Sputnik Planitia, covering an area equivalent to a quarter of Europe or the United States, is characterized by its three to four kilometer lower elevation compared to most of Pluto’s surface. Its bright appearance is due to its predominantly filled white nitrogen ice, which constantly smoothes the surface.
The nitrogen ice in Sputnik Planitia is in a constant state of renewal. As the nitrogen ice sublimates (turns directly from solid to gas) in the summer, it is replenished by nitrogen gas from the atmosphere, which condenses and falls to the surface in the winter. This process creates a geologically young and active surface, with few craters visible.