Toward Network-Based Planetary Biosignatures: Atmospheric Chemistry as Unipartite, Unweighted, Undirected Networks

Abstract

Previous examinations of astrophysical chemical reaction networks found that Earth's atmospheric network was distinct in its hierarchical organization and scale-free nature. If Earth's unique atmospheric network structure is due to the coevolution between the biosphere and atmosphere, it may hint at a novel planetary-scale biosignature. Here, we use updated chemical reaction networks of planetary atmospheres and explore their topologies using a plethora of diagnostic techniques from network science and graph theory, including global metrics, centrality metrics, community detection, and cluster analysis methods. We show that the topologies of atmospheric chemical reaction networks of different planetary bodies in the Solar System are distinct from one another. While we find that model networks of Earth's atmosphere do not display scale-free topology, Earth's chemical reaction network is nonetheless distinguishable from those of other Solar System atmospheres through various other metrics. Earth's network has the most nonrandom topology of all the planetary networks and, in some global metrics, is more similar to biological networks than are the other planetary networks. Finally, we discuss how further investigations of atmospheric chemical networks using more advanced network representations and novel network metrics may lead to the development of a network-based biosignature applicable to exoplanets. Plain Language Summary Atmospheric chemistry can be represented in graphical form by networks where nodes are chemical species and links are reactions. We show that the structures of atmospheric chemical reaction networks of different planetary bodies in the Solar System are distinct from one another. Of those examined, Earth's network is the most nonrandom planetary network. In certain metrics, Earth's network resembles the topologies of biological networks. We hypothesize that over billions of years of coevolution between biosphere and atmosphere, life may produce predictable patterns expressed in planetary-scale chemistry; if so, a network-based biosignature may be developed for exoplanet characterization to complement existing biosignature methods, providing a novel agnostic technique for identifying life as we do and do not know it. Testing this hypothesis will require extensive modeling of an ensemble of planetary atmospheres, combined with more advanced network representations and analysis.