Evidence for methane segregation at the surface of Pluto

In May 1995, a set of spectrophotometric curves of the system Pluto-Charon were recorded with the UKIRT telescope equipped with the spectrometer CGS4, The spectra cover the near-infrared range between 1.4 and 2.55 mu m with a resolution of approximately 700, The existence of solid methane is confirm ed by numerous absorption bands, and carbon monoxide and nitrogen ices are identified by their respective signatures at 2.35 and 2.15 mu m. We have mo deled the spectrum of May 15 that corresponds to the maximum of Pluto's vis ible lightcurve using a radiative transfer algorithm dealing with compact a nd stratified media. A geographical mixture of three distinct units is requ ired to explain all the significant structures of the analyzed spectrum. Th e first unit is a thin, fine-grained layer of pure CH4 covering a compact p olycrystalline substratum of N-2-CH4-CO, which are in a molecular mixture ( concentrations of CH4 and CO of the order of 0.5 and 0.1-0.2% respectively) . It covers about 70% of the observed area and corresponds to volatile depo sits that are sublimating under solar illumination. The second unit is eith er (a) a single thick layer of pure large-grained methane or (6) a unit wit h large-grained CH4 forming a substratum and the N-2-CH4-CO mixture a super ficial layer of fine grains covering 20% of the surface. Finally, the third unit is bright and spectrally neutral and is first modeled as a layer of v ery fine grains of nearly pure N-2. Tholin, suggested to explain the red sl ope in the visible, is also found to be spectrally compatible with this uni t. It covers the remainder of the surface (about 10-15%). All these results allow a better understanding of the processes of deposition, metamorphism, sublimation, and transport affecting the different ices detected on Pluto during its climatic cycles. (C) 1999 Academic Press.