FIGURE 2. Vertical–zonal map of ethane abundance.

This pressure–latitude cross-section of the ethane volume mixing ratio (indicated by the contours) is inverted by fitting the v₉-band of ethane (820 cm^-1) observed by the CIRS between 1 and 0.01 hPa. The temperature field used in the calculation is presented in Fig. 1. CIRS spectra measuring the temperature and the composition were obtained simultaneously at the same vertical and spectral resolution. Vertical profiles of the ethane volume mixing ratio were retrieved by means of a constrained inversion with strong low-pass filtering (see Supplementary Information).