Normalised radiance centred on the 9.6 µm O₃ features, for selected phase angles of the 1:1 SOR (a) and 3:2 SOR (b). We fit the continuum data (8–9.15 and 10.15–12.2 µm) with a quadratic polynomial and normalise the radiance. This approach allows us to isolate the O₃ feature as a function of orbital phase angle, illustrating the change from absorption to emission for the 1:1 SOR and the generally stronger absorption features for the 3:2 SOR.