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 confirmed 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 modeled the spectrum of May 15 that corresponds to the maximum of Pluto's visible lightcurve using a radiative transfer algorithm dealing with compact and stratified media. A geographical mixture of three distinct units is required to explain all the significant structures of the analyzed spectrum. The first unit is a thin, fine-grained layer of pure CH4 covering a compact polycrystalline 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 deposits that are sublimating under solar illumination. The second unit is either (a) a single thick layer of pure large-grained methane or (6) a unit with large-grained CH4 forming a substratum and the N-2-CH4-CO mixture a superficial 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 very fine grains of nearly pure N-2. Tholin, suggested to explain the red slope in the visible, is also found to be spectrally compatible with this unit. 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.