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Abstract
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.
Highlights
Visionaries dream of terraforming planets while program officers fret over ‘‘contaminating’’ them (Rummel, 2001; Mancinelli, 2003; Musk, 2018; Kminek et al, 2019)
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
This lack of observation of more complex/high-molecular-weight molecules could, be because the upper mass limit of the Ion and Neutral Mass Spectrometer (INMS) is only 99 Da (Waite et al, 2004), and the upper mass limit of the Cosmic Dust Analyzer (CDA) is nominally 200 Da, patterns in the observed CDA spectra indicate the presence of larger organic molecules (Postberg et al, 2018)
Summary
Visionaries dream of terraforming planets while program officers fret over ‘‘contaminating’’ them (Rummel, 2001; Mancinelli, 2003; Musk, 2018; Kminek et al, 2019). Prospective terraformers tend to believe that seeding another planet with life will require careful human or robotic (and usually Earth-assisted) cultivation. Planetary protection officers adopt a more conservative stance and assume a small, semi-sterilized spacecraft from Earth could spill life onto a planet, transforming it to a living world, in the same way that a small perturbation to a super cooled liquid would cause the entire volume to quickly crystallize. Both cases adopt an implicit assumption that Earth-life is viable outside the Earth. In the words of Morowitz et al (2008, p. 8), ‘‘the metabolic character of life is a planetary phenomenon, no less than the atmosphere, hydrosphere, or geosphere.’’ If this ‘‘metabolic character of life’’ is truly a planetary
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