title: Extra-terrestrial organics: organic matter in meteorites and martian analogues
Creators: name: O’Brien, Áine Clare
Publisher: name: University of Glasgow
Year: 2022

description: Astrobiology and the assessment of planetary habitability centres on the detection of organic matter as all known biological lifeforms on Earth are carbon-based. To understand how life began, and where else it may be found in the Solar System, requires knowledge of the origins of the building blocks of life (organic molecules). Mars is the most likely planet in our Solar System to host either extant or extinct life, thus the characterisation of its organics is key to improving our understanding of its habitability. Detecting organics has been a priority of Mars missions since the Viking era in the 1970s. There are two broad theories for the source of organic matter found both on Mars by rovers and in martian meteorites: martian carbon is exogenous, deposited on Mars from impacts of carbon-rich material, such as carbonaceous chondrites, or indigenous, originating from the reduced martian mantle. This thesis seeks to characterise the organic matter in martian meteorites, Mars analogues, carbonaceous chondrites, and ureilites, to determine the likely origin of martian organic material, and improve our understanding of the formation and processing of organics in the Solar System. In addition, by evaluating the protocols and techniques used to detect organic matter, this research demonstrates effective analytical workflows to maximise the scientific return from valuable planetary samples in anticipation of Mars Sample Return. This project has demonstrated that CHONPS, life’s building blocks, and available on Mars through the detection of sulfur, nitrogen, and oxygen-bearing organic molecules in small samples of martian meteorites, and orthophosphate in Nakhla. Despite these compounds being formed through abiotic processes, this demonstrates that Mars has some of the key essentials for life to form. Macromolecular carbon (MMC) inclusions detected in the shergottites Tissint, NWA 8159, and SaU 008 are likely indigenous to Mars. Similarities between the STXM XANES peaks in NWA 8159 and previously published data from ALH 84001 demonstrate that abiotic synthesis of carbon has continued on Mars for at least two billion years. Raman and STXM XANES data from Tissint and the ureilites Goalpara and Hajmah (a) demonstrate the sensitivity of meteoritic organic matter to thermal processing, both on planetary bodies and during laboratory analysis. Results from the analysis of the bulk organic content of the nakhlites Lafayette and Nakhla, as well as CM chondrites, using liquid chromatography mass spectrometry (LC-MS) indicate the presence of a mixture of extra-terrestrial organic compounds in these meteorites. Organic sulfonates detected in Lafayette and Nakhla are consistent with sulfur-bearing compounds detected at Gale Crater on Mars by Curiosity’s SAM instrument, as well as in Tissint, Nakhla, and NWA 1950 using time of flight secondary ion mass spectrometry. Numerous fatty acids detected with LC-MS in Nakhla and Lafayette show that Fischer-Tropsch-Type synthesis of organic matter probably occurred on planetary bodies. Results from the LC-MS analyses of Nakhla and Lafayette also reveal the presence of likely organic contaminants from the fall environments of these meteorites. Several plant-derived metabolites were detected in Lafayette, helping to constrain the fall scenario of this meteorite. The detectable presence of organic contaminants in small masses (&lt; 40 mg) of martian meteorites which fell ~a century ago demonstrates the importance of careful curation of extra-terrestrial samples to minimise the ingress of terrestrial compounds. Results from LC-MS analysis of the Winchcombe meteorite also show the urgency of collecting environmental control samples from the fall sites of meteorites, when possible, to distinguish between organics indigenous to the sample, and terrestrial contaminants. LC-MS analysis of martian analogues stored in polyethylene sample bags also show evidence of plastic contamination during storage, illustrating the importance of comprehensive curation records, and the avoidance of any hydrocarbon-based storage for extra-terrestrial samples. The protocols used in this project demonstrate the effectiveness of a correlative in-situ workflow when studying organics in thin sections of extra-terrestrial samples, starting with optical microscopy and Raman spectroscopy to identify macromolecular carbon, and finally analysing functional groups at the nanoscale with STXM XANES. Non-destructive microscopy at the mm/μm scale reveals the context of organic material whilst nanoscale analyses with whilst STXM XANES show molecular details of organics, whilst only destroying ~tens of microns of precious samples. LC-MS is highly effective at non-targeted analysis of the bulk organic content of igneous materials. The extraction protocol developed in this project captures a wide range of metabolite classes in all four meteorites studies, as well as the martian analogues.