Observation of Mars at 1212--1218 ¿ with the HST Goddard high-resolution spectrograph revealed both Mars' and Earth's D Ly-&agr; lines. The telluric line intensity is 17¿4 R (Rayleighs) at 600 km. We construct a model for H2, H, HD, and D from 85 to 500 km with escape velocities VH and VD as the model parameters. D is formed by photolysis of HDO and in reactions of HD with O, O+, and O(1D). The calculated intensity at solar minimum (F10.7 cm=70, T∞=736 K) agrees with the measured value for VD=80¿15 cm/s. We applied our model to previous detections of deuterium by means of deuterium absorption cells and ion mass spectrometry. Escapes of H and D are diffusion limited, isotope fractionation factor is 0.88, and deuterium enrichment in the escape processes is ineffective. ¿ 1998 American Geophysical Union