Black line: spectrum of Mars recorded on January 24, 2017, in the northern region between 1387.9 and 1389.4 cm⁻¹, recorded on January 24, 2017. Models: H₂O = 200 ppmv, HDO = 280 ppbv (green); H₂O = 250 ppmv, HDO = 350 ppbv (red, best fit); H₂O = 300 ppmv, HDO = 420 ppbv (blue). The surface pressure is 7.5 mb and the surface brightness temperature is 212 K. The temperatures at altitudes of 0, 10, 20, and 40 km are 205, 202, 200, and 185 K, respectively. The airmass is 2.0.