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Context. During the last 30 years the surface of Pluto has beencharacterized, and its variability has been monitored, through continuousnear-infrared spectroscopic observations. But in the visible range only fewdata are available. Aims. The aim of this work is to define the Pluto'srelative reflectance in the visible range to characterize the differentcomponents of its surface, and to provide ground based observations in supportof the New Horizons mission. Methods. We observed Pluto on six nights betweenMay and July 2014, with the imager/spectrograph ACAM at the William HerschelTelescope (La Palma, Spain). The six spectra obtained cover a whole rotation ofPluto (Prot = 6.4 days). For all the spectra we computed the spectral slope andthe depth of the absorption bands of methane ice between 0.62 and 0.90 $\mu$m.To search for shifts of the center of the methane bands, associated withdilution of CH4 in N2, we compared the bands with reflectances of pure methaneice. Results. All the new spectra show the methane ice absorption bands between0.62 and 0.90 $\mu$m. The computation of the depth of the band at 0.62 $\mu$min the new spectra of Pluto, and in the spectra of Makemake and Eris from theliterature, allowed us to estimate the Lambert coefficient at this wavelength,at a temperature of 30 K and 40 K, never measured before. All the detectedbands are blue shifted, with minimum shifts in correspondence with the regionswhere the abundance of methane is higher. This could be indicative of adilution of CH4:N2 more saturated in CH4. The longitudinal and secularvariations of the parameters measured in the spectra are in accordance withresults previously reported in the literature and with the distribution of thedark and bright material that show the Pluto's albedo maps from New Horizons.

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