Extended Data Fig. 4: Contribution function for the atmosphere as retrieved by ARCiS.

Shown in the contour colours are the relative contributions of various layers in the atmosphere to the transit spectrum as a function of wavelength. The dark horizontal line in this plot is located at the geometrically thin cloud layer, dominating the spectrum at all wavelengths. The spectral variation on top of this baseline predominantly comes from below the cloud layer at the wavelengths at which water features are present, whereas it is dominated by regions above the cloud layer for the spectral features of SO₂. This is consistent with the pressures at which we expect SO₂ to be abundant in the atmosphere from our photochemical modelling (see Fig. 3). The molecular contribution to the spectral variation in the 9.5–11.0-μm region is much fainter than at other wavelengths. This is the region in which the silicate feature is most prominent and thus also the spectral variation is dominated by the cloud layer.