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

TL;DR

This study provides temperature-dependent absorption coefficients for CH3D in methane and nitrogen ices, enabling remote sensing of D/H ratios on outer solar system bodies through spectroscopic measurements.

Contribution

It reports new laboratory measurements of CH3D absorption coefficients at various temperatures, facilitating remote D/H ratio determination in methane ice on celestial objects.

Findings

Absorption coefficients vary with temperature and ice matrix.

Data enable simulation of D/H ratios in methane ices.

Results support future telescopic D/H measurements on outer planets.

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 \mum to study CH3D bands at 2.47, 2.87, and 4.56 \mum, 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.