meteorite classes—have higher SOM concentration than the CH and CV chondrites—more thermally altered CC classes (Elsila et al., 2016). These differences could be attributed to different chemistries prior to parent body heating or varying degrees of degradation during parent body heating (Elsila et al., 2016). Meteorites with a more pristine petrologic type (closer to type 3.0) also have higher concentration of SOM as compared to more aqueously altered types (lower than 3.0) or thermally altered types (higher than 3.0) (Elsila et al., 2016).
Potential environments for synthesis of OM found in the carbonaceous chondrites include the interstellar medium (ISM), the pre solar nebula and early solar disk, and meteorite parent bodies. In the ISM, grain-surface chemistry is proposed to produce aldehydes, ketones, amino acids, amines, and hydroxy acids (Sandford et al., 2001; Elsila et al., 2007; Glavin et al., 2018). In the solar nebula and disk, it has been suggested that gas- grain reactions can enable Fischer-Tropsch type (FTT) synthesis; additionally, radiation- driven reactions, synthesis from meteorite impact (‘shock synthesis’), and aqueous syntheses, including hydrothermal FTT, Formose, Strecker, reductive amination, and aldehyde oxidation, could produce the SOM we observe on meteorites (Cronin et al., 1995; Kerridge, 1999; Cooper et al., 2011; Elsila et al., 2012; Glavin et al., 2018).
These scenarios of organic synthesis have been proposed and evaluated based on concentrations of key compounds, stable isotope abundance ratios (D/H, 13C/12C, etc.),
and relationships of these two variables to the degree and type of alteration and class of the meteorite sample from which the compound was extracted. Extraordinary
enrichments in abundances of rare, heavy isotopes (D, 13C, 15N), beyond the range observed in most terrestrial materials (100s of ‰ for δ13C and d15N and 1000s of ‰for δD)5.1 are observed in some meteoritic OM, and generally have been interpreted as evidence for formation in the ISM or inheritance from precursors from the ISM, because at the low relevant temperatures (10’s of K), zero-point energy effects of heavy isotope substitution in chemical bonds substantially increase the thermodynamic stabilities of substituted molecules (Pizzarello and Shock, 2010; Glavin et al., 2018). It is less clear whether this process is responsible for the subtle heavy isotope enrichments (10’s of ‰ for δ13C and d15N; 100’s of ‰ for δD) that characterize many meteoritic organic compounds.
Previous attempts to understand these more subtle heavy isotope enrichments of
meteoritic organics have examined H, C, and N isotope differences between compound classes (e.g., amino acids, MCAs, DCAs), molecular structural motifs (e.g., straight- or branched-chain), molecular size (e.g., carbon chain length), and host meteorite sample types. These efforts are complicated by the fact that isotopic composition of a given organic compound can differ between portions of a given sample and between studies.
Nevertheless, certain observations are consistent: the non-exchangeable hydrogens in most meteoritic OM is enriched in deuterium relative to terrestrial OM (Huang et al., 2007; Alexander et al., 2007; Glavin et al., 2018); most SOM—including α-amino acids, α-hydroxy acids, amines, sulfonic acids, dicarboxylic acids (DCAs), and aldehydes—are also enriched in 13C and 15N relative to terrestrial sources; and 13C enrichments in SOM
5.1hδX is equal to the ratio of the heavy isotope, h, in atom X relative to the light isotope, l, in a sample relative to that
ratio in a standard. It can be represented as: !δX =
""!
#!#
$%
""!
#!#
$%
− 1 and is typically multiplied by 1000 to be reported in
until of per mil (‰). We will use hR to represent the ratio of heavy to light isotopes ("#$$ ) and hF to represent the fractional abundance of the heavy isotope, #$"% $$" . We note that hF and hR are related such that !&= (% '"'" .
compounds generally decrease with the carbon chain length (Sephton, 2002; Glavin et al., 2018). These observations are suggestive evidence that many meteoritic compounds of prebiotic relevance formed in the ISM or from ISM-sourced precursors. However, the isotope enrichments in question are generally within a range that can be achieved by chemical isotope effects under a wide range of conditions, so this evidence alone is insufficient for a definitive association of these compounds with ISM chemistry.
Conversely, IOM and monocarboxylic acids (MCAs) tend to have 13Cabundances that are closer to terrestrial composition (Huang et al., 2005; Huang et al., 2007; Aponte et al., 2011). Finally, heavily altered meteorites (above petrologic type 3.0 or below type 2.5) have generally lower abundances of SOM and less pronounced heavy isotope
enrichments, suggesting parent-body processing and/or terrestrial contamination may influence the abundances and isotopic compositions of meteoritic OM (Elsila et al., 2016;
Glavin et al., 2018).
A commonly postulated scenario for meteoritic SOM synthesis is as follows: (1) ISM- sourced aldehydes, ketones, ammonia, and possibly CN are concentrated on meteorite parent bodies; (2) the aldehyde and ketones are aminated and either reduced into amines or react with CN to form α-aminonitriles and α-amino acids upon hydrolysis; (3) some aldehydes are oxidized into MCAs (Aponte et al., 2017). Alternatively, MCAs have been hypothesized to form by a kinetically controlled synthesis such as FTT or as a
combination of the two pathways (Yuen et al., 1984; Huang et al., 2005; Aponte et al., 2011; Glavin et al., 2018).
The scenario above provides a useful hypothetical framework for understanding the prebiotic synthesis of meteoritic SOM but has several shortcomings. Firstly, it requires that ketones are more 13C-enriched than aldehydes to create the consistent 13C
enrichments amongst α-CH3 amino and hydroxy acids and secondary amines and carboxylic acids relative to their α-H and primary analogs (Pizzarello et al., 2004; Elsila et al., 2012; Simkus et al., 2019). However, measurements of ketones are significantly lower in 13C than aldehydes of equivalent size (Simkus et al., 2019; Aponte et al., 2019)
so the ketones used to create the measured products are required to have formed from a separate pool as those in measurements.
Secondly, the scenario outlined above cannot explain the high abundances of α-CH3
compounds relative to their α-H analogs (Glavin et al., 2010; Elsila et al., 2016). For instance, the concentration of α-aminoisobutyric acid are equal to or higher than those of alanine. However, ISM measurements find that acetone (the ketone precursor to α-
aminoisobutyric acid) is only 1/15 as abundant as acetaldehyde (the aldehyde precursor to alanine) (Combes et al., 1987), and ketones are less reactive than aldehydes in both Strecker synthesis and in reductive amination (Van Trump, 1975; Gomez et al., 2002;
Lamm et al., 2013). Consequently, in addition to requiring another pool of ketones to explain the 13C abundance data, the scenario requires that α-CH3 and secondary compounds have a second synthetic pathway for formation. We note that while the abundance data could be explained by a large-scale degradation of α-H and primary compounds, this would result in the residual compounds being more 13C-enriched, which disagrees with the 13C abundance data.
A more general shortcoming of the scenario outlined above (and prior interpretations of the isotopic compositions of meteoritic organics generally) is that it is qualitative and has limited predictive capabilities. We note in particular that prior explanations of the stable isotope compositions of SOM compounds is that they do not predict isotopic structures (site-specific isotope ratios) in molecules, or quantitatively consider isotope effects in the chemistry called on to create specific compounds. For example, although dicarboxylic acids (DCAs) from the Murchison meteorite are higher in δ13C than alkanes of generally similar size, it is not obvious whether we should expect the carboxyl groups are host to exceptional 13C enrichments or, if so, whether the two carboxyl ‘ends’ of each compound are expected to be the same or different in δ13C. Several technologies are available for site-specific isotope ratio (SSIR) measurements of organic compounds, and Chimiak et al. (2020) has demonstrated that at least one of these methods (Orbitrap-based Fourier- transform isotope ratio mass spectrometry) is suitable for study of picomolar quantities of compounds extracted from a carbonaceous chondrite. This capability provides an
opportunity to test and substantially extend hypotheses regarding the origins of meteoritic organic matter, but such studies must be framed by quantitative predictions at the
compound-specific and site-specific isotope ratios implied by a hypothesized organic synthesis scenario.
This present study compiles previous measurements of compound-specific and site- specific isotopic compositions of meteoritic organics from several carbonaceous chondrites, and uses those data to derive and demonstrate the self-consistency of a quantitative model of prebiotic organic synthesis, including a specified and ordered network of reactions, consideration of isotope effects associated with those reactions, patterns of inheritance from precursors to products, and simultaneous consideration of H and C isotope compositions. In addition to creating a more comprehensive and specific hypothesized scenario of meteoritic prebiotic chemistry, the output of this model provides falsifiable predictions of site-specific and molecular-average isotopic compositions of both previously studied and previously unstudied chemical species; thus, this model can be disproven, or revised and refined, in response to future measurements.