Molecular concentrations in chemical equilibrium as a function of H, C, and O element abundances, calculated for T = 400 K and p = 1 bar. The central grey triangle marks the region within which H₂O, CH₄, and CO₂ coexist in chemical equilibrium. The thin grey lines indicate where two concentrations are equal: (dashed), (dash-dotted), and (dotted). n means particle densities. Blank regions have concentrations <10⁻⁴. In order to enhance the colour contrasts, a colour-map was chosen that is linear in from 0.01 to1.