Spatial variability of CH4-to-CO conversion pathways in irradiated gas giant atmospheres: implications for chemical relaxation and 1D quench schemes

Abstract Close-in gas giant atmospheres are in chemical disequilibrium. Solving 3D kinetics to capture one source of this disequilibrium, transport-induced quenching, is computationally demanding, so simplifications are used. General circulation models (GCMs) relax abundances on the timescale of a single conversion pathway. 1D models place a quench level and hold abundances vertically uniform above it. We test both simplifications on CH4 against aerosol-free, solar-composition GCM simulations with thermochemical kinetics for HAT-P-11b, HD 189733b, HD 209458b and WASP-17b. We find ten CH4-to-CO conversion pathways, varying with depth, longitude and planet. A single pathway controls CH4 quenching on the three cooler planets; on WASP-17b two compete. The CH4 quench level is 3D: where the atmosphere is hotter, or a faster pathway takes over, the conversion timescale shortens, the quench level rises and the quench-level abundance falls. On WASP-17b the pathway switch lifts the quench level over the substellar tropics, deepening equatorial CH4 depletion. A 1D quench treatment with the Zahnle & Marley timescale typically locates the quench level within a factor of two in pressure for CH4 and about four for CO on every planet. It fails above the quench level, where every quench scheme holds abundances vertically uniform. The minor carbon carrier, CH4 or CO depending on the planet, is wrong by up to two orders of magnitude, on HD 189733b in opposite directions at different latitudes. Quenching sets an abundance at the quench level but does not fix it thereafter; even without photochemistry, transport and conversion keep that abundance spatially structured.