The spectrum of Pluto, 0.40-0.93 mu m I. Secular and longitudinal distribution of ices and complex organics

Context. During the past 30 years the surface of Pluto has been characterized and its variability monitored through continuous nearinfrared spectroscopic observations. But in the visible range only a few data are available. Aims. The aim of this work is to define Pluto's relative reflectance in the visible range to characterize the different components of its surface, and to provide ground based observations in support of the New Horizons mission. Methods. We observed Pluto on six nights between May and July 2014 with the imager/spectrograph ACAM at the William Herschel Telescope (La Palma, Spain). The six spectra obtained cover a whole rotation of Pluto (P-rot = 6.4 days). For all the spectra, we computed the spectral slope and the depth of the absorption bands of methane ice between 0.62 and 0.90 mu m. To search for shifts in the center of the methane bands, which are associated with dilution of CH4 in N-2, we compared the bands with reflectances of pure methane ice. Results. All the new spectra show the methane ice absorption bands between 0.62 and 0.90 mu m. Computation of the depth of the band at 0.62 mu m in the new spectra of Pluto and in the spectra of Makemake and Eris from the literature, allowed us to estimate the Lambert coefficient at this wavelength at temperatures of 30 K and 40 K, which has never been measured before. All the detected bands are blueshifted with respect to the position for pure methane ice, with minimum shifts correlated to the regions where the abundance of methane is higher. This could be indicative of a dilution of CH4:N-2 that is more saturated in CH4. The longitudinal and secular variations in the parameters measured in the spectra are in accordance with results previously reported in the literature and with the distribution of the dark and bright materials that show the Pluto's color maps from New Horizons.