Triton's surface ices: Distribution, temperature and mixing state from VLT/SINFONI observations

Our analyses confirm the longitudinal variation of the N2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N2 layers in which CO and CH4 are diluted. In particular, we demonstrate that CO is present in diluted (as opposed to pure ice) form. In contrast, our models support the presence of small and longitudinally variable amounts of pure CH4 ice, providing an explanation to the enhanced atmospheric CH4/N2 ratio over expectations based on an ideal N2-CH4-CO mixture. They also suggest significant smaller particles of H2O and CO2 than reported previously in Quirico et al. (1999), with CO2 being probably distributed over a large area of the surface. We find that the 2.40 µm band is primarily due to 13CO ice, with a telluric value of the 13CO/12CO ratio, and not to ethane. We infer a N2 ice temperature of 37.5  ±  1K, suggesting that the atmospheric pressure over 2010-2013 was similar to that during the Voyager 1989 epoch.