Enhanced Evaluation of Martian Habitability in the Past and Present: The Search for Viable Electron Donors
Open Access
- Author:
- Wong, Gregory
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 09, 2020
- Committee Members:
- Christopher Howard House, Dissertation Advisor/Co-Advisor
Christopher Howard House, Committee Chair/Co-Chair
James Kasting, Committee Member
Julie Genevieve Cosmidis, Committee Member
Suvrath Mahadevan, Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- Mars
Astrobiology
Habitability
Geochemistry
Microbiology
Planetary Science - Abstract:
- Life requires energy for both growth and metabolism. On Earth, microorganisms have evolved diverse metabolisms to be able to harvest energy from oxidation-reduction (redox) reactions that involve the transfer of electrons from a reduced compound (electron donor) to an oxidized compound (electron acceptor). Because of life’s reliance on redox reactions, the search for habitable environments on other planets necessitates an investigation of the available electron donors and acceptors. On Mars, the environment today has abundant electron acceptors, such as sulfate and ferric iron. Possible electron donors on Mars, such as sulfide or reduced carbon, are less common near the surface and in the atmosphere, but represent a critical component of Martian habitability. This dissertation broadly focuses on understanding the distribution and viability of reduced sulfur and carbon as possible electron donors for a putative microbiology on Mars. Chapter 2 of the dissertation investigates the presence of reduced sulfur in Gale crater, Mars from the Mars Science Laboratory (MSL) Sample Analysis at Mars (SAM) evolved gas analysis (EGA) data. Simple mixtures of Mars-relevant compounds (including sulfides and sulfates) were investigated using laboratory SAM-like EGA. Select gases evolved from these mixtures were compared to SAM data using quadratic discriminant analysis (QDA). This novel analysis of SAM data found that reduced sulfur was likely present in numerous Martian samples from first 2300 sols of the MSL mission, implying a large spatial and temporal distribution of reduced sulfur in Gale crater. Chapter 3 expands upon Chapter 2 and investigates the distribution of reduced sulfur in the ‘clay-bearing’ (as seen from orbital data) region of Gale crater. This chapter combines EGA temperature interpretations, QDA comparisons of SAM and laboratory data, and sulfur isotope calculations as complementary methods to identify samples with reduced S. There is consistent evidence from these analyses to indicate that two samples include reduced sulfur in this area. The presence of sulfide in these samples, which represent different lithologies and times in Martian history, further supports the finding that reduced S was available for putative microbial metabolism in the ancient Martian environment. Chapter 4 investigates the origins of methane and chloromethane that are observed during thermal decomposition of solid Martian samples. The thermodynamics of potential oven reactions forming these compounds are considered, as are possible isotopic consequences. These estimates will aid future investigations of thermally-released methane and chloromethane on Mars. Chapter 5 investigates carbon monoxide, which is relatively abundant in the modern Martian atmosphere, as a possible electron donor. Numerous diverse microorganisms on Earth can oxidize CO for energy. We performed thermodynamic calculations and laboratory experiments to assess the plausibility of various CO-based metabolisms. While the thermodynamics of CO oxidation are promising, laboratory experiments suggest that microbial use of CO is likely limited by the desiccated Martian environment. The results of this dissertation expand the understanding of habitability on Mars in the past and present. Reduced sulfur, which can potentially serve as a microbial electron donor, has been identified on Mars from Curiosity rover data. This dissertation has also explored possible pools of reduced carbon that microbes could exploit for energy. Together, these results will improve interpretations of Martian habitability as exploration of the Red Planet continues.