The surface of Pluto is exposed to cosmic ray particles which slowly alter the reflectance of the condensed methane and the UV absorbed in the atmosphere may produce precipitates. Depending on the rates of the competing regolith processes and the rate of replenishment of the methane the surface can appear bright, 'red', or 'dark'. Here the relevant laboratory data show that, in the absence of any local particle precipitation, the amount of darkening occurring in one orbit is small. ¿ American Geophysical Union 1989