Vertical profiles of N₂, CH₄, Ar, and H₂ mole fractions through time (filled squares). Note that all colour scales are linear except for H₂. Mole fractions from flyby T64 (filled diamonds) come from Westlake et al. (2014).The solar longitude is indicated below the left graphs, with relevant longitudes corresponding to the years above. By convention the northern spring equinox corresponds to the solar longitude L_s = 0°.