Identifying signs of life on another planet would rank among the top scientific discoveries in history. Two of the most daunting challenges to achieving such a discovery are developing instrumentation to study exoplanet atmospheres and developing a quantitative framework to assess whether life creates distinctive atmospheric biosignatures. Here, David Catling will address the latter challenge, aiming to develop a quantitative framework to identify atmospheric signatures of life on distant planets. While billions have been invested in telescopes capable of studying exoplanet atmospheres, we lack robust methods to interpret this data for signs of life.   The project focuses on assessing how living organisms impact a planet’s atmosphere. As such, the research team makes assumptions about life. Firstly, they assume Earth-familiar microbial life because Earth contains the only known examples of life, and because it’s thought that if life exists elsewhere in the universe, it’s more likely to be microbial rather than plant- or animal-based. Secondly, they assume redox-based metabolism would be universal to any life form because reduction / oxidation (redox) reactions are the only class of chemical reactions that release enough energy to satisfy the high energy demands of organisms. Rather than looking for individual gases that might indicate living systems, Catling proposes examining chemical disequilibrium—multiple gases coexisting that should normally react and eliminate each other—as a more reliable biosignature.   The research team will build an integrated model simulating planetary evolution from lifeless to hosting various biospheres. They'll quantify two potential biosignatures: free energy dissipation (which should increase dramatically with biological activity) and the information content of atmospheric disequilibrium. The final step involves determining which measurable gas abundances and fluxes most reliably indicate biological activity.