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

•We present new optical constants of methane–nitrogen ice mixtures.•We study the mixtures spectral behavior at different temperatures and mixing ratios.•These data enable us to account for thermodynamic equilibrium on Pluto’s surface. 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 90K, in the wavelength range 1.1–2.7μm 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.