What does this equation mean?

If no gas can be trusted individually, the natural move is to look for a property of the whole atmosphere. The most general candidate is thermodynamic disequilibrium: life is a persistent chemical engine, so a biosphere should hold its planet’s atmosphere away from the composition it would relax to if all permitted reactions ran to completion. The quantity is the available Gibbs energy — the difference between the free energy of the observed composition and that of its equilibrium state at the same elemental abundances, temperature and pressure: $\Phi = G(\mathbf{n}_{\mathrm{obs}}) - G(\mathbf{n}_{\mathrm{eq}})$. Krissansen-Totton, Olson and Catling computed this across Earth history and reported values that make the appeal obvious and the limitation equally obvious. Earth’s purely gas-phase disequilibrium is small, about 1.5 joules per mole of atmosphere; when the full atmosphere-ocean fluid envelope is included, the figure rises to roughly 2,326 joules per mole, dominated by the coexistence of nitrogen, oxygen and liquid water, which would react to nitric acid at equilibrium. For the maximum Archean case — an anoxic atmosphere in which the disequilibrium is carried by the coexistence of carbon dioxide, nitrogen, methane and liquid water — they obtain about 234 joules per mole [ 2 ] .