Pluto has a density of about two times that of water and it is estimated that Pluto may have a core of silicate rock about 1700 km in diameter, which is surrounded by ices of water, methane, and carbon monoxide. The crust of Pluto may be a thin coating of nitrogen, methane, and carbon monoxide ice. Hubble Space Telescope photographs (taken in infrared) show light and dark patches on the surface of Pluto that may represent terrains of different composition and perhaps different ages as well. It is considered likely that Pluto has polar caps. While Pluto may have had some internal heating early in its history, that is likely long past and the planet is quite cold and geologically inactive. There is no reason to expect that Pluto has a magnetic field.