The Prebiotic Synthesis of Soluble Organic Matter in Carbonaceous Chondrites and the Influence of Asteroidal Aqueous Alteration

• Author / Creator

  Simkus, Danielle N

• Carbonaceous chondrite meteorites are derived from primitive asteroids in the solar system and often contain prebiotic organic compounds including amino acids, carboxylic acids, nucleobases, and smaller precursor molecules. These soluble organic compounds, synthesized via non-biological chemical reactions in space, are key precursors to several biologically-relevant compounds including proteins, membrane lipids, DNA and RNA. Asteroidal aqueous alteration processes have been shown to play an important role in the synthesis and preservation of soluble organic compounds in carbonaceous chondrites, though the chemical processes associated with parent body aqueous alteration are not fully understood. This Ph.D. thesis incorporates method development with meteorite analyses to gain insight into the influence of aqueous alteration during prebiotic organic synthesis. The organic contents of two specific carbonaceous chondrites were investigated: (1) the Murchison meteorite, selected as a reference standard due to the large dataset of organic analyses available in the literature for comparison, and (2) the Tagish Lake meteorite, selected for its pristine nature and unique alteration-derived heterogeneity. Method optimization was carried out for the extraction, derivatization and analysis of meteoritic aldehydes and ketones, and the procedure was applied to the analysis of both the Murchison and Tagish Lake meteorites. This thesis reports the first compound-specific δ13C compositions for meteoritic aldehydes and ketones. The relatively 13C-depleted compositions (particularly for the Tagish Lake samples) indicate that the aldehydes and ketones are not primordial unreacted Strecker synthesis precursors, but rather products of secondary alteration reactions on the parent body asteroid. Potential alteration-driven sources of aldehydes and ketones (e.g. degradation of amino acids or oxidation of insoluble organic matter) are discussed. In addition to the aldehyde and ketone analyses, previously unstudied Tagish Lake specimens (TL1, TL4 and TL10a) were investigated for indigenous amino acids, carboxylic acids, and aliphatic and aromatic hydrocarbons. The analyses revealed distinct organic contents likely indicating distinct parent body aqueous alteration histories. Specimen TL1 exhibited relatively high abundances of organics and a soluble organic composition that was similar to those of previously analyzed specimens TL5b and TL11h. Specimen TL4 appeared to be minimally aqueously altered and exhibited an overall low soluble organic content, attributed to the absence of water during key synthesis reactions. Specimen TL10a contained a unique and unusual soluble organic content, apparently devoid of carboxylic acids and weakly polar aliphatic and aromatic compounds, but exhibiting low abundances of amino acids and aldehydes and ketones. Overall, these analyses support a theory of a “sweet spot” of aqueous alteration that allows for optimal synthesis and preservation of soluble organic compounds. The data also suggest that the absence of key precursor compounds in certain Tagish Lake specimens (e.g. TL10a) may have precluded subsequent organic synthesis reactions. Lastly, time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis was applied to the analysis of both the Murchison and the Tagish Lake meteorites to demonstrate the potential utility of the TOF-SIMS method for detecting amino acids in situ on meteorite fresh fracture surfaces, without the need for bulk solvent extractions. Although amino acids and characteristic amino acid fragments were positively identified in the Tagish Lake meteorite, the compounds were not confidently identified in the Murchison samples, possibly due to mass interferences between organic species. The amino acids detected in the Tagish Lake meteorite did not appear to be associated with specific minerals or textures in the meteorite, but rather appeared to be homogeneously distributed throughout the matrix.

• Subjects / Keywords

  • Prebiotic chemistry
  • Meteorites
  • Carbonaceous chondrites
  • Aqueous alteration
  • Tagish Lake meteorite
  • Soluble organic matter
  • Murchison meteorite

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36M33J8B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.