Pluto is very dense, so astronomers believe it is mostly rock, unlike its neighboring gas giants.
It seems to be covered with ice, perhaps composed of methane and nitrogen.
Bright and dark areas appear on the surface, and Pluto has polar ice caps.
Most astronomers expect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice.
Observations by the Hubble Space Telescope suggest that Pluto’s internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.
