The William Herschel Telescope (WHT) has participated in 2014 and 2015 in a worldwide campaign to spectroscopically follow up Pluto from the ground in support of the encounter of NASA's New Horizons spacecraft with Pluto.
Constant monitoring of the surface of Pluto is necessary because it is known to be spectrally and photometrically variable from season to season, and probably during the whole secular calendar. By gathering data at different wavelengths astronomers are able to characterize the distribution of the materials which make up the surface and atmosphere in different ways, from the layers of volatile ices (bright, whitish areas made up of methane, nitrogen, and carbon monoxide) to the more complex organic residues, which are reddish.
Last year Pluto was already observed for six nights using the WHT. The spectra, obtained using ACAM and planned as a series of overrides, showed two principal characteristics of the surface of Pluto, the clearest being the absorption bands due to methane ice. The second characteristic is the continuum slope of the spectrum which is an indicator of the colour of the surface. This colouring agent is not uniformly distributed over Pluto's surface, but changes significantly during its rotation period, which is 6.4 Earth days.
