Characterizing Chemical Signatures of Life via Mass Spectrometry

Abstract

The search for signs of life beyond Earth has been fueled by a natural curiosity about whether we are alone in the universe. Organic molecules, as the primary chemical components of terrestrial living organisms, are major targets in life detection missions. However, organic compounds have also been found in abundance in meteorites. They can be synthesized via abiotic processes such as lightning strikes, and naturally degraded with time. Searching for chemical signatures of life requires analog experiments to constrain chances of false positive detection and advancement of instrumentation to reduce possibilities of false negative measurements. The capacity to synthesize organics via abiotic mechanisms is influenced strongly by the redox condition. In this dissertation, the redox state of early Earth is estimated using trace element systematics in zircon grains. The Earth's surface is found to have reached a habitable redox condition as early as 4 billion years ago, coinciding with a time when Earth was still routinely bombarded by meteorites. Simulations of such high-speed impacts using high-power laser beams demonstrate the feasibility of synthesizing simple organic compounds from carbonates and nitrogen salts. Laboratory experiments and numerical modeling suggest that crater-forming impacts could have synthesized a considerable concentration of organics on the surface of Earth, Mars, and Ceres. The detection and characterization of organic molecules requires sophisticated analytical instrumentation. Laser desorption mass spectrometry (LDMS) and OrbitrapTM mass analysis are examples of next-generation techniques under development for conducting comprehensive chemical investigations in space. Although a combination of these two techniques enables the determination of the atomic composition of organic molecules, even complex polymers such as peptides, such an approach fails to recognize the 3D structure and sequencing of polymers. To facilitate the ionization and sequencing capability of peptides via LDMS, silicon nanoparticles are incorporated in the substrate as an alternative to conventional organic matrices but with reduced risk of forward contamination. The simultaneous measurement of mass to charge ratio and collision cross-section via Orbitrap mass analysis allows for rapid separation of organic molecules by their class, structure, and composition in sample mixtures. The techniques developed here are valuable for astrobiological exploration beyond Earth.