The Solar System is becoming increasingly accessible to exploration by robotic missions to search for life. However, astrobiologists currently lack well-defined frameworks to quantitatively assess the chemical space accessible to life in these alien environments. Such frameworks will be critical for developing concrete predictions needed for future mission planning, both to determine the potential viability of life on other worlds and to anticipate the molecular biosignatures that life could produce. Here, we describe how uniting existing methods provides a framework to study the accessibility of biochemical space across diverse planetary environments. Our approach combines observational data from planetary missions with genomic data catalogued from across Earth and analyzed using computational methods from network theory. To demonstrate this, we use 307 biochemical networks generated from genomic data collected across Earth and “seed” these networks with molecules confirmed to be present on Saturn's moon Enceladus. By expanding through known biochemical reaction space starting from these seed compounds, we are able to determine which products of Earth's biochemistry are, in principle, reachable from compounds available in the environment on Enceladus, and how this varies across different examples of life from Earth (organisms, ecosystems, planetary-scale biochemistry). While we find that none of the 307 prokaryotes analyzed meet the threshold for viability, the reaction space covered by this process can provide a map of possible targets for detection of Earth-like life on Enceladus, as well as targets for synthetic biology approaches to seed life on Enceladus. In cases where biochemistry is not viable because key compounds are missing, we identify the environmental precursors required to make it viable, thus providing a set of compounds to prioritize for detection in future planetary exploration missions aimed at assessing the ability of Enceladus to sustain Earth-like life or directed panspermia.
The concept of the origin of life implies that initially, life emerged from a non-living medium. If this medium was Earth's geochemistry, then that would make life, by definition, a geochemical process. The extent to which life on Earth today could subsist outside of the geochemistry from which it is embedded is poorly quantified. By leveraging large biochemical datasets in conjunction with planetary observations and computational tools, this research provides a methodological foundation for the quantitative assessment of our biology's viability in the context of other geospheres. Investigating a case study of alkaline prokaryotes in the context of Enceladus, we find that the chemical compounds observed on Enceladus thus far would be insufficient to allow even these extremophiles to produce the compounds necessary to sustain a viable metabolism. The environmental precursors required by these organisms provides a map for the compounds which should be prioritized for detection in future planetary exploration missions. The results of this framework have further consequences in the context of planetary protection, and hint that forward contamination may prove infeasible without meticulous intent.It is probable that the geochemical process known as life had already commenced when 2 today's oldest minerals began to crystallize. While there is widely accepted evidence 3 that the process of life has been present on Earth continuously for the past 3.4Gy [1], 4 the lack of evidence prior to this date has more to do with the paucity of 5 fossil-preserving rocks than concrete evidence of life's absence [14,32]. Despite the 6 biosphere's apparent interminable coexistence with the geosphere, there remain many 7 open questions on the matter of life persisting in Earth's absence [3,35], not to mention 8 the questions of Earth persisting in life's absence [21,23,24]. For example, Visionaries 9 dream of terraforming planets while program officers fret over "contaminating" 10 them [25,31,34]. While the terraformers tend to believe that seeding another planet 11 1/16 would require careful human or robotic (and usually Earth-assisted) cultivation, 12 planetary protection officers take the more conservative stance that a small, 13 semi-sterilized spacecraft of Earth origin could cause life to spill onto a planet in the 14 same way that a small perturbation to a super cooled liquid would cause the entire 15 volume to quickly crystallize. In both cases, there is the predominately implicit 16 assumption that Earth-life would be viable outside of the Earth. 17When life is viewed as a geologic process, this is a somewhat surprising assumption. 18 In the words of Morowitz et al., "the metabolic character of life is a planetary 19 phenomenon, no less than the atmosphere, hydrosphere, or geosphere" [30]. If this 20 "metabolic character of life" is truly a planetary phenomenon, does that imply that life 21 is inextricable from the planet through which it emerged? Or is it possible that an 22 97We ran the network expansion algorithm on the subs...
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