Remote sensing D/H ratios in methane ice: Temperature-dependent absorption coefficients of CH 3D in methane ice and in nitrogen ice

The existence of strong absorption bands of singly deuterated methane (CH ₃D) at wavelengths where normal methane (CH ₄) absorbs comparatively weakly could enable remote measurement of D/H ratios in methane ice on outer Solar System bodies. We performed laboratory transmission spectroscopy experiments, recording spectra at wavelengths from 1 to 6 μm to study CH ₃D bands at 2.47, 2.87, and 4.56 μm, wavelengths where ordinary methane absorption is weak. We report temperature-dependent absorption coefficients of these bands when the CH ₃D is diluted in CH ₄ ice and also when it is dissolved in N ₂ ice, and describe how these absorption coefficients can be combined with data from the literature to simulate arbitrary D/H ratio absorption coefficients for CH ₄ ice and for CH ₄ in N ₂ ice. We anticipate these results motivating new telescopic observations to measure D/H ratios in CH ₄ ice on Triton, Pluto, Eris, and Makemake.