The Problem Life Solves (Invited)

After forming, planets start the long process of dissipating energy into space. Early on, accretionary processes provide sufficient kinetic energy to raise temperatures enough to drive chemical systems rapidly toward equilibrium, maximizing the release of chemical energy. Eventually heat is dissipated, temperatures drop, and outer portions of planets cool enough to slow the rates of chemical reactions. As reaction rates slow to the scale of geologic time, chemical energy becomes trapped in assemblages of planetary materials far from equilibrium. Numerous examples are provided by chondritic meteorites, which show that activation energy barriers allow chemical energy to remain trapped for most of the age of the solar system even if heat dissipation is efficient -- and perhaps as a direct consequence. Activation energies that inhibit favorable reactions can be overcome by catalysis, which permits chemical systems to attain lower energy states. Catalysis in planets serves to continue the release of energy into space begun by heat dissipation. This implies that there is an overall thermodynamic drive for catalysis to appear as planets cool. Reasons why catalysis emerges in some cases and not others may depend on interactions of cooling rates and compositions but the specifics are murky at present. Life is a particularly efficient catalyst, and its emergence on a planet helps solve the problem generated by the catastrophic decrease in reaction rates during cooling. The single example we have of life on Earth got its start catalyzing oxidation-reduction reactions arranged in states far from equilibrium by geologic processes. On the pre-photosynthetic Earth the boldest biosignatures were redox processes occurring at rates that could only be explained by catalysis, and specifically by catalytic processes that have no abiotic mechanism. Biologically enhanced rates of redox reactions persist to the present, and maintain the biogeochemical cycles that permit the photosynthetic primary production that is now Earth's most aggressively profound signature of life. But, phototrophy may be far more rare than the chemotrophy on which it depends, and from which it apparently emerges. Consequently, the search for biosignatures on other planets will become the search for reaction rates that can not be explained by abiotic processes. This means that the search for extraterrestrial life will depend on remote methods of determining rates of oxidation-reduction reactions, and that much needs to be understood about abiotic catalysis of redox processes to avoid false positives.