Heat, Aromatic Units, and Iron-Rich Phyllosilicates: A Mechanism for Making Macromolecules in the Early Solar System

Abstract

The major organic component in carbonaceous chondrites is a highly aromatic macromolecular material. Aromatic organic matter and phyllosilicates are colocated in these meteorites, and it is possible that the physical association represents a synthetic chemical relationship. To explore the potential reactions that could take place to produce the aromatic macromolecular material, we heated various simple aromatic units in the presence of montmorillonite with different exchanged cations. The majority of cation-exchanged montmorillonites tested, sodium-, aluminum-, iron-, nickel-, and cobalt-rich montmorillonites, do not produce polymerization products. By contrast, Fe(3+) cation-exchanged montmorillonite readily facilitates addition reactions between aromatic hydrocarbons. A feasible mechanism for the process is oxidative coupling, which involves a corresponding reduction of the Fe(3+) cation to its Fe(2+) counterpart. A similar reduction process for the other metal cations does not take place, highlighting the importance of iron. This simple process is one feasible mechanism for the construction of aromatic macromolecules such as those found in carbonaceous chondrites. The search for a relationship between Fe(3+)-rich phyllosilicates and aromatic organic structures (particularly dimers, trimers, and more polymerized forms) in carbonaceous chondrites would represent an effective test for constraining the role of clay catalysis in the early Solar System.

Artificial Maturation of Iron- and Sulfur-Rich Mars Analogues: Implications for the Diagenetic Stability of Biopolymers and Their Detection with Pyrolysis–Gas Chromatography–Mass Spectrometry

“…It is well known that oxidizing minerals such as jarosite can result in complications during the detection of organic matter because of the evolution of O 2 during thermal decomposition (Hecht et al, 2009;Lewis et al, 2015), as well as mineral surface catalyzed transformative reactions (Watson and Sephton, 2015). Our results are consistent with the literature; the much greater diversity and magnitude of organic responses observed post-alkali/acid treatment reveals the extent to which oxidizing minerals interfere with and obfuscate the detection of organic compounds during py-GC-MS.…”

“…The origins of some specific PAHs could also be inferred; for example, phenanthrene is likely the product of the side chain removal and aromatization of abietic acid (Severson et al, 1972). However, many of the PAHs observed in the pyrolysis products of our samples may also have been produced by the iron-oxide-catalyzed pyrolysis of simple organic compounds such as oleic acid (Watson and Sephton, 2015). PAHs can also be formed as secondary pyrolysis products via free radical substitutions from even simple aromatic compounds like benzene (Moldoveanu, 2010b).…”

“…The PAH formation process is exacerbated by the presence of iron oxides, as Fe 3+ will act as an electron acceptor, oxidizing the hydrocarbon molecule by scavenging H + ions. Free radicals are formed, which leads to cyclization and aromatization of aliphatic molecules as well as the formation of aryl free radicals which then react with other molecules to form dimers and further, higher-molecular-weight polymers, through oxidative coupling reactions (Grzybowski et al, 2013;Watson and Sephton, 2015). It is difficult to determine the origin of the PAHs formed in this sample, and to do so is outside the scope of this study.…”

Organic Records of Early Life on Mars: The Role of Iron, Burial, and Kinetics on Preservation

“…Even smectite clays that do not naturally contain ferric iron can hasten organic reactions via electron acceptors such as Al 3+ , generating highly reactive free radicals that catalyze various organic reactions (Solomon and Rosser, 1965;Solomon, 1968;Almon and Johns, 1975;Johns, 1979). While this can include preservation-enhancing reactions such as oxidative polymerization (Watson and Sephton, 2015), in these iron-rich environments, the primary reaction promoted by clay surfaces is the oxidation of organic matter by iron oxides.…”

Influence of phyllosilicates on the hydrothermal alteration of organic matter in asteroids: Experimental perspectives

“…In this study, we compared the impact of two synthetic smectites of different composition on the HMT transformation in asteroidal conditions. We choose an aluminium-magnesium-bearing smectite, frequently used to study the organo-clay interaction (Wang and Lee, 1993;Saladino et al, 2010;Montgomery et al, 2011;Fuchida et al, 2014;Jaber et al, 2014;Watson and Sephton, 2015), as well as an iron-bearing smectite, to investigate the roles of the smectite composition and structure.…”