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

Triton, the largest Solar System satellite beyond Saturn, was probably captured from the Transneptunian population by Neptune. It is mainly covered by N-2, CO, CO2, CH4 and H2O in solid state and, except for H2O and CO2, these species are also present in gas phase. Sublimation and recondensation of the volatile species may lead to geographical and temporal variation on the surface composition, and could participate to the formation of complex chemical compounds formed from photochemistry occurring in the atmosphere (Krasnopolsky and Cruikshank, 1999) or from irradiation of N-2 : CH4 : CO layers (Moore and Hudson, 2003). We present new near-IR observations performed at the VLT-ESO with SINFONI in 2010, 2011 and 2013, from which band depths, areas and positions of the main ice features are determined at different longitudes. We also re-reduce and re-analyze earlier data obtained during the last 20 years. Models based on the Hapke theory (Hapke, 1993) are developed to constrain the abundance, grain size, temperature, and state of mixing of the different ices (N-2, CH4, CO, CO2, H2O) as well as attempt to identify other species. For this purpose, we present and use new optical constants of CO2 measured at 35 and 54 K. Our analyses confirm the longitudinal variation of the N-2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N-2 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/N-2 ratio over expectations based on an ideal N-2-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 pm band is primarily due to (CO)-C-13 ice, with a telluric value of the (CO)-C-13/(CO)-C-12 ratio, and not to ethane. We infer a N-2 ice temperature of 37.5 +/- 1K, suggesting that the atmospheric pressure over 2010-2013 was similar to that during the Voyager 1989 epoch.