Remote Sensing D/H Ratios in Methane Ice: Temperature-Dependent Absorption Coefficients of CH₃D in Methane Ice and in Nitrogen Ice

Published in 2011: Icarus 212, 941-949.

W.M. Grundy¹, S.J. Morrison², M.J. Bovyn³, S.C. Tegler³, and D.M. Cornelison⁴

(1) Lowell Observatory, Flagstaff AZ.

(2) Cornell University, Dept. of Astronomy, Ithaca NY; 2010 summer REU student at Northern Arizona University.

(3) Northern Arizona University, Dept. of Physics and Astronomy, Flagstaff AZ.

(4) Missouri State University, Dept. of Physics, Astronomy, and Materials Science, Springfield MO; formerly at Northern Arizona University Dept. of Physics and Astronomy.

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Abstract

The existence of strong absorption bands of singly deuterated methane (CH₃D) at wavelengths where normal methane (CH₄) 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 CH₃D 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 CH₃D is diluted in CH₄ ice and also when it is dissolved in N₂ ice, and describe how these absorption coefficients can be combined with data from the literature to simulate arbitrary D/H ratio absorption coefficients for CH₄ ice and for CH₄ in N₂ ice. We anticipate these results motivating new telescopic observations to measure D/H ratios in CH₄ ice on Triton, Pluto, Eris, and Makemake.

Fig. 4. Effective absorption coefficients of three absorption bands of CH₃D diluted in methane ice, as a function of temperature in 10 K increments starting at 40 K on the bottom and ending with 90 K on top. Each successive spectrum is shifted upward by 25 cm^-1 in the left two panels and by 200 cm^-1 in the right panel. Absorption coefficients for an arbitrary ratio of CH₃D/CH₄ can be simulated with an appropriately weighted combination of these absorption coefficients and those of normal methane.

• 2.5 µm band of CH3D in CH4 at 40K
• 2.5 µm band of CH3D in CH4 at 50K
• 2.5 µm band of CH3D in CH4 at 60K
• 2.5 µm band of CH3D in CH4 at 70K
• 2.5 µm band of CH3D in CH4 at 80K
• 2.5 µm band of CH3D in CH4 at 90K
• 2.9 µm band of CH3D in CH4 at 40K
• 2.9 µm band of CH3D in CH4 at 50K
• 2.9 µm band of CH3D in CH4 at 60K
• 2.9 µm band of CH3D in CH4 at 70K
• 2.9 µm band of CH3D in CH4 at 80K
• 2.9 µm band of CH3D in CH4 at 90K
• 4.5 µm band of CH3D in CH4 at 40K
• 4.5 µm band of CH3D in CH4 at 50K
• 4.5 µm band of CH3D in CH4 at 60K
• 4.5 µm band of CH3D in CH4 at 70K
• 4.5 µm band of CH3D in CH4 at 80K
• 4.5 µm band of CH3D in CH4 at 90K
• 2.5 µm band of CH3D in N2 at 40K
• 2.5 µm band of CH3D in N2 at 50K
• 2.5 µm band of CH3D in N2 at 60K
• 2.9 µm band of CH3D in N2 at 40K
• 2.9 µm band of CH3D in N2 at 50K
• 2.9 µm band of CH3D in N2 at 60K
• 2.9 µm band of CH3D in N2 at 40K
• 2.9 µm band of CH3D in N2 at 50K
• 2.9 µm band of CH3D in N2 at 60K
• ADS catalog of refereed citations to this work.