Absorption coefficients of the methane-nitrogen binary ice system: Implications for Pluto

The methane-nitrogen phase diagram of Prokhvatilov and Yantsevich (1983. Sov. J. Low Temp. Phys. 9, 94-98) indicates that at temperatures relevant to the surfaces of icy dwarf planets like Pluto, two phases contribute to the methane absorptions: nitrogen saturated with methane N2‾:CH4 and methane saturated with nitrogen CH4‾:N2. No optical constants are available so far for the latter component limiting construction of a proper model, in compliance with thermodynamic equilibrium considerations. New optical constants for solid solutions of methane diluted in nitrogen (N2:CH4) and nitrogen diluted in methane (CH4:N2) are presented at temperatures between 40 and 90 K, in the wavelength range 1.1-2.7 μm at different mixing ratios. These optical constants are derived from transmission measurements of crystals grown from the liquid phase in closed cells. A systematic study of the changes of methane and nitrogen solid mixtures spectral behavior with mixing ratio and temperature is presented.