Black line: spectrum of Mars recorded on March 24, 2016, in the northern region between 1387.9 and 1389.3 cm⁻¹. Models: H₂O = 250 ppmv, HDO = 400 ppbv (green); H₂O = 375 ppmv, HDO = 550 ppbv (red, best fit); H₂O = 500 ppmv, HDO = 700 ppbv (blue). The surface pressure is 6 mb and the surface brightness temperature is 255 K. The temperature profile is the same as that used for the integrated spectrum (see Sect. 2.2). The airmass is 2.0.