Absorption Coefficients of the Methane-Nitrogen Binary Ice System: Implications for Pluto

TL;DR

This study provides new optical constants for methane-nitrogen ices at cryogenic temperatures, enabling more accurate modeling of their spectral properties relevant to icy dwarf planets like Pluto.

Contribution

The paper introduces the first optical constants for nitrogen saturated with methane and methane saturated with nitrogen, measured at 40-90 K, filling a key gap for thermodynamic models.

Findings

Optical constants for N2:CH4 and CH4:N2 ices are measured between 40-90 K.

Spectral behavior varies systematically with temperature and mixing ratio.

Results improve understanding of surface compositions of icy dwarf planets.

Abstract

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.