  The methane-nitrogen phase diagram of Prokhvatilov and Yantsevich (1983)
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 $\bf{\bar{N_{2}}}$:CH$_{4}$ and methane saturated with
nitrogen $\bf{\bar{CH_{4}}}$:N$_{2}$. 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 (N$_{2}$:CH$_{4}$) and nitrogen
diluted in methane (CH$_{4}$:N$_{2}$) are presented at temperatures between 40
and 90 K, in the wavelength range 1.1-2.7 $\mu$m 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.
