Life in the Universe: How Carbon-Based Chemistry Shapes the Boundaries of Habitability

Whether the Earth is the only planet to sustain habitable conditions is one key question in STFC's Science Challenges. During the last two decades, aqueous environments have been discovered throughout the Solar System. However, more than liquid water is required for life. One element crucial to habitable conditions is carbon. It is one of the six (CHNOPS) elements that constitute the minimal inventory of elements required to build life as we know it and of these six elements, it plays a dominant role as the backbone of all known biomolecules. On Earth, much of the carbon required by life in the present-day comes from other life or ancient carbon that was once part of biomass. However, on other planetary bodies where life is emerging, the carbon will be available from abiotic sources including carbon dioxide gas and organic carbon, the latter produced from a range of processes including hydrothermal activity and photolytic reactions (tholins). We know that carbon dioxide gas can be fixed by life in 'autotrophy', a common process on present-day Earth, yet the accessibility of a range of organic carbon in the Solar System remains unknown. If this organic carbon is biologically accessible, then it suggests a route for the earliest life to have used other pathways, such as fermentation, to emerge. It would also reveal fundamental insights into whether the wide distribution of organic carbon in the universe implies a similar abundance of habitable conditions. A second question raised is whether this organic carbon would always be beneficial to life. We tend to assume that organic carbon, must, by default, imply available carbon. However, in some extraterrestrial environments (e.g. Titan), these organics include nitriles (-CN), which are extremely toxic to known biochemistry. In this three-year project, we seek to address the question of whether key forms of abiotic organic carbon known to exist in our Solar System are accessible to life, whether they might make some environments uninhabitable, and thus we seek to understand how this carbon shapes habitability in the wider universe. We have three objectives which will: 1) address the availability of organic carbon in meteorites (abiotic carbon produced in protoplanetary discs), 2) investigate the toxicity of nitriles and similar extraterrestrially abundant organic compounds proposed for the origin of life, but which may be toxic for life, and 3) investigate the biological accessibility and use of 'tholins', complex abiotic organics produced in locations such as Titan and elsewhere. This project will significantly advance our understanding of how extraterrestrially-produced organic carbon influences the habitability of the universe as well as providing data that will better allow us to assess mission data.