Equilibrium chemical abundances for a 3 M _⊕ planet with 0.1% soot for isothermal atmospheres at different temperatures. Across the entire temperature range considered, methane (black) or acetylene (violet) persists at high concentration through the observable portion of the atmosphere (∼1 mbar in transmission spectroscopy). As shown in our photochemical modeling, methane photolysis brings about the formation of organic hazes, and acetylene is an even more direct haze precursor. Circles at the bottom of the plot panels show the bottom-of-atmosphere chemical abundances from our mantle/atmosphere equilibrium model. The plotted abundance profiles would exactly match the circle symbols at the bottom-of-atmosphere temperature and pressure values derived in the mantle/atmosphere equilibrium model (Table 1).