Example of FP1 spectrum (after average). The top left panel shows the whole spectrum while the three other panels are a close-up around the relevant spectral bands. Measured spectra are in black. Red solid lines show the synthetic spectra calculated during the retrieval process. Data were acquired at a spectral resolution of 0.5 cm^-1. Error bars of the FP1 spectrum have been corrected because the initial error bars were too small with respect to the radiance variations caused by noise (see Sects. 2 and 3.3). C₄H₂, C₂N₂, and C₃H₄ bands are visible, and allow us to retrieve the volume mixing ratios of these species. The CH₄ bands in FP1 are used to retrieve the temperature profile between 20 mbar and 10 mbar.