Pluto's surface is composed of a diverse mix of ices and rock. The ices include nitrogen, methane, and carbon monoxide, which can exist as gas, liquid, or solid on Pluto due to its extreme temperature variations. These ices form a thin atmosphere when Pluto is closest to the Sun, and then freeze onto the surface as it moves away. The surface also shows large regions of water ice. Notably, Pluto displays a range of surface features, including plains, mountains, valleys, and craters. The dwarf planet's iconic heart-shaped region, known as Tombaugh Regio, is a large basin filled with nitrogen and carbon monoxide ices.