The structure of Pluto is not very well understood at present. Nevertheless, spectroscopic observation from Earth in the 1970s has revealed that the planet surface is covered with methane ice. Surface temperature is -230 degrees C, and the frozen methane exhibits a bright coloration. However, with the exception of the polar caps, the frozen methane surface is seen to change to a dark red on the basis of observation of eclipse by its moon Charon.
Image Credit: Lunar and Planetary Institute
Credit: Lunar and Planetary Institute